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The photolysis of potassium cobaltioxalate in dilute acid solution Doering, Juergen Gerhard Walter 1961

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THE PHOTOLYSIS OF POTASSIUM  COBALTIOXALATE  IN DILUTE ACID SOLUTION by JUERGEN GERHARD WALTER DOERING B.Sc,  The U n i v e r s i t y  A THESIS SUBMITTED  o f B r i t i s h Columbia,  1959  IN PARTIAL FULFILMENT OF  THE REQUIREMENTS FOR THE DEGREE 01? MASTER OF SCIENCE i n the Department of Chemistry  We a c c e p t t h i s t h e s i s as conforming required  t o the  standard  THE UNIVERSITY OF BRITISH COLUMBIA July,  1961  In p r e s e n t i n g the  this thesis i n p a r t i a l fulfilment of  requirements f o r an advanced degree a t t h e U n i v e r s i t y o f  British  Columbia, I agree t h a t the  a v a i l a b l e f o r reference  and study.  L i b r a r y s h a l l make i t f r e e l y I f u r t h e r agree t h a t  permission  f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by t h e Head o f my Department o r by h i s It i s understood t h a t f i n a n c i a l gain  copying o r p u b l i c a t i o n o f t h i s t h e s i s f o r  s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n  Department o f  CHEMISTRY  The U n i v e r s i t y o f B r i t i s h Vancouver 8, Canada.  Date  representatives.  J u l y 15,  1961  Columbia,  permission.  ii  ABSTRACT A new s e n s i t i v e s p e c t r o p h o t o m e t r y d i r e c t d e t e r m i n a t i o n o f Co C o ( I I I ) was d e v e l o p e d .  +  method f o r the  i o n i n the presence o f e x c e s s  U s i n g t h i s method the p r i m a r y quantum  y i e l d s f o r the p h o t o c h e m i c a l d e c o m p o s i t i o n o f p o t a s s i u m c o b a l t i o x a l a t e were determined a t pH 3 and 0°C over a wide range o f wavelengths.  The v a l u e s f o r the v a r i o u s mercury  l i n e s were found t o "be:  280 mu,  .369;  302 mu,  .329;  313  mu,  .362; 334 mu, .279; 365 mu, .244; 405 mu, .108; 4-35 mu, .067; 578 mu, —  .00033*  The quantum y i e l d f o r c o b a l t o u s i o n  f o r m a t i o n i s t w i c e the p r i m a r y quantum y i e l d . o f t h e quantum y i e l d on the c o b a l t i o x a l a t e detected.  No dependence  c o n c e n t r a t i o n was  The r e s u l t s are e x p e c t e d t o be e s s e n t i a l l y t h e  same i n n e u t r a l cobaltioxalate  s o l u t i o n because the i n s t a b i l i t y c o n s t a n t o f i s extremely  small.  C o r r e l a t i o n o f the quantum y i e l d s w i t h the a b s o r p t i o n spectrum  showed t h a t p h o t o a c t i v i t y i s c o n s i d e r a b l e b o t h f o r the  e l e c t r o n t r a n s f e r peak ( i n t h e U.V.) ( i n the b l u e s p e c t r a l r e g i o n ) . a b s o r p t i o n bands shows t h a t  and the f i r s t  d - d  Gaussian e x t r a p o l a t i o n  band  o f the  a constant l e v e l of p h o t o a c t i v i t y  cannot be d i r e c t l y a s s i g n e d t o each band.  F u r t h e r treatment  of the d a t a i n d i c a t e s t h a t the quantum y i e l d s can be r e p r e s e n t e d  as two d i f f e r e n t f u n c t i o n s of the l o g a r i t h m o f the  light  energy which however cannot he d i r e c t l y r e l a t e d t o the d i f f e r e n t types of t r a n s i t i o n s .  The r e s u l t s are  two  finally  e x p l a i n e d on the b a s i s o f i n t e r n a l c o n v e r s i o n between the d - d  and charge t r a n s f e r  bands.  Abstract  approved  ACKNOWLED GEMENT I wish, to express my sincere gratitude and thanks to Dr. G. B. Porter f o r h i s generous encouragement, supervision and enlightening discussions throughout the course of the work. I also wish to thank Dr. A. D. K i r k f o r h i s many h e l p f u l suggestions and i n t e r e s t i n t h i s work. I am g r a t e f u l to the U n i v e r s i t y of B r i t i s h Columbia f o r Student Assistantships during the 1959-60 and 1960-61 sessions. F i n a l l y I wish to thank the glassblowing and workshop s t a f f f o r t h e i r assistance i n the construction of parts of the apparatus.  iv  TABLE 01 CONTENTS Page CHAPTER I . A. B. C.  D.  INTRODUCTION  P r e p a r a t i o n and P r o p e r t i e s o f P o t a s s i u m C o b a l t i o x a l a t e The A b s o r p t i o n Spectrum o f ( p 4 ^ 3 ^ ~ The Decomposition o f K-Co o x ^ o H p O 1. P r i m a r y I o n i z a t i o n ^ 2. Secondary I o n i z a t i o n , L i g a n d Exchange and R a c e m i z a t i o n 3. The Thermal Decomposition 4. The P h o t o c h e m i c a l Decomposition The O b j e c t o f the I n v e s t i g a t i o n C o  CHAPTER I I . A.  1 c  0  EXPERIMENTAL  Materials 15 1. Reagents 15 2. The P r e p a r a t i o n and A n a l y s i s o f K,Co ox -»3.5H 0 .. 15 3. S o l u t i o n s < < f . . . . 16 a. Solutions Required f o r the A n a l y s i s of Cobalt 17 b. Actinometer Solutions 17 c. Light F i l t e r Solutions 18 Apparatus 18 1. O p t i c a l Systems IS a. F o r X = 313 and Above 18 b. F o r X = 302 mu and Below 21 2. F i l t e r s 21 3* Other Equipment 22  CHAPTER I I I . A. B. C.  6 8 10 13 15  7  B.  1 ^ 5 5  THE ANALYSIS OF C o ( I I ) IN THE PRESENCE OF EXCESS C o ( I I I )  The Requirements f o r the A n a l y s i s The C o b a l t - I r o n - P h e n o n t h r o l i n e System 1. The B u f f e r System 2. The A b s o r p t i o n a t 510 mu E x p e r i m e n t a l Procedure 1. C a l i b r a t i o n 2. A p p l i c a t i o n t o C o b a l t i o x a l a t e  P  25 25 26 27 27 29 29 31  V  Page CHAPTER 17. A.  B.  C.  THE  PHOTOLYSIS OF K^Co  ox^J.^O  E x p e r i m e n t a l Procedure 1. Runs at 313 mji and Above a. Calibration b. Photolysis 2. Runs at 302 mu and Below Results 1. Treatment o f Data ~, a. Quantum Y i e l d s f o r Co Ion P r o d u c t i o n b. C o r r e c t i o n s f o r Systematic E r r o r s c. P r i m a r y Quantum Y i e l d s d. Random E r r o r s 2. T a b u l a t i o n of Results Discussion 1. Comparison o f the R e s u l t s w i t h E a r l i e r Work 2. The Quantum Y i e l d as a F u n c t i o n o f Wavelength ... 3. The A c t i v a t i o n and D e a c t i v a t i o n o f Co ox,*" 4. E m p i r i c a l Treatment o f Data a. Assignment o f Constant L e v e l s o f Photoa c t i v i t y t o Both the d - d and the Charge T r a n s f e r Bands b. Comparison w i t h K,Fe ox, 5. T h e o r e t i c a l Interpretations-^ 6. S u g g e s t i o n s f o r F u r t h e r Work  BIBLIOGRAPHY  33 33 33 33 34 35 36 36 36 38 39 39 40 44 44 46 4-6 53 53 54 59 64 69  vi  LIST OF TABLES Table I II III IV V VI  Page F i l t e r s Used f o r the I s o l a t i o n of Mercury Lines  ..  22  Absorbancies of a Cobalt Analysis Solution as a Function of Time  30  Primary Quantum Y i e l d s and t h e i r Estimated Errors  41  Experimental Parameters and Primary Quantum Yields  42  Comparison of Quantum Y i e l d s of Recent I n v e s t i gations of the Photolysis of Cobaltioxalate .... 45 The E x t i n c t i o n of the F i r s t d - d and the Charge Transfer Band as Related to the Total E x t i n c t i o n and the Calculated Primary Quantum Yields  54  vii  LIST. OF FIGURES to follow Figure  page  1  Schematic Diagram o f t h e O p t i c a l System  2  P e r Cent T r a n s m i s s i o n o f t h e F i l t e r Combin a t i o n s as a F u n c t i o n o f Wavelength The Spectrum o f K-Co ox,*3.5H 0 i n pH 3 Acetate - HCl Buffer S o l u t i o n  3 4 5  6 7 8  9  10  11  19 23  p  28  The P r i m a r y Quantum Y i e l d s as a F u n c t i o n o f the C o n c e n t r a t i o n o f K^Co ox^  43  The P r i m a r y Quantum Y i e l d s as a F u n c t i o n o f Energy Shown i n R e l a t i o n t o t h e A b s o r p t i o n Spectrum  47  The G a u s s i a n E x t r a p o l a t i o n s o f the Charge T r a n s f e r Band and o f the F i r s t d - d Band ...  48  Schematic Diagram o f t h e R e l e v a n t o f the Term System o f Co5+  50  Features  The P r i m a r y Quantum Y i e l d s o f P o t a s s i u m F e r r i o x a l a t e and o f P o t a s s i u m C o b a l t i o x a l a t e as a F u n c t i o n o f t h e L o g a r i t h m o f t h e Wavenumbers  56  The A b s o r p t i o n S p e c t r a o f P o t a s s i u m F e r r i o x a l a t e and o f P o t a s s i u m C o b a l t i o x a l a t e i n Aqueous S o l u t i o n  57  Schematic Diagram o f the E n e r g y R e l a t i o n s h i p s between the V a r i o u s S t a t e s o f C o b a l t i o x a l a t e ; Energy o f d - d Band i s H i g h i n R e l a t i o n t o the Charge T r a n s f e r Band  62  Schematic Diagram o f t h e E n e r g y R e l a t i o n s h i p s between the V a r i o u s S t a t e s o f C o b a l t i o x a l a t e ; Energy o f the d - d Band i s Low i n R e l a t i o n t o the Charge T r a n s f e r Band  65  CHAPTER I INTRODUCTION A,  Preparation and Properties of Potassium Cobaltioxalate The preparation of a green cobalt ( I I I ) oxalato  complex was f i r s t reported i n 1 8 3 5 by Winkelblech ( 1 ) * without an a n a l y s i s .  Kehrmann ( 2 ) i n 1886 prepared and analyzed a  complex of the composition KjCcKCgO^j^HgO.  Later i n v e s t i -  gators were successful i n preparing the complex from Co(II) compounds using a v a r i e t y of o x i d i z i n g agents (3 - 6) and also by anodic oxidation (7 - 1 0 ) . The racemate can only be obtained i f the c r y s t a l l i z a t i o n i s c a r r i e d out below 13.2°C ( 1 1 , 1 2 ) . Above t h i s temperature Recoder (13)  found that c r y s t a l s of the d- and  1 - enantiomorphic forms are deposited.  The s o l u b i l i t y of  the racemate i s equal to 3 4 . 5 gm per 1 0 0 gm of water at 0°C. The complex i s diamagnetic.  The compound used i n t h i s investi-  gation c o n s i s t e n t l y had the formula K ^ C o ( C 2 0 ^ ) ^ » 3 . 5 H 2 0 .  This  i s the usual formula found although some i n v e s t i g a t o r s report a value of 1 0 ^ 0  (15»  16).  The c r y s t a l l i n e racemate i s  t r i c l i n i c and shows dark green well formed c r y s t a l s up to  * Numbers i n parenthesis r e f e r to bibliography.  2 1 cm i n length. B.  Thin sheets d e f i n i t e l y show dichroism.  The Absorption Spectrum of CoCC^C^)^" Because of i t s photochemical a p p l i c a t i o n the  absorption spectrum of the Co ox^  - • ion* has always been  of considerable i n t e r e s t and has been determined by several workers as e a r l y as 1893 (11, 12, 16-21).  The l a t e s t  determination of the near U.V. and v i s i b l e spectrum was made by Copestake and u r i (22) i n 1954. 24-5 mu  £ . 21400  420 mu  e=  218  596 mu  6=  165  They found three maxima:  where 6 i s the molar e x t i n c t i o n c o e f f i c i e n t . Beer's Law was —6 —1 obeyed within a concentration range of 10 to 10 molar. In 1933 Johnson and Mead (14) found that the o p t i c a l r o t a r y d i s p e r s i o n curves f o r the d- and 1- forms were not complementary although the racemate showed no net r o t a tion.  Kuhn and Bein (23) went so f a r as to postulate and  c a l c u l a t e d i f f e r e n t absorption curves i n the v i s i b l e part of the spectrum f o r the d- and 1- forms from the d i f f e r e n t r o t a r y d i s p e r s i o n curves.  Symmetry considerations however  show that t h i s i s not possible and that the observed d i f ferences must therefore be a t t r i b u t e d to erroneous measurements or impure compounds.  * ox = C 0^. 2  3 Mead (24) has  determined the  a b s o r p t i o n maxima o f K^Co the o x a l a t e  ox  on p r o g r e s s i v e l y r e p l a c i n g  i o n s w i t h e t h y l e n e - d i amine:  + Co en£  ox^  s h i f t s i n the  Co  ox^",  3+ , Co en^  .  By comparing these r e s u l t s w i t h  o f a s i m i l a r s e r i e s o f chromium compounds, he  those  showed t h a t i n  the m a j o r i t y o f c a s e s the p o s i t i o n s o f b o t h a b s o r p t i o n  bands  i n the v i s i b l e are i n f l u e n c e d by changing the m e t a l o r  one  or more o f the l i g a n d s . S h i b a t a e t a l (25) who  and  m a i n t a i n e d t h a t one  complex s a l t was by c o o r d i n a t e d  Reconsideration  o f the work o f  a l s o o f L i f s c h i t z and Rosenbohm of the  two  (20)  a b s o r p t i o n bands of a  c o n t r o l l e d by the m e t a l i o n and  the  other  groups shows t h a t the r e s u l t s a c t u a l l y f a l l  i n l i n e w i t h Mead's  observations.  Mead p o s t u l a t e s t h a t the two  bands  represent  d i f f e r e n t s t a t e s o f e x c i t a t i o n o f the c o o r d i n a t i o n e l e c t r o n s . The  energy o f these e l e c t r o n s i s m o s t l y determined by  energy o f c o o r d i n a t i o n which w i l l be m e t a l i o n and  the  s p e c i f i c to a p a r t i c u l a r  a p a r t i c u l a r coordinated  radical.  T h i s view  i s c l o s e l y a k i n t o the modern approach o f the c r y s t a l t h e o r y which i s e s s e n t i a l l y the view used by Graddon In a r e v i e w paper on the a b s o r p t i o n o x a l a t e s he  The  found t h a t the a b s o r p t i o n  s p e c t r a of  oxalate  i r o n are  o f the m e t a l atoms r a t h e r t h a n o x a l a t e  partially filled  obscure the  char-  groups.  s h e l l of d e l e c t r o n s leads to  of l i g h t by the m e t a l atom which may  (26).  s p e c t r a o f complex  complexes o f t r i v a l e n t chromium, c o b a l t and acteristic  field  absorption  absorption  by c o o r d i n a t e d  groups  The f u r t h e r a p p l i c a t i o n o f t h e Bethe-Schlapp-Penny or c r y s t a l f i e l d  t h e o r y (27, 28) has r e s u l t e d i n s t r i k i n g  s u c c e s s e s i n t h e assignment o f these weaker a b s o r p t i o n p ( 6 =  bands  1 - 10 ) which l i e i n t h e v i s i b l e and n e a r u l t r a v i o l e t  t o d e f i n i t e w e l l c h a r a c t e r i z e d t r a n s i t i o n s i n t h e case o f octahedral  complexes  and t h e i r a s s o c i a t e d t e t r a g o n a l and  rhombic d i s t o r t e d forms.  The c r y s t a l f i e l d  approximation  w i l l g i v e q u i t e good v a l u e s f o r the elnergies ( A E ) o f t h e s e t r a n s i t i o n s which a r e more o r l e s s l o c a l i z e d on the c e n t r a l m e t a l , t h e l i g a n d s m e r e l y behaving as the source o f an e l e c t r o s t a t i c d i s t u r b i n g p o t e n t i a l which p a r t i a l l y removes, i n t h e case o f c o b a l t , the degeneracy o f t h e 3d l e v e l s . f i e l d the d ^ , d _ . d „ „ , ( t ) o r b i t a l s xy' x z * y z ' e g whose a r e a s o f maximum e l e c t r o n d e n s i t y a r e d i r e c t e d away from Thus f o r an o c t a h e d r a l  Q  the l i g a n d p o s i t i o n s a r e lowered i n energy and the d 2 2 and x —y d 2 (e _) o r b i t a l s , whose areas o f maximum e l e c t r o n d e n s i t y z g are d i r e c t e d toward t h e l i g a n d s , a r e d i s t u r b e d i n energy.  The l i g h t a b s o r p t i o n  and r a i s e d  i s thought t o cause a L a p o r t e  p r o h i b i t e d p r o m o t i o n o f an e l e c t r o n from t h e t p ^ t o t h e e. o r b i t a l s as f o l l o w s : Co  3 +  5 The Van Vleck - Mulliken  or ligand f i e l d theory  (28-30) i s best used f o r those complexes where there i s strong i n t e r a c t i o n between i o n and l i g a n d o r b i t a l s the t r a n s i t i o n being e i t h e r of the Mulliken (31)  change t r a n s f e r type  or else i n v o l v i n g o r b i t a l s having a large degree of  d e r e a l i z a t i o n over the ligands and the c e n t r a l i o n .  This  intense charge t r a n s f e r t r a n s i t i o n can be represented as Co^  +  + ox  >  -  Co"** + ox" 2  -5For Co oxj  t h i s i s the band s i t u a t e d i n the  and corresponds to an oxidation reduction process  U.V.  (32).  These t r a n s i t i o n s are very intense and i t i s c l e a r that they are . Lap'orte allowed and f o r the 3d s e r i e s at l e a s t spin allowed.  I t i s these charge t r a n s f e r t r a n s i t i o n s that are  responsible C.  f o r the photochemical decomposition.  The Decomposition of K^Co  ox^'3.3H20  Four types of decomposition can be considered: primary i o n i z a t i o n , secondary i o n i z a t i o n and ligand exchange, the thermal decomposition and f i n a l l y the photochemical decomposition. solution.  A l l these decompositions occur i n aqueous  The thermal and photochemical decomposition have  also been observed i n the c r y s t a l l i n e s t a t e . 1.  Primary  Ionization  A l l K Co ox, i n s o l u t i o n i s e s s e n t i a l l y d i s s o c i a t e d x  i n t o potassium  and c o b a l t i o x a l a t e i o n s :  V « J 2.  *  3K  +  C o o ^ -  +  Secondary I o n i z a t i o n , L i g a n d Exchange and The c o b a l t i o x a l a t e i o n can be imagined  stepwise  Sacemization  to d i s s o c i a t e  i n the f o l l o w i n g way: Co ox,^"" 3  *  Co^  +  + 3ox~  I t has been found t h a t t h i s i o n i z a t i o n i s e x t r e m e l y small i n extent.  The f i r s t d i s s o c i a t i o n c o n s t a n t f o r  Co ox-,^*" 3  T  *  Co ox ~~ + ox" d 0  -20 has a v a l u e o f about 1 x 10  l e a d i n g t o an o v e r a l l d i s —60  s o c i a t i o n c o n s t a n t o f 1 x 10  •  T h i s i s an e x t r e m e l y  low  v a l u e compared t o the o x a l a t o complexes o f Mn(II) and E e ( I I I ) which have the o v e r a l l c o n s t a n t s 3.8 x 10*"  and 6.3 x 10"  respectively. Long (33i 34) i n an i s o t o p i c exchange study u s i n g  5carbon 11 l a b e l e d o x a l a t e showed t h a t the s t a b i l i t y o f Co ox^ i s n o t o n l y a dynamic one but a l s o a s t a t i c one. significant.  This i s  They found the C o ( I I I ) and C r ( I I I ) complexes t o  undergo no o b s e r v a b l e exchange w i t h i n the s h o r t time i t was p o s s i b l e t o f o l l o w the r e a c t i o n w i t h C  1 1  interval  i n spite of  the f a c t t h a t r a c e m i z a t i o n o f these compounds i s e x t e n s i v e under s i m i l a r c o n d i t i o n s .  C l e a r l y a simple  dissociative  7 r a c e m i z a t i o n mechanism i s not o p e r a t i v e . and H a r r i s (35)  who  Graziano  by u s i n g the l o n g e r l i v e d C ^ 1  L o n g s f i n d i n g s n o t w i t h s t a n d i n g the r a p i d 1  thermal  I t was  confirmed  irreversible  decomposition. At t h i s p o i n t i t i s c o n v e n i e n t t o c o n s i d e r t o what  e x t e n t p h o t o c a t a l y z e d l i g a n d exchange and p h o t o r a c e m i z a t i o n Adamson and S p o r e r (36)  occur.  that d - d  t r i e d t o v e r i f y the h y p o t h e s i s  t r a n s i t i o n s l e a d t o s u b s t i t u t i o n and r a c e m i z a t i o n  p r o c e s s e s by d i s p l a c e m e n t o f the l i g a n d s .  I n the case o f  3—  Co 0 X j  "the i n t r a m o l e c u l a r r a c e m i z a t i o n r e a c t i o n i s con-  s i d e r a b l y f a s t e r t h a n l i g a n d exchange o r the t h e r m a l decomp o s i t i o n (14, 3 3 ) •  The r a c e m i z a t i o n can be imagined t o  p r o c e e d v i a a t r i g o n a l b i p y r a m i d i n t e r m e d i a t e where o n l y one end o f a l i g a n d i s l o o s e n e d .  As the p r o c e s s i s s e n s i t i v e t o  the p r e s e n c e o f d i and t r i v a l e n t p o s i t i v e i o n s one expect t h a t i t c o u l d be r e a d i l y i n d u c e d by l i g a n d from the e  orbitals.  The  would repulsion  above workers, however, found  t h a t l i g h t a b s o r p t i o n i n the r e g i o n o f the c r y s t a l f i e l d band l e d to o x i d a t i o n r e d u c t i o n decomposition r a t h e r than to racemization although racemization i s favoured thermally. An i n v e s t i g a t i o n o f the p h o t o a q u a t i o n o f the CrC^O)^"*" i o n u s i n g i s o t o p i c a l l y l a b e l l e d water m o l e c u l e s (37)  showed the quantum y i e l d s were v e r y low,  temperature wavelength  extremely  dependent and p r a c t i c a l l y independent used.  o f the  T h i s a g a i n does not a l l o w assignment  s t r o n g p h o t o a c t i v i t y t o the d - d  bands.  of  3.  The  Thermal Decomposition  Copaux (38) above 120°C under C 0 powder.  The  found t h a t E^Co  development l e a v i n g an orange  2  »  Vraneck (19)  ordinary  2Co  ox + 3 K o x + 2 C 0 2  2  found t h a t t h i s r e a c t i o n i s f i r s t  v e r y temperature dependent:  I t proceeds slowly  temperatures but r a p i d l y on h e a t i n g ,  the p r e s e n c e o f f r e e o x a l i c a c i d (2, 7> oxalate  red  aqueous s o l u t i o n decomposes e a s i l y :  2K^Co ox^  o r d e r and  ox^*3.5H2^ decomposes  9»  especially i n  38).  i n t e r f e r e s w i t h the p r e c i p i t a t i o n o f Co  Excess ox by  the  2f o r m a t i o n of c o b a l t o  oxalate  complexes Co  ox  4— and  2  I n a more i n t e n s i v e i n v e s t i g a t i o n o f t h i s Copestake and energies  U r i (22)  f o r the  s u p p l i e d r a t e c o n s t a n t s and  d e c o m p o s i t i o n i n n e u t r a l and  They f o u n d the r e a c t i o n to be i n n e u t r a l s o l u t i o n but not substrates  l i k e ethanol The  mechanism o f the  e s t i n g here because the t o be  involved  position. not no  The  .  system  strength  Organic rate.  thermal r e a c t i o n i s i n t e r -  same i n t e r m e d i a t e  species  i s thought  as t h a t p r o p o s e d f o r the p h o t o c h e m i c a l decomabove workers e s t a b l i s h e d t h a t the r a t e  a f u n c t i o n o f the  oxalate  (33-35)•  was  i o n c o n c e n t r a t i o n , a n d as t h e r e  exchange d i r e c t p a r t i c i p a t i o n o f the  r u l e d out  ox^  acid solution.  i n acid solution. a f f e c t the  Co  activation  independent o f i o n i c  apparently  at  oxalate  i o n can  be  They thought a pseudomonomolecular course  i  would f i t the Co  data ox^"+  best: H0  »  2  HCo  ox ~  + 0H~  2  +  ox"  To them a d i r e c t u n i m o l e c u l a r mechanism seemed u n l i k e l y a l s o because the be  a c t i v i t y o f Co  dependent on i o n i c s t r e n g t h .  the f i r s t  step  i s o x i d i z e d by  The  ox^  would t h e n  r a d i c a l ox"  formed i n  another molecule o f  cobalti-  oxalate Co  ox^~  + ox"  *  Co  T h i s r e a c t i o n i s exothermic and n o t S t r a n k s and on a s t u d y o f the  Co  Co  oxj-^  >  C 0^~ 2  step  and  Co  ox  based t h e i r argument  l i b e r a t e d from  2  + C0  2  The  reversible i n i t i a l  ox*  ^  the  +  2  Co  ox  2  observed nonexchange  process 2-  0  C0  2  +  would be  ox  -  tenable  o n l y i f the  i s slow s i n c e r a p i d e l e c t r o n exchange between  C 0^"  i s very  2  The  2  determining,  c o n s i s t e n t w i t h the  3  + 2C0  following unidirectional reaction  proposed by Adamson e t a l (40) reverse  who  r a d i o a c t i v i t y of C0  T h i s r e a c t i o n would be of o x a l a t e .  rate  coworkers (39)  decomposing system p r e f e r the  + 3ox"  2 +  likely.  complete scheme proposed by S t r a n k s  coworkers i s as  follows:  and  ( i ) Co  ox^~  ( i i ) Co  + H0  *  Co  ox *OC Oj»H 0 "*  ox *0C 0^*H 0^*"  •  Co  ox  ox  •  2  2  ( i i i ) Co  2  5 5  -  +  x  C0 ~ 2  C o  5  2  o x  2  2 2  3  2  " + C0 +  C 0  2  + C0 ~  +  2  HQ 2  2  R e a c t i o n ( i ) a l l o w s f o r r a p i d monoaquation oxygen exchange and  r a c e m i z a t i o n o f the c o b a l t a t e  other processes. p a t h f o r the  independent  of  A l l three r e a c t i o n s provide a u n i d i r e c t i o n a l  d e c o m p o s i t i o n by  r e s u l t s o f G r a z i a n o and  internal electron transfer.  Harris  (35)  are c o n s i s t e n t  The  with t h i s  scheme* 4.  The  P h o t o c h e m i c a l Decomposition  Beacom (41) K^Co  only r e c e n t l y reported  ox^*3.5H 0 are p h o t o s e n s i t i v e 2  light.  that c r y s t a l s of  e s p e c i a l l y to u l t r a v i o l e t  A l l investigations excluding  the p r e s e n t i n q u i r y have  neglected t h i s f a c t . The  r e s u l t s o f e a r l y i n v e s t i g a t i o n s of the  r e a c t i o n i n aqueous s o l u t i o n (16, each o t h e r and The  l e a d t o few  19,  42,  43)  consistently v a l i d  o v e r a l l r e a c t i o n i s the  light  conflict  with  conclusions.  same as t h a t f o r the  thermal  reaction: 2Co  ox^~  *•  2Co  I f s u f f i c i e n t oxalate w i l l p r e c i p i t a t e e i t h e r not  ox  + 3ox~  + 2C0  2  i o n s are p r e s e n t the  Co  at a l l or only incompletely  ox (44)  11 w i t h the f o r m a t i o n o f the c o b a l t o complexes Co o x Co  and  2  ox^ The  complex i s o n l y p h o t o s e n s i t i v e i n the "blue and  u l t r a v i o l e t p o r t i o n s o f the spectrum  and i r r a d i a t i o n w i t h  monochromatic l i g h t c o n f i r m s t h a t the r e a c t i o n i s of z e r o order.  The  quantum y i e l d i n c r e a s e s w i t h f r e q u e n c y .  o r d i n a r y methods i t was effect.  n o t p o s s i b l e t o observe  an  after  Bhagwat and Dhar (16) found t h a t a d d i t i o n o f a c i d s  d i d n o t change the quantum y i e l d . temperature  They observed  a small  coefficient.  In a f a i r l y competent s t u d y o f the system (43)  Using  Murgulescu  f o r m u l a t e d the r e a c t i o n as: Co ox^  x_  *  Co o x  22  Co oxj-' + ox^2^4"  -  2-  x_  The  + ox  r  a  (  3  -i  ^ c  a  l  i°  n  Co o x  2  P-  + ox  + 2C0  2  which a c c o r d i n g t o Weiss  (45)  would be expected t o have b o t h o x i d i z i n g and r e d u c i n g p r o 3-  p e r t i e s would, b e s i d e s b e i n g a b l e t o be o x i d i z e d by Co ox^ be e x p e c t e d t o reduce T h i s was C 0^~ 2  such substances  shown t o be the c a s e .  The  as m e r c u r i c  ,  chloride.  o x i d i z i n g p r o p e r t i e s of  have n o t been demonstrated d i r e c t l y but i t i s known t h a t  the i o n f a c i l i t a t e s the p o l y m e r i z a t i o n o f v i n y l  compounds.  Copestake and U r i (22) r e c e n t l y measured the f o l l o w i n g quantum y i e l d s (  ) r e l a t i n g t o the C o  2 +  ion  formation i n n e u t r a l s o l u t i o n and at room temperature where 313 mu,  the thermal decomposition i s almost n e g l i g i b l e : 365 mu,  . 6 9 ; 405 mu,  .17;  435 mu,  0.12.  With complete  absorption the decomposition was of zero order.  .73;  light  Furthermore  the rate was unchanged i f oxygen free solutions were used. lowered pH only increased the thermal r e a c t i o n .  A  In t h i s case  the rate of decomposition i s represented by them as follows:  d  [  C  o  o x  dt  5 " 3  = k [Co ox, "] 5  +  ^  0  where k i s the rate constant of the thermal r e a c t i o n and N the number of quanta absorbed per u n i t time and u n i t volume.  They  established that the quantum y i e l d i s independent of the l i g h t 2+ Pi n t e n s i t y , the Co i o n concentration and the ox i o n conc e n t r a t i o n . An induction period was not observed. The following mechanism was proposed: (i) (ii) (iii) (iv)  Co oxj  » Co oxj  Co ox,^ *3  -  Co ox,^5>* ^ Co ox^"* + o x  -  photoexcitation  > Co ox,-^ ~w — 3  primary dark back r e a c t i o n  > r\ _ » fCo 0 X 02 - + ox  d i s s o c i a t i o n of exited complex  » Co  As the a d d i t i o n of Co  2 +  +  + 3ox ~ + 2C0 2  2  i o n has no e f f e c t they  concluded that there i s no s i g n i f i c a n t back r e a c t i o n . In 1958 Parker and Hatchard (46) d i d some preliminary experiments on t h i s system with f l a s h p h o t o l y s i s .  Their  r e s u l t s p o i n t t o a s l i g h t m o d i f i c a t i o n . o f the above  scheme  p o s s i b l y i n v o l v i n g a d i o x a l a t o c o b a l t i c oxalate r a d i c a l i n step ( i v ) . 3-  I n c o n s i d e r i n g the spectrum o f Co ox^ to  i n relation  the quantum y i e l d s Copestake and U r i were l e d t o  conclude  t h a t the l i g h t a b s o r p t i o n peak i n the b l u e has i t s e l f measure o f p h o t o c h e m i c a l  activity.  a  This i s of considerable  i n t e r e s t as t h i s peak can h a r d l y he i n t e r p r e t e d as an e l e c t r o n t r a n s f e r peak as i s the case w i t h the peak i n the u l t r a v i o l e t . I n an attempt t o c o r r e l a t e the s p e c t r a o f complex c o b a l t and chromium compounds w i t h quantum y i e l d s Adamson and Sporer at  (36)  measured the quantum e f f i c i e n c i e s  ($o) o f Co  ox^~  370 and 550 mu and found v a l u e s o f 1 . 0 and .0069 r e s p e c -  tively.  A l s o t h e y came t o t h e g e n e r a l c o n c l u s i o n t h a t the  photochemical  r e a c t i o n s f o r the compounds they c o n s i d e r e d do  n o t depend upon whether the a b s o r p t i o n band i s c o n s i d e r e d t o he o f c r y s t a l f i e l d o r e l e c t r o n t r a n s f e r t y p e . D.  The O b j e c t  o f the I n v e s t i g a t i o n  The p r e s e n t  i n v e s t i g a t i o n was undertaken i n an  attempt t o throw new l i g h t on the c o n t r o v e r s y over the a s s i g n ment o f p h o t o a c t i v i t y t o d e f i n i t e a b s o r p t i o n bands. t h i s i t was n e c e s s a r y  to obtain accurate  a wide range o f wavelengths. to  insure t h i s :  To  quantum y i e l d s  The f o l l o w i n g s t e p s were  achieve over taken  14 As K^Go  ox^*3»5  i s photochemically active i n  the s o l i d s t a t e , r e c r y s t a l l i z a t i o n s were c a r r i e d out. i n the dark and the compound was s t o r e d i n l i g h t t i g h t c o n t a i n e r s . A l l p r e v i o u s work r e l i e d on t h e decrease  o f the  3-  absorbance o f Co ox^  , as the c o n c e n t r a t i o n o f t h i s s p e c i e s  d e c r e a s e s , f o r the d e t e r m i n a t i o n o f quantum y i e l d s .  Besides  e r r o r s t h a t a r e i n t r o d u c e d b y the t h e r m a l r e a c t i o n and the photochemical  decomposition  by the m o n i t o r i n g l i g h t t h e r e i s  the s y s t e m a t i c e r r o r t h a t a r i s e s from the b a s i c i n a b i l i t y t o determine  a c c u r a t e l y t h e amount o f d e c o m p o s i t i o n  d i f f e r e n c e o f two l a r g e q u a n t i t i e s .  from the  Considerable e f f o r t  was  t h e r e f o r e expended t o d e v i s e a q u i c k and a c c u r a t e means o f d e t e r m i n i n g s m a l l amounts o f C o ( I I ) i n the presence  of a large  excess o f C o ( I I I ) . L i g h t i n t e n s i t i e s were measured u s i n g the new s e n s i t i v e f e r r i o x a l a t e a c t i n o m e t e r d e v e l o p e d by Hatchard and Parker ( 4 7 ) . A l l quantum y i e l d s were measured a t 0°C i n o r d e r t o s t a n d a r d i z e t h e r e s u l t s and t o minimize  the thermal  reaction.  I n a d d i t i o n the s o l u t i o n s were o f c o n s t a n t i o n i c s t r e n g t h and b l a n k s were r u n t o c a n c e l t h e e f f e c t s o f the thermal and o t h e r s y s t e m a t i c An attempt  reaction  errors. i s made t o c o r r e l a t e the r e s u l t s  f o r the p h o t o d e c o m p o s i t i o n  obtained  w i t h t h e a b s o r p t i o n spectrum.  work i s examined c r i t i c a l l y i n t h i s r e s p e c t .  Earlier  CHAPTER I I EXPERIMENTAL A.  Materials 1.  Reagents A l l r e a g e n t s used f o r t h e p r e p a r a t i o n o f o t h e r  compounds and f o r the p r e p a r a t i o n o f s o l u t i o n s were o f a n a l y t i c a l r e a g e n t grade.  The A.R. Co SO^TE^O used as a  s t a n d a r d i n t h e C o ( I I ) a n a l y s i s was r e c r y s t a l l i z e d t h r e e times t o i n s u r e p u r i t y and p r o p e r water c o n t e n t .  Potassium  f e r r i o x a l a t e f o r t h e a c t i n o m e t e r s o l u t i o n s was p r e p a r e d as recommended by H a t c h a r d and P a r k e r ( 4 7 ) . 2.  The P r e p a r a t i o n and A n a l y s i s o f K,Co ox,*3«5H20 The method used was e s s e n t i a l l y the one employed  by Sorensen  (4) w i t h a few m o d i f i c a t i o n s :  A m i x t u r e o f 1 2 , 5 gms CoCO^, 125 ml o f s a t u r a t e d K o x s o l u t i o n and 110 ml s a t u r a t e d HgOX s o l u t i o n were c a r e 2  f u l l y h e a t e d i n a water b a t h u n t i l a l l CoCO^ d i s s o l v e d . m i x t u r e was c o o l e d t o 40°C,  The  t r e a t e d w i t h 15 gm PbOg and  25 ml 50% a c e t i c a c i d i n s m a l l p o r t i o n s , c o o l e d and f i l t e r e d . E t h a n o l was s l o w l y added t o t h e f i l t r a t e u n t i l  precipitation  16 o f t h e complex was complete. the  The supernate was d e c a n t e d and  c r y s t a l s were washed s e v e r a l t i m e s w i t h 98% e t h a n o l and  filtered off.  The r e a c t i o n s were as f o l l o w s :  H o x + CoCO,  >  2  4K ox + Pb0 2  2  Co ox + H 0 2  + 2Co ox + 4 H C H * 0 2  i> 2KjCo ox* + 2 K C H 0 2  The  5  2  +  + C0  2  2  FbCCgHjOg^ + 2H 0 2  p r o d u c t was r e c r y s t a l l i z e d t h r e e  dark a t i c e temperature. adding the ethanol these p r e c a u t i o n s  times i n t h e  Slow c r y s t a l l i z a t i o n was i n s u r e d by  r e q u i r e d o v e r a p e r i o d o f two h o u r s . A l l i n s u r e d t h a t t h e p r o d u c t was pure racemic  d - 1 K*Co o x « 3 . 5 H 0 . 2  2  A n a l y s i s o f t h e compound u s i n g the method  described  i n c h a p t e r I I I showed t h a t i t c o r r e s p o n d e d t o t h e s t a t e d f o r m u l a w i t h i n 0.1%.  The water o f h y d r a t i o n was checked  i n d e p e n d e n t l y by a b s o r b i n g  t h e water e v o l v e d  on h e a t i n g t h e  complex i n a stream o f d r y n i t r o g e n by means o f magnesium perchlorate  tubes.  Temperatures ranged f r o m 1 0 0 t o 2 0 3 ° C .  A g a i n i t was c o n f i r m e d t h a t t h e water o f h y d r a t i o n  corresponded  to 3 * 5 molecules. 3.  Solutions For the p r e p a r a t i o n  water was employed.  of a l l solutions  redistilled  S o l u t i o n s o f c o b a l t i o x a l a t e were  pre-  p a r e d i n a b u f f e r s o l u t i o n (PH = 3*0) o f i o n i c s t r e n g t h 0 . 7 9 -  As the concentration of cobaltioxalate was u s u a l l y of the order 1 x 10""^ M/1  or l e s s , these solutions can be considered  as being of approximately constant i o n i c strength. a.  Solutions Required f o r the Analysis of Cobalt  The pH3 buffer was required i n order to f a c i l i t a t e the C o l l a n a l y s i s .  The acetate - HCl b u f f e r used was pre-  pared according to i n s t r u c t i o n s from Vogel (48). r e d i s t i l l e d reagent grade.  The HCl was  A c a l i b r a t e d Beckman zeromatic  pH meter was used to confirm the value of the pH. Using t h i s b u f f e r as a solvent a . 0 0 0 5 M s o l u t i o n of FeCl^ and a 0.1  N s o l u t i o n of 1:10 phenanthroline mono-  hydrate were prepared.  The above solutions were stable f o r  r e l a t i v e l y long periods of time. b.  Actinometer Solutions  Quantities of l i g h t were measured using the f e r r i o x a l a t e actinometer (47).  A c t u a l l y a procedure  slightly  modified from that recommended by Hatchard and Parker was used.  Three solutions were prepared: (i)  A s o l u t i o n .006 M i n K^Fe ox^ and 0.1 H i n I^SO^  as an actinometer s o l u t i o n f o r wave-  lengths of 365 mu and below. (ii)  A s o l u t i o n .15 M i n K^Fe ox^ and 0.1 N i n ^SO^  as an actinometer s o l u t i o n f o r wave-  lengths of 405 mu and above.  A d e v e l o p i n g b u f f e r s o l u t i o n 0.18  (iii)  H S0 2  4  and 0.30  1% o f 1 : 1 0  N i n NaC H^0 2  2  N in  and c o n t a i n i n g  phenanthroline.  These s o l u t i o n s were s t o r e d i n b l a c k b o t t l e s i n the dark u n t i l t h e y were r e q u i r e d .  The  special buffer  s o l u t i o n ( i i i ) i f mixed w i t h a c t i n o m e t e r  solutions ( i ) or ( i i )  i n the p r o p e r p r o p o r t i o n s a v o i d s the use  o f two  s o l u t i o n s as recommended by H a t c h a r d  Parker.  c.  Light F i l t e r Solutions  In  o r d e r t o i s o l a t e a f a i r l y narrow wavelength band  c l o s e t o the 3 1 3 0 0.0132  and  developing  1 mercury l i n e two  light f i l t e r  solutions,  M p o t a s s i u m hydrogen p h t h a l a t e and 0.0229 M K C r 0 2  were p r e p a r e d .  The  former s o l u t i o n i s u n s t a b l e towards  and has t o be r e p l a c e d a f t e r s e v e r a l r u n s .  One  4  light  centimeter  t h i c k n e s s e s o f the above s o l u t i o n s were combined w i t h a C o r n i n g No.  9863 broadband U.V.  filter.  s o l u t i o n as p r e s c r i b e d by Hunt and Davies B.  (49)  was  chloride omitted.  Apparatus 1.  313  The n i c k e l  mu  O p t i c a l Systems X  = 513  a.  For  The  apparatus  mu  and Above  used f o r i r r a d i a t i o n s w i t h wavelengths  and above i s s c h e m a t i c a l l y shown i n f i g u r e 1 .  medium p r e s s u r e mercury a r c lamp was  A B.T.H.  employed f o r these  runs.  FIGURE 1  Schematic Diagram o f the O p t i c a l S y s t  KEY  1. 2. 3h.  56. 78. 9-  10. n.  12. 13Ik. l j. c  16. 1718.  mercury arc l i g h t "beam doubly convex quartz . shutter l i g h t t i g h t box c i r c u l a r aperture filters quartz p l a t e side beam side beam actinometer main beam collimator evacuated tube 1"1. beaker c i r c u l a r aperture reaction c e l l main beam actinometer i c e water mixture  Fig.  T h i s lamp was n o t c o o l e d .  The beam, made approximately-  p a r a l l e l by means o f a d o u b l y convex q u a r t z l e n s , p a s s e d a l i g h t t i g h t box c o n t a i n i n g two  compartments.  I n the  into  first  the f i l t e r s were accommodated and i n the second a q u a r t z window was  p l a c e d i n the beam at an angle o f 4-5° such t h a t  a p p r o x i m a t e l y 6% o f the beam was  deflected at r i g h t  angles.  An a c t i n o m e t e r p l a c e d i n t o t h i s s i d e beam measured the lamp output i n d e p e n d e n t l y from an a c t i n o m e t e r i n the main beam once the r a t i o o f the two beams had been e s t a b l i s h e d . The i n t o a one  l i g h t beam now  l i t e r beaker h o l d i n g the r e a c t i o n c e l l  actinometer.  D u r i n g runs t h i s beaker was  c r u s h e d i c e water m i x t u r e . of  passed through a c o l l i m a t o r and  and  an  f i l l e d with a  I n o r d e r t o a v o i d the  condensation  m o i s t u r e on the q u a r t z window o f the beaker i t was r e p l a c e d  by an evacuated tube  ( ~ 10"^  mm  Hg)  a p p r o x i m a t e l y 10 cm l o n g  and h a v i n g two p l a n e p a r a l l e l q u a r t z windows. c e l l was  reaction  p l a c e d i n c l o s e c o n t a c t w i t h the i n n e r q u a r t z window  i n o r d e r t o minimize a p e r t u r e 1.4-5  light  a b s o r p t i o n by water.  A  circular  cm i n diameter j u s t i n f r o n t o f the r e a c t i o n  c e l l d e f i n e d the l i g h t beam. on an o p t i c a l bench.  The  e n t i r e apparatus was  aligned  A movable t r o l l e y i n s u r e d c o r r e c t  p o s i t i o n i n g o f the beaker beam.  The  and the r e a c t i o n c e l l  i n the  light  A l l p a r t s o f the apparatus were made l i g h t t i g h t  by  e n c l o s u r e and p a i n t i n g w i t h d u l l b l a c k p a i n t . The  o p t i c a l system was  oxalate actinometer.  c a l i b r a t e d u s i n g the  ferri-  V a l u e s f o r the quantum y i e l d s were t a k e n  21  from H a t c h a r d  and P a r k e r  (4-7)  o r e x t r a p o l a t e d from  v a l u e s i f t h e y were n o t d i r e c t l y a v a i l a b l e . e x t i n c t i o n c o e f f i c i e n t of f e r r o i n at 510  mu  The  these  molar  were  determined  ZL  to  be 1 . 1 0  x 10  by Dr. A. D. K i r k o f t h i s l a b o r a t o r y u s i n g  the Unicam SP 600  visible  spectrophotometer.  c l o s e l y w i t h the v a l u e g i v e n by H a t c h a r d b.  For  A = 302  mu  and  and  This  agrees  Parker.  Below  Runs below and i n c l u d i n g 3 0 2  mu  were c a r r i e d  out  w i t h the l i g h t beam emerging from a Bausch and Lomb monochromator.  The  light  source was  an a i r c o o l e d Hanovia h i g h  p r e s s u r e compact mercury argon a r c lamp o f type F o r these e x p e r i m e n t s no  s i d e beam was  10-A-l.  a v a i l a b l e and  b r a t i o n s were c a r r i e d out by u s i n g the average l i g h t  caliinten-  s i t i e s r e c o r d e d by a p h o t o c e l l b e f o r e and a f t e r each r u n i n c o n j u n c t i o n w i t h an 2.  actinometer.  Filters The  filters  used f o r i s o l a t i n g the v a r i o u s mercury  l i n e s are l i s t e d i n t a b l e I .  The  glass f i l t e r s  were o b t a i n e d  from C o r n i n g G l a s s Works i n s t a n d a r d t h i c k n e s s e s . cent t r a n s m i s s i o n of the f i l t e r combinations a f u n c t i o n o f wavelength i n f i g u r e 2 . filters  were e f f e c t i v e  The  per  i s plotted  as  I t shows t h a t a l l  i n e x c l u d i n g the n e i g h b o u r i n g mercury  l i n e s and t r a n s m i t t i n g o n l y a t the d e s i r e d wavelength.  22 TABLE I FILTERS USED FOR THE ISOLATION OF MERCURY LINES  -1 cm  Relative line intensity  2804  35663  2  monochromator  3020  33112  6  monochromator  3130  31949  13  Mercury l i n e (*)  Corning color specification  F i l t e r s employed  1 cm .0132 MKHCoH.O. 1 cm .0228 MK C?0£ Corning No. 9863 2  3340  29940  -  3650  27397  22  7-83  Corning No. 7380 No. 5860  4046  24-716  7  5-62  Corning No. 3060 No* 4308 No. 597Q  4358  22946  16  5-74  Corning No. 3389 No. 5113  5780  17301  30  3-110  Corning No. 3480 No. 4303  3.  Schott interference filter  Other Equipment A Cary 14 recording spectrophotometer was used f o r  the p l o t t i n g of spectra and a Unicam SP 600 manual spectrophotometer was employed f o r the measurement o f absorbancies i n the v i s i b l e region at s p e c i f i e d wavelengths. The  cobaltioxalate solutions were i r r a d i a t e d i n  FIGURE 2  The p e r c e n t t r a n s m i s s i o n o f the combinations as a f u n c t i o n o f f o r the f o l l o w i n g wavelength from l e f t t o r i g h t ,  (in  313.0 334.0 365.0 404.6 435.8 578.0  mu.)  filter  wavelength i n order  .CHAPTER I I I THE ANALYSIS OF C o ( I I ) IN THE PRESENCE OP EXCESS C o ( I I I ) A.  The Requirements  f o r the A n a l y s i s  P r e v i o u s i n v e s t i g a t o r s had used the decrease i n absorbance  o f the Co ox-r  position.  But t h i s method was  i s extremely d i f f i c u l t  i o n as a measure o f the decomfelt  t o determine  q u a n t i t y as the d i f f e r e n c e o f two In  accurately a small  large  quantities.  the s e l e c t i o n o f the t e s t i t was  i t be s p e c i f i c t o C o ( I I ) . c o n s i d e r a t i o n s was result.  t o be u n s u i t a b l e as i t  I t was  essential  that  B e s i d e s t h i s one o f the main  the l e n g t h o f time i n v o l v e d t o o b t a i n the  e s s e n t i a l t o minimize  the number o f maul-  's— p u l a t i o n s because For  o f the t h e r m a l d e c o m p o s i t i o n o f Co ox^  •  t h i s r e a s o n any method i n v o l v i n g a s e p a r a t i o n had t o be  ruled  out. A s p e c t r o p h o t o m e t r i c method seemed most d e s i r a b l e .  A s u i t a b l e a b s o r b i n g s p e c i e s would have t o have a h i g h e x t i n c t i o n c o e f f i c i e n t i n o r d e r t o make the method s e n s i t i v e . Chemical p r e t r e a t m e n t would have t o be kept t o a minimum: A c o l o r e d complex s h o u l d p r e f e r a b l y be d e v e l o p e d by the simple a d d i t i o n o f a few r e a g e n t s t o the r e a c t i o n m i x t u r e . D e v e l o p i n g time s h o u l d be s h o r t .  The wavelength  o f maximum  a b s o r p t i o n o f the C o ( I I ) complex s h o u l d be a t a wavelength where the a b s o r p t i o n of Co ox^ minimum: c l o s e t o 358 complexing  agent  3-  /  is l°  or 4°A mu.  w  °r p r e f e r a b l e a t a  I f the a d d i t i o n of the  f o r C o ( I I ) a l s o r e s u l t s i n the c o n v e r s i o n o f  3Co ox^  i n t o some o t h e r complex, then a l s o t h i s complex must  not absorb  a p p r e c i a b l y a t the wavelength o f maximum a b s o r p t i o n  of the C o ( I I ) complex.  I n any case d i f f e r e n t i a l s p e c t r o -  photometry u s i n g a b l a n k d e v e l o p e d under the same c o n d i t i o n s w i l l p r o v i d e an a c c u r a t e d e t e r m i n a t i o n o f C o ( I I ) . B.  The  Cobalt-Iron-Phenonthroline Vydra  and P r i b i l  (50)  System  s t u d i e d the o x i d a t i o n o f  C o ( I I ) by F e ( I I I ) i n the presence  o f 1:10  p o t e n t i o m e t r i c a l l y and c o l o r i m e t r i c a l l y .  phenanthroline  both  They deduced t h a t  the r e a c t i o n can "be r e p r e s e n t e d as f o l l o w s : (Fe Phen 0H) 2  4 +  4  2FePhen*  2+  + 2CoPhen  + 2CoPhen  5 + 5  2 + 2  + 2HPhen  >  + 2HpO  The r e a c t i o n i s r a p i d and  stoichometrically  q u a n t i t a t i v e p r o v i d e d the pH i s e q u a l t o 3 and the r a t i o 1:10  p h e n a n t h r o l i n e t o the combined t o t a l c o b a l t and  c o n c e n t r a t i o n i s a t l e a s t 6:1. t h a t the system may  The  above a u t h o r s  of  iron  suggest  be used f o r the d e t e c t i o n o f v e r y s m a l l  q u a n t i t i e s o f C o ( I I ) by measuring the amount o f  intensely  c o l o r e d f e r r o i n produced. The  s u i t a b i l i t y of t h i s method o f a n a l y s i s f o r the  27 Co  2+  —  Co oxj  1.  3-  system was  determined:  The Buffer System In order to analyze f o r C o  to carry out t h e i r r a d i a t i o n s of K^Co -  solution.  quickly i t was ox^ i n the pH3  necessary buffer  This also made a l l the solutions approximately  constant i n i o n i c strength. pH3  2 +  The spectrum of K^Co  ox^ i n a  acetate - HCl buffer s o l u t i o n (48) i s shown i n f i g u r e  3.  It was found to be v i r t u a l l y i d e n t i c a l with the spectrum i n neutral solution.  This i s not very s u r p r i s i n g when one  considers the low i n s t a b i l i t y constant of Co ox^  •  An increase i n the hydrogen i o n concentration does, however, increase the rate of thermal decomposition of Co ox^3" (22).  F e r r o i n i s s t i l l f a i r l y stable at pH3 but the  pH must not be lowered as i t then decomposes with the formation of the phenanthrolium i o n 2.  Absorption at 510  (51)• mu  In the presence of an excess of 1:10  phenanthroline  e s s e n t i a l l y a l l ferrous i o n produced i s transformed into f e r r o i n and the formation of the intermediate complexes FePhen  2+  and F e P h e ^  2+  i s negligible.  JTerroin has an intense e l e c t r o n t r a n s f e r absorp t i o n peak at 510 mu with an e x t i n c t i o n c o e f f i c i e n t of 4 31.1 x 10 . At t h i s wavelength Co ox^ shows an e x t i n c t i o n c o e f f i c i e n t of only 10.8 and a pH3  s o l u t i o n doubly as  FIGURE 3  The spectrum o f K^Co  o x ^ O ^ H ^ O i n pH3  acetate - HCl b u f f e r  solution.  locations  o f the peaks and t r o u g h s are  242  23210  358  38  422 494 604  The  210.2 5.50 158  29 c o n c e n t r a t e d i n 1:10  phenanthroline  as i n c o b a l t i o x a l a t e  showed an e x t i n c t i o n of 95 f o r Co ox-^~  5 A ferric  c h l o r i d e s o l u t i o n o f pH3  and CoPhen-,^"" combined. 3  t o which 1:10  phenanthroline  had been added showed an e x t i n c t i o n c o e f f i c i e n t o f 334 510  mu.  at  T h i s r e l a t i v e l y h i g h e x t i n c t i o n of f e r r i c phenan-  t h r o l i n e w i l l n o t i n t e r f e r e as i t s c o n c e n t r a t i o n i n the developed  s o l u t i o n w i l l be v e r y low. 3-  o f Co ox*/  The  final  combined e x t i n c t i o n s  3+ , Co Phen^  and f e r r i c p h e n a n t h r o l i n e w i l l  still  be low i n comparison t o the v a l u e f o r f e r r o i n and a l l o w s f o r a f a i r degree o f s e n s i t i v i t y f o r the method i f d i f f e r e n t i a l spectrophotometry i s employed. C. E x p e r i m e n t a l Procedure 1.  Calibration C o n s i s t e n t r e s u l t s were o b t a i n e d when the r a t i o  the o - p h e n a n t h r o l i n e  c o n c e n t r a t i o n t o the t o t a l  c o b a l t and i r o n c o n c e n t r a t i o n was  of  combined  i n the range 8-13  to  1.  With r a t i o s h i g h e r t h a n 13 a marked decay o f the absorbance a t 510  mu w i t h time was  observed.  A t y p i c a l example where the r a t i o i s 11.6  to 1 i s  shown i n t a b l e I I . The f o l l o w i n g s o l u t i o n s were used: a.  sample  solution:  2 ml 0.000904 M C o S 0 « 7 H 0 i n b u f f e r 4  0.5  ml 0.1  M 1:10  2  phenanthroline  5 ml 0 . 0 0 0 5 M F e C l ^  i n buffer  i n buffer  made up t o 50 ml w i t h b u f f e r  30 TABLE I I ABSORBABLES OP A COBALT ANALYSIS SOLUTION AS A FUNCTION OF TIME  Time a f t e r mixing (mins.)  Absorbance at 510 mu blank v s . buffer  Absorbance a t 510 mu sample v s . buffer  Absorbance at 510 mu sample v s . blank  10  .021  .422  .402  34  .022  .422  .404  48  .021  .422  .404  62  .024  .425  .402  90  .024  .424  .403  120  .026  .425  .402  170  .026  .424  .400  500  .033  .434  .404  1110  .042  .442  .402  Average  b.  .403  blank s o l u t i o n : 0.5  ml 0 . 1 M 1:10  phenanthroline i n buffer  5 ml 0 . 0 0 0 5 M F e C l j  i n buffer  made up t o 50 ml w i t h b u f f e r E x a m i n a t i o n o f t a b l e I I shows t h a t t h e r e i s a s l i g h t increase  i n the absorbance  with time.  This increase  o f b o t h the sample and the b l a n k i s however a p p r o x i m a t e l y e q u a l i n  both c a s e s and the r e s u l t a n t o p t i c a l d e n s i t y r e a d i n g i s  31 c o n s t a n t w i t h i n the s p e c t r o p h o t o m e t r y The  error  c o b a l t c o n t e n t o f CoSO^yB^O c a l c u l a t e d  these d a t a i s w i t h i n 1.1% o f the t h e o r e t i c a l . w i t h i n the i n s t r u m e n t a l e r r o r range.  from  This error i s  H a t c h a r d and P a r k e r  found t h a t Beer's Law i s obeyed f o r the f o r m a t i o n o f f e r r o i n and t h e same i s t h e case h e r e . 2.  Application to Cooaltioxalate F o r the a n a l y s i s o f the C o  2 +  produced by the  3—  p h o t o l y s i s o f Co ox^  an a l i q u o t o f unexposed K^Co ox^  s o l u t i o n e q u a l t o t h a t o f the exposed the same temperature  s o l u t i o n and s t o r e d a t  as the l a t t e r was used as a b l a n k .  I t was found t h a t the b e s t d e v e l o p i n g time i s about t e n minutes.  A s h o r t e r d e v e l o p i n g time i s i n a d v i s a b l e as  f e r r o i n forms r a t h e r s l o w l y a t pH3.  Longer development  will  i n t r o d u c e e r r o r s through unequal t h e r m a l d e c o m p o s i t i o n o f sample and b l a n k s o l u t i o n s . absorbance  Experiments  a l s o show t h a t the  o f the s o l u t i o n s i n c r e a s e s s t e a d i l y f o r s e v e r a l  h o u r s and then drops s h a r p l y f a r below the i n i t i a l v a l u e . The  spectrum o f K^Co ox^ w i t h added o - p h e n a n t h r o l i n e  shows t h a t C o ( I I I ) a t l e a s t p a r t i a l l y complexes w i t h o-phenanthroline.  S u f f i c i e n t o - p h e n a n t h r o l i n e must t h e r e f o r e  be added t o complex a l l C o ( I I I ) i . e . i t must be t a k e n i n t o account when the r a t i o o f o - p h e n a n t h r o l i n e t o c o b a l t p l u s i r o n i s considered.  A n a l y s i s of a s e r i e s of c o b a l t i o x a l a t e t o which, s t a n d a r d amounts o f Co  +  solutions  i o n had been added showed  t h a t the r e s u l t s were c o n s i s t e n t l y h i g h by about  0,9%.  CHAPTER IV THE PHOTOLYSIS OF K Co ox -3.5^0 2  A.  T  Experimental Procedure 1.  Runs at 313 mu and Above a.  Calibration  The mercury arc was allowed to warm up f o r h a l f an hour.  The c a l i b r a t i o n runs f o r each wavelength were c a r r i e d  out as follows: The r e a c t i o n c e l l was f i l l e d with pH3 acetate - HCl buffer and the actinometers containing s o l u t i o n were placed i n p o s i t i o n : c e l l and one i n the side beam.  3«5 ml of actinometer  one behind the r e a c t i o n  The beaker was f i l l e d with  ice water mixture and then the actinometers were exposed f o r a s u f f i c i e n t length of time so that the developed actinometer solutions had absorbancies i n the range 0 . 2 - 0 . 8 . The exposed actinometer solutions were t r a n s f e r r e d to 25 ml low a c t i n i c f l a s k s , 1 . 5 ml of unexposed actinometer s o l u t i o n and 5 ml of the s p e c i a l buffer s o l u t i o n were added and the s o l u t i o n was made up to the mark with d i s t i l l e d water.  The actinometer  solutions were d i l u t e d to l a r g e r volumes i f required but enough unexposed actinometer and s p e c i a l buffer s o l u t i o n were  34 added t o keep the r a t i o s o f s p e c i a l b u f f e r , s o l u t i o n and water c o n s t a n t .  actinometer  A f t e r the s o l u t i o n had been  a l l o w e d t o develop i n the dark f o r 30 minutes i t s absorbance a t 510  mu was  measured a g a i n s t a i r .  The  absorbance  against  a i r o f a b l a n k p r e p a r e d i n an i d e n t i c a l manner except f o r the exposure Use  t o l i g h t was  s u b t r a c t e d from the above measurement.  o f the same a b s o r p t i o n c e l l o b v i a t e d any c o r r e c t i o n s f o r  path length. The r a t i o o f the a b s o r b a n c i e s o f the two  actinometer  s o l u t i o n s i s e q u a l t o the r a t i o o f the l i g h t i n t e n s i t y i n the two  l i g h t beams a t the p o s i t i o n s o f the a c t i n o m e t e r s .  Slight  v a r i a t i o n i n the p o s i t i o n w i l l l e a d t o the same r e s u l t i f the beams are r e a s o n a b l y p a r a l l e l .  A knowledge o f t h i s r a t i o  now  a l l o w s the d e t e r m i n a t i o n o f quantum y i e l d s u s i n g t o t a l l y a b s o r b i n g s o l u t i o n s o f K^Co i s not  ox^ i . e . a main beam a c t i n o m e t e r  essential. b.  Photolysis  A sample o f K^Co  o x ^ ^ ^ B ^ O was  i n t o a low a c t i n i c f l a s k i n a darkened made up t o the mark w i t h pH3  buffer.  s o l u t i o n s were i n the range 0 . 7  to 30.  weighed d i r e c t l y  room, d i s s o l v e d The  a b s o r b a n c i e s o f the  3.2  ml of the  were q u i c k l y t r a n s f e r r e d t o the r e a c t i o n c e l l i n the A s i m i l a r sample was  and  s t o r e d i n a 50 ml low a c t i n i c  and  beaker.  flask  kept a t 0°C i n an i c e water mixture i n the dark room. an i c e water mixture i n the r e a c t i o n beaker  solution  with  With  35 a c t i n o m e t e r s i n p o s i t i o n the sample was exposed f o r a time varying  from 10 minutes t o 3 hours depending on the i n t e n s i t y  of the mercury l i n e used and the p h o t o s e n s i t i v i t y o f Co ox ^~  3  a t t h i s wavelength. For purposes o f a n a l y s i s i t was pose not more t h a n 5% o f the K^Co otherwise the b l a n k valid.  d e s i r a b l e t o decom-  ox^ o r i g i n a l l y p r e s e n t as  Co ox^ s o l u t i o n would no l o n g e r be  Decomposition o f a l a r g e f r a c t i o n o f c o b a l t i o x a l a t e  would a l s o change the absorbance t h u s g i v i n g r i s e t o an e r r o n e o u s l y h i g h v a l u e o f the quantum y i e l d . A f t e r exposure the a c t i n o m e t e r s were developed as previously described.  The c o n t e n t s o f the r e a c t i o n c e l l were  t r a n s f e r r e d t o a 50 ml low a c t i n i c f l a s k .  To b o t h the  sample  and b l a n k s o l u t i o n s 1.0 ml o f 1.0 N o - p h e n a n t h r o l i n e i n pH3 b u f f e r and 5 ml o f 0 . 0 0 0 5 M F e C l * i n pH-3 b u f f e r were  added.  The s o l u t i o n s were made up t o the mark w i t h pH3 b u f f e r the  and  absorbance o f the sample v e r s u s the b l a n k was measured a t  510 mu 2.  a f t e r a d e v e l o p i n g time o f 10 minutes. Runs a t 302 mu and Below The procedure was  s i m i l a r t o the one used above  except t h a t a monochromator was used f o r the i s o l a t i o n o f the mercury l i n e s and no s i d e beam was c a l i b r a t i o n was  available.  The  c a r r i e d out by means o f a p h o t o c e l l i n  c o n j u n c t i o n w i t h the main beam a c t i n o m e t e r .  The i n t e n s i t y o f  the beam was measured w i t h the p h o t o c e l l b e f o r e and a f t e r  the  c a l i b r a t i o n run.  S i m i l a r measurements w i t h the  b e f o r e and a f t e r the p h o t o l y s i s  photocell  t h e n p r o v i d e d a v a l u e o f the  l i g h t i n t e n s i t y d u r i n g the p h o t o l y s i s .  Exposure  times had  t o be known f o r b o t h r u n s . B.  Results 1.  Treatment  a.  o f Data  Quantum Y i e l d s  f o r Co  2+  Ion P r o d u c t i o n  Because the same e x t i n c t i o n that  o f f e r r o i n a t 510 mu  and C o ( I I ) a n a l y s i s  c o e f f i c i e n t , namely  appears b o t h i n the a c t i n o m e t r y  calculation,  a s i m p l i f i e d method f o r the  c a l c u l a t i o n o f quantum y i e l d s c o u l d be  A  Co  A„  Co  a  V  •  ps  Co  V  *  found:  $Pe  .  ox, "" 3  CR  /, 1  ps  antilog A  G o  oy  .3-  where: (f) ^Co AQ  q  .2+  =  quantum y i e l d f o r Co  =  absorbance oxalate  V"co A^  00  =  ox^  =  0Xj *" 3  =  cobalti-  solution.  absorbance  solution  o f the o r i g i n a l unexposed  cobaltioxalate ^j?e  ion formation.  o f developed exposed  volume o f the above 3-  +  solution.  quantum y i e l d f o r f e r r o u s i o n f o r m a t i o n f o r the wavelength used as  37 r e p o r t e d by H a t c h a r d and P a r k e r ( 4 - 7 ) . Ap  g  =  absorbance o f the d e v e l o p e d s i d e beam actinometer s o l u t i o n , p h o t o l y s i s run.  Vp  S  CR  =  volume o f the above  solution.  =  the r a t i o o f t h e absorbance  ( A ^ ) o f the cm  d e v e l o p e d main beam a c t i n o m e t e r s o l u t i o n (volume the  = V  C  M  ) t o t h e absorbance  (A  c g  ) of  s i d e beam a c t i n o m e t e r s o l u t i o n  (volume  = V„_).  I t i s e q u a l t o the r a t i o  of the two beams. CR  = A  cm  V  .  cs  cm m  cs  The absorbance o f the c o b a l t i o x a l a t e  solution  be determined by t h r e e methods: (i)  from the known e x t i n c t i o n c o e f f i c i e n t o f KjCo ox^ a t the wavelength u s e d .  (ii)  "by measuring the absorbance by means o f a spectrophotometer and u s i n g an a l i q u o t o f the  (iii)  same s o l u t i o n t h a t was exposed.  from the r a t i o s o f the l i g h t beams d u r i n g the  c a l i b r a t i o n and p h o t o l y s i s  A  Co  ox 33  -  log  1 0  CR  runs:  could  38 where PR i s c a l c u l a t e d s i m i l a r t o CR: PR  =  V  _p_m >s  V  -  .7 _p_m ps  T h i s method s i m u l t a n e o u s l y p r o v i d e d a check on the a c c u r a c y o f the a c t i n o m e t r y . U s u a l l y method ( i i ) was employed except f o r t o t a l l y a b s o r b i n g s o l u t i o n s when method ( i ) was u s e d . F o r t h e runs u s i n g the monochromator quantum y i e l d s were c a l c u l a t e d i n t h e u s u a l manner as the r a t i o o f the number o f Co  +  i o n s produced t o t h e number o f quanta absorbed. b.  Corrections  The  quantum y i e l d s t|)  were s u b j e c t e d (i)  f o r Systematic  C  t o the f o l l o w i n g Consideration  Errors  c a l c u l a t e d i n the above ways  q  corrections:  o f f i g u r e 1 shows t h a t the l i g h t  i n t e n s i t y i n the r e a c t i o n c e l l i s a c t u a l l y h i g h e r t h a n t h a t r e c o r d e d by the main beam a c t i n o m e t e r because o f r e f l e c t i o n s and  s c a t t e r i n g from q u a r t z l i q u i d s u r f a c e s  Several  cell,  d e t e r m i n a t i o n s o f the r a t i o o f a c t i n o m e t e r s w i t h and  without the b u f f e r f i l l e d r e a c t i o n c e l l culated  o f the r e a c t i o n  showed t h a t the c a l -  quantum y i e l d s s h o u l d a c t u a l l y be 6.1% (ii)  smaller.  C a l i b r a t i o n o f t h e a n a l y s i s o f Co  +  i n the  KjCo o x j system showed t h a t the r e s u l t s were c o n s i s t e n t l y too h i g h by about 0.9%.  This requires  another r e d u c t i o n  of the  quantum y i e l d by 0.9% b r i n g i n g the t o t a l t o 7%« Another s y s t e m a t i c e r r o r might a r i s e from the f a c t t h a t d u r i n g runs the main "beam a c t i n o m e t e r was kept a t 0°C w h i l e t h e s i d e beam a c t i n o m e t e r was a t room temperature.  The  temperature c o e f f i c i e n t f o r the p h o t o l y s i s o f f e r r i o x a l a t e i s however n e g l i g i b l e .  I t becomes a p p r e c i a b l e o n l y a t l o n g  wavelengths. c.  Primary  Quantum  Yields  The c o r r e c t e d quantum y i e l d s f o r c o b a l t o u s i o n f o r m a t i o n were h a l v e d i n o r d e r t o o b t a i n t h e p r i m a r y yields  quantum  (j)p a c c o r d i n g t o the mechanism on page 12. d.  Random E r r o r s  A c c i d e n t a l e r r o r s r e s u l t i n g from the t o l e r a n c e s o f balances  and v o l u m e t r i c equipment were e s t i m a t e d t o amount  t o about 1%.  The random d e v i a t i o n due t o the a b s o l u t e p h o t o -  metric error AT  i s however more important  The r e l a t i v e e r r o r  i n this instance.  A C i n the c o n c e n t r a t i o n measured by the c  d e t e r m i n a t i o n o f the t r a n s m i t t a n c e T o f our a b s o r b i n g stance c a n be e x p r e s s e d AC c  =  as  Q.4-54-5 AT T log T  Por t h e instrument used 0.328%.  sub-  T was determined  T h i s r e s u l t s i n a photometric  t o be  e r r o r o f about 1% f o r  absorbance v a l u e s i n the range 0.2 t o 0 . 8 5 . range, however, t h e e r r o r i n c r e a s e s r a p i d l y .  Outside  this  I n the quantum  y i e l d c a l c u l a t i o n f o r absorbance measurements are d i r e c t l y i n v o l v e d l e a d i n g t o an average  d e v i a t i o n o f 4% from  this  source. The expected e r r o r was c a l c u l a t e d f o r each quantum y i e l d and compared w i t h the s t a n d a r d d e v i a t i o n o f t h e e x p e r i mentally obtained values. 2.  Tabulation of Results The p r i m a r y quantum y i e l d s o b t a i n e d on e x p o s i n g  c o b a l t i o x a l a t e s o l u t i o n s t o l i g h t o f v a r i o u s wavelengths a r e summarized i n t a b l e s I I I and IV.  I t s h o u l d be remembered t h a t  a l l runs were c a r r i e d out a t 0°C and t h a t the s o l u t i o n s o f c o b a l t i o x a l a t e were a d j u s t e d t o pH3 w i t h a c e t a t e - H C l b u f f e r . T h i s gave t h e s o l u t i o n s the c o n s t a n t i o n i c s t r e n g t h o f 0.79* As h a s been found b y o t h e r i n v e s t i g a t o r s duction of C o  2 +  quanta absorbed light  (22, 43) the p r o -  i o n i s d i r e c t l y p r o p o r t i o n a l t o the number o f and t h e quantum y i e l d i s independent  o f the  intensity. In t a b l e I I I a comparison o f t h e average  estimated  e r r o r s w i t h the s t a n d a r d d e v i a t i o n s shows t h a t the s p r e a d o f v a l u e s o b t a i n e d i s f a i r l y c o n s i s t e n t w i t h the e s t i m a t e d a c c u r a c y o f the method.  The d e t e r m i n a t i o n a t 5780 X i s  s u b j e c t t o a p a r t i c u l a r l y l a r g e e r r o r because o f the extremely low a b s o r b a n c i e s o f the developed  actinometer  solutions.  41 TABLE I I I PRIMARY QUANTUM YIELDS AND THEIR ESTIMATED ERRORS  X(A)  Estimated e r r o r (%)  <f) ^ +  2804  9  .369  3020  4.2 3.7  .356 .302  + i T  3130  6.8 5.9 5.2  .340 .376 .370  +  3340  4.7 4.7  .276 .282  + i  3650  5.0 4.8 5.0 4.9  .229 .244 .258 .244  4046  7.2 7.2 5.4  .114 .119 .093  +  4358  9.3 5.5 7.7  -.0650 .0680 .0720  +  5780  50  low temperature  + +  .0332  .369  .0151 .011  .329  .0231 .022 .0194  .362  + +  +  +  .013 + .0132  .279  +  .0114 .0118 .0127 .0120  .244  +  .0082 .0086 .00505  .108  +  .00605 .00368 .00556  .0670  +  + + +  +  + +  ^ . 00033  At t h i s wavelength r e l i a b l e because  <£) average  p  +  .000165 -£.00033  +  Standard deviation  .033  -  .013  .027  .022  .016  .013  .003  .012  .010  .0073  .010  .0051  .0032  .00017  the f e r r i o x a l a t e actinometer i s not very  o f the low e x t i n c t i o n c o e f f i c i e n t and t h e dependent quantum y i e l d o f f e r r i o x a l a t e .  G e n e r a l l y i t can be s a i d t h a t t h e e x p e c t e d e r r o r i n c r e a s e s a t low l i g h t i n t e n s i t i e s .  Runs l o n g e r t h a n t h r e e hours were n o t  f e a s i b l e as t h e c o o l i n g mixture was t h e n s p e n t .  42 TABLE IV EXPERIMENTAL PARAMETERS AND PRIMARY QUANTUM YIELDS  6  K^Co  0Xj  2804  7670  3020  1990  3130  844  3340  136  3650  41.8  4046  174  4358  181  5780  133  [K,Co ox,] x 10 5 +  17.0 0.366 0.491 23.8 14.2 19.7 8.78 8.24 11.52 9.04 43.2 38.2 4.25 20.7 3.48  3  Light Exposure intensity time (quanta/sec. (mins.) x ICT) 0.36  100  .369  0.964  100 110  .356 .302  1.08 1.07 .846  121 130 120  .340 .376 .370  180  .276 .282  .37 .37 21.4 21.6 20.6 19.9 5.08 5.08 5.32  + + +  +  90 60 90  .114 .119 .093  +  .0650 .0680 .0720  8.06  1.66  180  ^1.00033  .013  + .013 +  100 110 90  .023  + .022 + .019  .229 .244 .258 .244  2.44 3-98 2.22  .014  + .011  19.6 21 7.5 7.5  7.38 21.6 154  .033  .011  + .012 + .013 + .012  .0082  + .0086 + .0050 +  .0060  + .0037 + .0056 +  .00017  Examination of table IV shows that the quantum y i e l d s at 3650, 4046 and 4358 A have a tendency to be higher the higher the cobaltioxalate concentration. P l o t s of the quantum y i e l d s versus concentration (Figure 4) show that t h i s increase i s f o r t u i t o u s as i t i s w i t h i n the experimental e r r o r .  FIGURE 4  The primary quantum y i e l d s as a function of the concentration of K^Co ox^ f o r the wavelengths 3650, 4046 and 4358 A .  More a c c u r a t e r e s u l t s would be r e q u i r e d t o see whether t h e r e is a definite C.  relationship.  Discussion 1.  Comparison o f the R e s u l t s w i t h E a r l i e r Work The  quantum y i e l d s f o r Co  by Adamson and.Sporer (36)  +  i o n formation  obtained  and Copestake and U r i (22)  and  those  o b t a i n e d i n the p r e s e n t i n v e s t i g a t i o n are compared i n  table  V. The p r e v i o u s work was  c a r r i e d out i n n e u t r a l  s o l u t i o n whereas the p r e s e n t experiments were conducted s o l u t i o n b u f f e r e d t o a pH o f 3.  in a  There s h o u l d , however, be  no  d i f f e r e n c e i n quantum y i e l d f o r t h i s r e a s o n as the s p e c t r a are i d e n t i c a l i n both media.  T h i s i s so because o f the  3extremely  s m a l l i n s t a b i l i t y c o n s t a n t o f Co ox^  f o r m a t i o n o f o x a l a t e i o n and  "bb,e  subsequent f o r m a t i o n o f u n d i s -  sociated oxalic acid i s negligible. the m o l e c u l e s  :  I t i s a l s o expected  that  and i o n s of the b u f f e r are n o t more e f f e c t i v e  i n the quenching o f e x c i t e d c o b a l t i o x a l a t e m o l e c u l e s the s o l v e n t m o l e c u l e s .  At the same time  than  are  i t i s unlikely that  i r r a d i a t i o n o f the b u f f e r produces a c t i v a t e d s p e c i e s t h a t would i n t e r f e r e w i t h the r e a c t i o n s t u d i e d .  On the c o n t r a r y , the  b u f f e r can o n l y be o f advantage i n p r o v i d i n g a c o n s t a n t  ionic  environment. The  r e s u l t s o b t a i n e d by Adamson and S p o r e r are about  TABLE V COMPARISON OP QUANTUM YIELDS OP RECENT INVESTIGATIONS OF THE PHOTOLYSIS OF COBALTIOXALATE  $Co Adamson and Sporer 2804 3020 3130 3340  -  -  3650  -  3700  1.0  4046 4358 5500 5780  Copestake and U r i  -  . .0069  -  twice the values found here.  Present work  -  0.658  0.73  0.724  -  0.558  0.69  0.487  0.17  0.217  0.12  0.134  -  -  +  0.738  +  0.066 0.026  +  0.043  +  0.026  +  0.024  + +  0.015 0.010  ^ 0.00066  +  0.00033  Better agreement e x i s t s with the  values of Copestake and U r i except that i n t h e i r work there i s a rather abrupt f a l l i n quantum y i e l d on going from 3650 A to 4046 A •  For reasons mentioned before, the values obtained  i n the present research are thought to be more accurate than those of previous i n v e s t i g a t o r s and to be v a l i d within the error l i m i t s indicated.  46 2.  The Quantum Y i e l d as a F u n c t i o n o f Wavelength. It  is  i s r e a s o n a b l e t o assume t h a t the quantum y i e l d  some f u n c t i o n o f the energy  o f the absorbed  radiation.  It  i s f o r t h i s r e a s o n t h a t f o r the c o r r e l a t i o n o f the wavelength w i t h quantum y i e l d s , wave numbers were a c t u a l l y Examination  used.  o f f i g u r e 5 shows t h a t t h e r e i s a r a t h e r  g r a d u a l i n c r e a s e i n the quantum y i e l d w i t h energy  and t h a t a t  h i g h e n e r g i e s i t seems t o approach a c o n s t a n t v a l u e . p h o t o a c t i v i t y o n l y f o r the charge  Assuming  t r a n s f e r band the Gaussian  e x t r a p o l a t i o n o f t h i s band i n f i g u r e s 5 and 6 shows t h a t on g o i n g t o lower wave numbers the quantum y i e l d s h o u l d much more r a p i d l y than i t does.  decrease  I n analogy t o Adamson*s and  S p o r e r ' s f i n d i n g s on o t h e r C o ( I I I ) complexes i t i s then here apparent spectrum  t h a t the d - d  also  band i n the b l u e p a r t o f t h e  does p o s s e s s a c o n s i d e r a b l e amount o f p h o t o a c t i v i t y  not w i t h s t a n d i n g the c o n t e n t i o n o f t h e simple c r y s t a l theory to the c o n t r a r y .  A t t h e same time  field  i t i s interesting  to note t h a t t h e expected p h o t o r a c e m i z a t i o n does n o t o c c u r . examining  the p e r cent t r a n s m i s s i o n p l o t f o r the f i l t e r s i n  f i g u r e 2 the r e a d e r can s a t i s f y h i m s e l f t h a t a t these wavelengths t h e r e i s no leakage  longer  o f s h o r t wavelengths r a d i a t i o n  so as t o cause u n e x p e c t e d l y h i g h quantum y i e l d s .  3.  By  3-  The A c t i v a t i o n and D e a c t i v a t i o n o f Co o x / In  o r d e r t o i n t e r p r e t the r e s u l t s f u r t h e r i t i s  n e c e s s a r y t o c o n s i d e r the f a t e o f the c o b a l t i o x a l a t e i o n on  FIGURE 5  The p r i m a r y quantum y i e l d s as a f u n c t i o n o f energy shown i n r e l a t i o n t o the a b s o r p t i o n spectrum.  The dashed l i n e s are G a u s s i a n  extrapolations  o f the charge t r a n s f e r band  and o f the f i r s t d - d "band. Key: 13  p r i m a r y quantum y i e l d s from Adamson and S p o r e r (36)•  A  p r i m a r y quantum y i e l d s from  Copestake  and U r i (22). ©  p r i m a r y quantum y i e l d s from t h e p r e s e n t investigation.  FIGURE 6  The Gaussian e x t r a p o l a t i o n s o f the charge t r a n s f e r band and o f the f i r s t d - d  band.  Key: <?> observed p r i m a r y quantum y i e l d s ^  p r i m a r y quantum y i e l d s on the assumption  calculated  t h a t the charge  t r a n s f e r band has a c o n s t a n t quantum y i e l d o f .35  and the d - d  band has  a c o n s t a n t quantum y i e l d o f  0.07.  exposure t o l i g h t . Comparison o f the s p e c t r a  of various  complexes shows t h a t the prominent f e a t u r e s Co^  Co(III)  are due t o the  i o n : t h e y a l l show two d - d bands i n e s s e n t i a l l y the  same p o s i t i o n s more o r l e s s overshadowed by the charge t r a n s f e r band which depends more on the type o f l i g a n d . more e a s i l y o x i d i z a b l e will  the l i g a n d s  t h i s charge t r a n s f e r band be  the f a r t h e r i n t h e v i s i b l e positioned.  I n t h e case o f c o b a l t i o x a l a t e d - d  absorption  The  the f i r s t and second  bands c o r r e s p o n d t o t h e L a p o r t e  forbidden  e l e c t r o n i c t r a n s i t i o n s from the ground s t a t e """A-^ t o the excited  s t a t e s """^g Figure  part  "^lg  res  ec  b:  v  e  7 i s a schematic p r e s e n t a t i o n  o f the e s s e n t i a l  o f the term system o f C o ( I I I ) a c c o r d i n g t o c a l c u l a t i o n s  from Tanabe and Sugano (52). field  The s t r e n g t h  i s p l o t t e d a l o n g the a b s c i s s a .  depends on the d i s t a n c e and  P ' '• ' ^y•  l i g a n d charges.  o f the  This f i e l d  octahedral strength  between the c e n t r a l i o n and the l i g a n d A U i s o b t a i n e d as  The energy d i f f e r e n c e  follows: = where the  A E + £. - £  b  (£ ^ O ) b  A E r e p r e s e n t s the e l e c t r o s t a t i c energy s p l i t t i n g o f  d o r b i t a l s i n the f i e l d  o f the d i p o l a r l i g a n d m o l e c u l e s ;  £ . r e p r e s e n t s the i n c r e a s e  i n o r b i t a l energy r e s u l t i n g from  cr a n t i b o n d i n g w i t h the l i g a n d o r b i t a l s and 6^ r e p r e s e n t s t h e 7V bonding e f f e c t due t o back d o n a t i o n o f e l e c t r o n s  into  FIGURE 7  Schematic diagram o f the r e l e v a n t o f the term system o f Co^  features  Fig. 7 free  o c t a h e d ra 1  Co  field  '  —  -—  /  x  >-*  \ .  /  /.?.<;  s  \  Fieldstrength  (Dq)  T g(t2g)(eg) 2  'A  l g  (t  2 g  )  6  i  51 ligand o r b i t a l s . It  can be  seen t h a t below the  critical  field  strength  5 a t p o i n t P the  T  2g  '  ^  t e r m  ^ke  3  srouncL s t a t e .  The  only  magnetic complex known t o c o r r e s p o n d t o t h i s s t a t e o f i s K^CoFg.  A l l o t h e r C o ( I I I ) complexes i n c l u d i n g K^Co  have f i e l d  s t r e n g t h s l a r g e r t h a n P and  the  T h i s ground s t a t e i s d i a m a g n e t i c .  configurations  o f the  d  (xy)  d  (xy) (xy)  2  d  (xy)  2  d  (xy)  d  d  (yz)  2  d  (yz)  2  d  (yz) (yz)  2  d  (yz)  2  (xz)  2  d  (yz)  d  (xy)  2  d  (yz)  (zx)  d  (zx)  d  2  ox^  electron  are: \g  2  2  d  (x  - y )  2  2  \  e  (zx)  d  d  The  2  (zx)  d  d  d  d  The i n various  2  affairs  term i s the  ground s t a t e .  singlet states  para-  (zx) (zx) d  2  2  d  (z)  2  (zx)  excited cobaltioxalate  i o n can  l o s e i t s energy  ways: (i)  Energy t r a n s f e r t o n e i g h b o u r i n g m o l e c u l e s through i n e l a s t i c c o l l i s i o n s o f the  second k i n d ) .  This  be m o s t l y done by s o l v e n t  (collisions  quenching molecules.  will  52 (ii)  Internal conversion. t r a n s i t i o n may  A radiationless  o c c u r from the  excited  s t a t e t o the h i g h v i b r a t i o n a l l e v e l s o f the ground s t a t e or t o o t h e r e x c i t e d states. (iii)  Fluorescence. w i t h the  R e t u r n t o the  emission of r a d i a t i o n a f t e r a  time i n t e r v a l o f about I O has  ground s t a t e  -  sec.  This  so f a r n o t been observed i n t r a n s i t i o n  m e t a l complexes. (iv) The r e a c t i o n and  Photochemical r e a c t i o n . p r o c e s s e s i and i i compete w i t h the  the  quantum y i e l d t h e r e f o r e  o f these r e a c t i o n s .  I t i s here t h a t the  e x c i t e d s t a t e becomes i m p o r t a n t .  The  the  e x c i t e d s t a t e s can be  the  i n t e g r a t e d experimental absorption  g i v e s the  photochemical  depends on the l i f e t i m e of  The  ( i n sec.)  the  n a t u r a l l i f e t i m e s of  c a l c u l a t e d from the magnitude curve.  Piatt  f o l l o w i n g approximate r e l a t i o n  f  =  rates  -4  10 € max.  l i f e t i m e s o f the t r a n s i t i o n s t h e n  are:  of  (53)  53  \  »  e  \ g  \  — •  charge  f  e  = 6 x 10"^ s e c .  T = 5 x 10"^  \  sec.  e  transfer  X = 4- x 10"^ s e c .  Because o f the competing p r o c e s s e s ( i ) , ( i i i ) and (iv)  the a c t u a l l i f e t i m e s are much s h o r t e r t h a n the n a t u r a l  l i f e t i m e s s t a t e d above, 4.  i . e . F l u o r e s c e n c e does n o t o c c u r .  E m p i r i c a l Treatment a.  Assignment  o f Data  o f Constant L e v e l s  t o Both the d - d  of P h o t o a c t i v i t y  and the Charge T r a n s f e r  Bands  An e x a m i n a t i o n o f f i g u r e 5 s u g g e s t s t h a t the charge t r a n s f e r band and the f i r s t l e v e l s of p h o t o a c t i v i t y . overlap,  d - d  band might have c o n s t a n t  I n the r e g i o n where the two  bands  the observed quantum y i e l d would be e x p e c t e d t o be  e q u a l t o the sum  o f the c o n t r i b u t i o n s  o f the quantum y i e l d s  from "both bands depending on the p e r c e n t a g e o f each band i n the  total extinction coefficient. To check t h i s h y p o t h e s i s the two bands were e x t r a -  polated for  on a G a u s s i a n s c a l e g i v i n g the e x t i n c t i o n c o e f f i c i e n t s  each band (Table V I ) .  assuming  U s i n g the r e l a t i v e p e r c e n t a g e s  the v a l u e s 0.350 and 0.07  f o r the c o n s t a n t quantum  y i e l d s f o r the charge t r a n s f e r and the f i r s t d - d r e s p e c t i v e l y the h y p o t h e t i c a l  and  band  quantum y i e l d s were c a l c u l a t e d .  54 TABLE V I THE EXTINCTION OF THE FIRST d - d AND THE CHARGE TRANSFER BAND AS RELATED TO THE TOTAL EXTINCTION AND THE CALCULATED PRIMARY QUANTUM YIELDS  A CA) 6KjCo ox*  % d-d  -  %  CT.  -  -  -  3020  2300  3130  844  .00356  99.99  334-0  136  • 735  99.26  .348  .000515  .3485  24.9  .0871  .0526  .1397  3650  41.8 75.1  100  -  404-6  174  99.77  .231  .000808  .0698  .0706  4358  181  99.98  .0188  .000119  .0699  .070  F i g u r e 6 compares the c a l c u l a t e d quantum y i e l d s w i t h e  the e x p e r i m e n t a l ones.  At 3650 A the c o n t r i b u t i o n s from  both  bands are o f comparable o r d e r o f magnitude which s h o u l d r e s u l t i n a r e a s o n a b l y c l o s e agreement "between t h e c a l c u l a t e d and the observed  quantum y i e l d s .  The c a l c u l a t e d v a l u e i s however o n l y  about h a l f t h e e x p e r i m e n t a l l y observed  one.  A t lower  energies  the agreement i s somewhat b e t t e r . A l t h o u g h these r e s u l t s do n o t f o l l o w t h e h y p o t h e s i s as expected i t i s p o s s i b l e t o e x p l a i n t h e d i s c r e p a n c i e s u s i n g the scheme p r e s e n t e d i n the t h e o r e t i c a l s e c t i o n below. b.  Comparison w i t h K^Fe ox^  I n f i g u r e 8 t h e quantum y i e l d s o f b o t h  ferrioxalate  and  cobaltioxalate  of the  are p l o t t e d  energy of the  as a f u n c t i o n  radiation.  energy f u n c t i o n s o f the  For  quantum y i e l d are  quantum y i e l d as the  almost c o n s t a n t . rapidly.  At  Straight  low  e n e r g i e s the be  distinct  c l e a r l y apparent. smaller increase  energy i n c r e a s e s :  l i n e s can  logarithm  f e r r i o x a l a t e two  At h i g h e r e n e r g i e s t h e r e i s a p r o g r e s s i v e l y of the  of the  i t becomes  quantum y i e l d drops o f f  drawn through b o t h s e t s  of  p o i n t s which i n t e r s e c t a t 4046 A. I n the are no  spectrum o f f e r r i o x a l a t e  d i s t i n c t a b s o r p t i o n peaks t o which these two  photoactivity e x i s t s that  can  be  assigned.  active d - d  The  a t s h o r t wavelengths.  due  The  of the  t r i - o x a l a t o complex was  polating  p o s i t i o n o f the  the  of  the  broad the  t o pure K^Fe  ox^;  T h i s i s so because at  a p p r e c i a b l y t o form the  charge t r a n s f e r  the  spectrum i n t h i s  mono-oxalato complexes, o x a l a t e i o n and  (54).  di  free oxalic  acid  a b s o r p t i o n peak  t h e r e f o r e e s t i m a t e d by  extra-  spectrum from measurements t a k e n a t h i g h  ferri-  o x a l a t e c o n c e n t r a t i o n s and dotted l i n e ) . 297  obscured by  c o n c e n t r a t i o n s n e c e s s a r y t o measure the  region ferrioxalate dissociates and  types  Another d i f f i c u l t y i s t h a t  spectrum as p r e s e n t e d i s d e f i n i t e l y not a t l e a s t not  there  p o s s i b i l i t y however  t r a n s i t i o n s are  i n t e n s e charge t r a n s f e r band.  low  ( f i g u r e 9a)  i n neutral  T h i s p u t s the  solution  (figure  9a  charge t r a n s f e r peak a t about  mu. For  clear cut.  cobaltioxalate  The  p o i n t s can be  ( f i g u r e 8b) interpreted  the  case i s not  i n two  ways:  so  FIGURE 8  The p r i m a r y quantum y i e l d s o f p o t a s s i u m ferrioxalate  and o f p o t a s s i u m  as a f u n c t i o n o f the l o g a r i t h m numbers. Key: a = K^Fe ox^ b = K-zCo ox,  cobaltioxalate o f the wave  FIGURE 9  The  absorption  oxalate  spectra of potassium  and o f p o t a s s i u m c o b a l t i o x a l a t e i n  aqueous s o l u t i o n . Key: a = KjFe b = K,Co  ferri-  o  x  j  ox,  A s t r a i g h t l i n e can be drawn t h r o u g h the i n d i c a t i n g complete m i x i n g o f the bands. the  This  d - d and  charge  On  the  points.  o t h e r hand the  s i t u a t i o n can be i n t e r p r e t e d  as b e i n g analogous t o t h a t o f f e r r i o x a l a t e .  This  quantum y i e l d s a t 2804 A and  c o r r e c t i f the Yet,  as i n the  ferent functions and  transfer  i n t e r p r e t a t i o n i s somewhat s u s p e c t because o f  extreme s c a t t e r o f the  as v a l i d .  points  seems t o be  3020 A are  case o f f e r r i o x a l a t e the  taken  two d i f -  cannot be a s s i g n e d d i r e c t l y to the d - d  charge t r a n s f e r bands r e s p e c t i v e l y .  The assignment would  however a p p l y i f the whole spectrum were s h i f t e d , by 260 A toward l o n g e r wavelengths.  T h i s apparent s h i f t has  Adamson (55) o b t a i n e d a s i m i l a r  observed f o r o t h e r systems. pattern  f o r the It  photolysis  oxalate.  The  respective However the of the oxalate  o f Co(NH*)^ (SCU) " " and 2  i s i n t e r e s t i n g t o see  quantum y i e l d s i s the  Co(NH*)^ Br'  whether t h i s s h i f t i n  d i f f e r e n c e i n the  overlap  the  points  for  the  c u r v e s i n f i g u r e 8 c o r r e s p o n d s t o about 3400 cm" . difference  i n the  charge t r a n s f e r a b s o r p t i o n  the  extrapolated  peak f o r  i n f i g u r e 9a amounts t o about 7650 cm" . 1  i n r e a l i t y the  "band i s i n p a r t due o n l y 3400 cm""  1  a t 264 mu.  o f the  t o an obscured d - d  would put  peaks  ferri-  T h i s means  t r i o x a l a t o f e r r i c peak i s a t lower  wavelengths or t h a t the p h o t o a c t i v i t y  oxalate  1  same f o r f e r r i o x a l a t e a s f o r c o b a l t i -  two compounds u s i n g  either that  a l s o been  the  charge t r a n s f e r  band.  A shift of  charge t r a n s f e r peak f o r  ferri-  T h i s would c o r r e s p o n d c l o s e l y t o a  59 p o s i t i o n o f the chaxge t r a n s f e r peak a t 260 mu hy B i s i k a l o v a 5.  (56).  Theoretical Interpretations In  to  as r e p o r t e d  o r d e r f o r the d e c o m p o s i t i o n  of c o b a l t i o x a l a t e  o c c u r t h e r e must be a t r a n s f e r o f an e l e c t r o n from  o x a l a t e i o n t o the c e n t r a l Go^ imagined  t o be  ion.  The  i n t e r m e d i a t e can be  a t r i g o n a l b i p y r a m i d where o n l y one 2-  oxalate i o n r a d i c a l i s attached: molecule  00020^  an  end o f the  OO^Oj *  ^  a  o f water a t t a c h e s i t s e l f at the p o s i t i o n where  end of the l i g a n d i s l o o s e n e d b e f o r e the t r i g o n a l  one  bipyramid  i s formed t h e n the o c t a h e d r a l c o n f i g u r a t i o n i s m a i n t a i n e d . The  a d d i t i o n o f t h i s molecule  o f water would lower the  energy  o f the i n t e r m e d i a t e c o n s i d e r a b l y and r a c e m i z a t i o n would not o c c u r upon d e a c t i v a t i o n . At l o n g wavelengths ( d - d  absorption)  t h e r e i s p r o b a b l y not enough energy a v a i l a b l e f o r t h i s mediate t o be formed. one  inter-  T h i s type o f i n t e r m e d i a t e where o n l y  end of an o x a l a t e group i s detached  i s quite probable  because i t s f o r m a t i o n i n v o l v e s o n l y a r e l a t i v e l y s m a l l i n c r e a s e in  entropy. The  light  e x c i t e d s t a t e r e s u l t i n g from the a b s o r p t i o n o f  i n the middle  o f the charge  vibrationally excited. v i b r a t i o n a l energy molecules  t r a n s f e r band w i l l  I t i s very l i k e l y that t h i s  i s r a p i d l y passed  excess  on t o n e i g h b o u r i n g  i n the form o f thermal energy and  equilibrium i s quickly attained,  be  (collision  vibrational frequency =  lO^sec"" ) 1  60 At the the  same time i n s p i t e o f the f a i r l y s h o r t  excited  t o the  s t a t e t h e r e w i l l he  ground s t a t e .  t h a t the  The  l i f e t i m e of  extensive i n t e r n a l conversion  e f f e c t o f b o t h these p r o c e s s e s i s  f r a c t i o n of m o l e c u l e s r e a c t i n g w i l l he  constant.  This  y i e l d i n the  i n e f f e c t then explains  l e v e l o f the  c o n s t a n t quantum  charge t r a n s f e r hand.  I f however i t i s p o s s i b l e p r o c e e d not  the  fairly  o n l y by way  o f the  f o r the  d i s s o c i a t i o n to  vibrationally equilibrated  charge t r a n s f e r band but  also v i a a v i b r a t i o n a l l y  e x c i t e d s t a t e , t h e n the r e s u l t s can be  explained  in  the  f o l l o w i n g manner. Following  charge t r a n s f e r the  t e n d t o d i s s o c i a t e almost immediately. energy i s a v a i l a b l e the through the may  result.  solvent  cage and  supplied solvent  quantum y i e l d would then i n c r e a s e  will  enough k i n e t i c able  to break  deactivation  again a f a i r l y  However on g o i n g t o v e r y s h o r t  t o b r e a k through the  species  r e c o m b i n a t i o n and  enough k i n e t i c energy would be are able  I f not  fragments w i l l not be  Thus t h e r e w i l l be  quantum y i e l d .  excited  constant wavelengths  so t h a t the  fragments  cage immediately.  r a p i d l y and  approach  The unity  once a c e r t a i n c r i t i c a l wavelength i s r e a c h e d . So considered. d - d o f the  f a r o n l y charge t r a n s f e r a b s o r p t i o n F o r the  band two "'"To  explanation  has  been  of the p h o t o a c t i v i t y o f  mechanisms are proposed depending on the  s t a t e i n r e l a t i o n t o the e x c i t e d e l e c t r o n  the energy  transfer  levels.  Only the  A  »  l g  because quantum y i e l d s i n the extremely  T g 2  band i s c o n s i d e r e d here  ^"A^  *  "^lg  b a n ( i  a r e  low. I f the r e g i o n  o f maximum a b s o r p t i o n  i n the  d - d  band l i e s h i g h e r t h a n the l o w e s t v i b r a t i o n a l l e v e l o f charge t r a n s f e r band the f i g u r e 10. the  I t i s quite  r e s u l t s can be l i k e l y t h a t the  charge t r a n s f e r band l i e s i n the  v e r y edge o f the  o f the  using  o - o t r a n s i t i o n of  r e d or a t l e a s t at  Gaussian e x t r a p o l a t i o n  equilibrium configurations  explained  the  the  o f t h i s band as  e x c i t e d and  ground  the  state  m o l e c u l e s are undoubtedly q u i t e d i f f e r e n t . I f i t i s assumed t h a t d i s s o c i a t i o n o c c u r s o n l y a v i b r a t i o n a l l y e q u i l i b r a t e d s t a t e c l o s e to the a d d i t i o n o f a molecule of water t o the  o - o "band t h e n  t r i g o n a l bipyramidal  i n t e r m e d i a t e would lower t h i s l e v e l even f u r t h e r .  The  d i f f e r e n c e between the  should  ground s t a t e and  c o r r e s p o n d t o about 3 3 « 6 K c a l . , the the  thermal  from  this level  energy  a c t i v a t i o n energy o f  reaction.  In t h i s case i t i s assumed t h a t the no p h o t o a c t i v i t y o f i t s own  as such and  the  d - d  There i s t h e r e f o r e  that  charge  In order f o r i n t e r n a l conversion to occur  s t a t e s i n v o l v e d must have s i m i l a r n u c l e a r  has  decomposition  i s observed r e s u l t s from i n t e r n a l c o n v e r s i o n t o the t r a n s f e r band.  band  the  configurations.  an energy b a r r i e r t o overcome and t r a n s i -  t i o n s t o the h i g h e r v i b r a t i o n a l l e v e l s w i l l have a  better  62  FIGURE 10  Schematic diagram showing the energy r e l a t i o n s h i p s "between t h e v a r i o u s  excited  s t a t e s among  each o t h e r and w i t h the ground s t a t e o f the cobaltioxalate  i o n f o r t h e case t h a t the r e g i o n  o f maximum a b s o r p t i o n  o f the d - d band  lies  higher than the v i b r a t i o n a l l y e q u i l i b r a t e d l e v e l o f t h e charge t r a n s f e r band.  The h o r i -  z o n t a l l i n e s correspond t o v i b r a t i o n a l l e v e l s . Key: a - e x c i t a t i o n by r a d i a t i o n b - i n t e r n a l conversion e - collisional  deactivation  d - collisional activation e - dissociation f - energy d i f f e r e n c e thermal  reaction  corresponding t o the  c barge  d-d  tra ns f e r  F i g . 10 Co  H^Q  O X 2 O C 2 O 3 "  b  e <-  >»  v.  C  UJ  i  e «-  a  ground state  chance to do so. This explains why the quantum y i e l d s at »  a  0  4046 A and 3650 A are considerably higher than at 435© A . A marked temperature dependence i s expected f o r quantum y i e l d s at wavelengths "corresponding to d - d t r a n s i t i o n s up to 5 K c a l lower than the v i b r a t i o n a l l y e q u i l i b r a t e d charge t r a n s f e r s t a t e .  A range greater than 5 E c a l  i s s t a t i s t i c a l l y improbable.  Outside t h i s range there  should be no appreciable temperature c o e f f i c i e n t .  I f the  o - o charge t r a n s f e r t r a n s i t i o n i s assumed to l i e at the end of the Gaussian extrapolation of t h i s band i . e . at about 23000 cm*~\ then the temperature dependence should be at approximately t h i s energy and down to 21500 cm . -1  Below the  l a t t e r energy the quantum y i e l d should also be very small indeed i . e . the curve i n f i g u r e 5 should approach zero much sooner than i n d i c a t e d . d i c t e d by f i g u r e 8b.  This i s exactly what i s also preIn t h i s figure the i n t e r s e c t i o n of the  two l i n e s corresponds to the point where t r a n s i t i o n s into the charge t r a n s f e r band are s u f f i c i e n t l y allowed to make them e f f e c t i v e i n producing excited states i n quantity and extrap o l a t i o n to zero quantum y i e l d should give the approximate p o s i t i o n of the o - o band.  This however places the o - o  band f o r charge t r a n s f e r at 16200 cm" or 617 mu. I f the region of maximum absorption of the d - d band i s below the v i b r a t i o n a l l y e q u i l i b r a t e d l e v e l of the charge t r a n s f e r band then an intermediate of lower energy than t h i s l e v e l must be postulated to explain the photo  64 a c t i v i t y i n the region of the d - d  band.  presented schematically i n figure 11.  The  situation i s  As before charge  t r a n s f e r absorption r e s u l t s i n a constant quantum y i e l d of about 0.35*  Transitions  into the c e n t r a l v i b r a t i o n a l l e v e l s  of the d - d  band are not  s u f f i c i e n t l y energetic to overcome  the b a r r i e r to i n t e r n a l conversion.  Collisional  deactivation  then brings the excited molecule to the lowest v i b r a t i o n a l level.  A r a d i a t i o n l e s s t r a n s i t i o n to high v i b r a t i o n a l l e v e l s  of the ground state may  then occur.  This molecule would then  have a tremendous amount of v i b r a t i o n a l energy which may lead to e l e c t r o n t r a n s f e r .  The  e f f i c i e n c y may  a constant quantum y i e l d of say 0.07 higher d - d  t r a n s i t i o n s may  results.  then  he such that Some of  the  however have enough energy such  that i n t e r n a l conversion of molecules i n these l e v e l s into the charge t r a n s f e r band may  occur.  This would explain  rather large discrepancy i n the observed quantum y i e l d  the and  the c a l c u l a t e d quantum y i e l d s using the assumed constant quantum y i e l d s of the two  bands.  I f t h i s l a t t e r scheme  applies then one would also expect a marked temperature dependence i n the region from 405 6.  to 365  mu.  Suggestions f o r Further Work The  question as to which of the hypotheses presented  corresponds to r e a l i t y has  to he decided by further  ments as both schemes explain the p h o t o a c t i v i t y of the d - d  band.  experi-  i n the  region  65  FIGURE 11  Schematic diagram showing the r e l a t i o n s h i p o f the ground s t a t e o f the c o b a l t i o x a l a t e i o n w i t h the e x c i t e d s t a t e s and the r e l a t i o n s h i p between the e x c i t e d s t a t e s f o r the case t h a t the r e g i o n o f maximum a b s o r p t i o n o f the d - d  band l i e s lower i n energy t h a n the  v i b r a t i o n a l l y e q u i l i b r a t e d l e v e l o f the charge t r a n s f e r band.  The h o r i z o n t a l l i n e s c o r r e s p o n d  to v i b r a t i o n a l l e v e l s . Key: a - e x c i t a t i o n by r a d i a t i o n b - i n t e r n a l conversion c - collisional  deactivation  d - collisional activation e - dissociation  charge transfer  More a c c u r a t e quantum y i e l d s would have t o be e s t a b l i s h e d , p a r t i c u l a r l y i n the s h o r t and l o n g wavelength regions.  Values  a t h i g h e n e r g i e s would i n d i c a t e t o what  e x t e n t the cage e f f e c t i s o p e r a t i v e i n case the  decomposition  proceeds v i a a v i b r a t i o n a l l y e x c i t e d intermediate.  At l o n g  wavelengths i t would be i n t e r e s t i n g t o see whether the quantum y i e l d s f o l l o w the b e h a v i o r p r e d i c t e d by f i g u r e  8b.  I t would a l s o be i n t e r e s t i n g to determine the extent of photoracemization  a t s h o r t wavelengths as a c c o r d i n g  t o the hypotheses p r e s e n t e d i t i s more p r o b a b l y a t s h o r t than a t l o n g wavelengths i f the i n t e r m e d i a t e i s a t r i g o n a l bipyramid. t o observe  T h i s may  e x p l a i n why  the p h o t o r a c e m i z a t i o n  Adamson and S p o r e r  a t l o n g wavelengths p o s t u l a t e d  by them on the grounds o f the c r y s t a l f i e l d The most important  failed  theory.  t e s t f o r the hypotheses l i e s i n  the a c c u r a t e d e t e r m i n a t i o n o f the temperature c o e f f i c i e n t s quantum y i e l d s over the e n t i r e v i s i b l e r e g i o n .  As p r e v i o u s l y  d i s c u s s e d the dependence o f these temperature c o e f f i c i e n t s wavelength would c o n f i r m which view, i f any,  of  on  i s applicable.  U s i n g the p r e s e n t method t h e f o l l o w i n g improvements may  lower the e r r o r l e v e l s t o such a degree so as t o a l l o w a  r e a l i z a t i o n o f the above aims: (i)  F o r more a c c u r a t e p h o t o m e t r i c  work i t  would be advantageous t o make the  67 monitoring  l i g h t beam o f t h e  spectrophoto-  meter more monochromatic by u s i n g an i n t e r ference  f i l t e r f o r 510 mu.  voltage  s t a b i l i z e r s i t may t h e n be p o s s i b l e  to obtain readings  With the a i d o f  significant to  1 1000  o f an o p t i c a l d e n s i t y u n i t . (ii)  The o p t i c a l system c o u l d be improved by making the l i g h t beam more n e a r l y p a r a l l e l and  employing a s t e a d y mercury a r c .  ference  filters  f o r the exact  mercury l i n e s may p r o v i d e c h r o m a t i c l i g h t beam.  Inter-  i s o l a t i o n of  a b e t t e r mono-  F o r the d e t e r m i n a t i o n  o f temperature c o e f f i c i e n t s i t would be advisable  t o use an arrangement where t h e  beam i s s p l i t  so t h a t two samples a t  d i f f e r e n t temperatures c o u l d be i r r a d i a t e d simultaneously.  T h i s would t e n d t o c a n c e l  out e r r o r s due t o lamp f l u c t u a t i o n s . (iii)  The exact cell  l i g h t i n t e n s i t y i n the r e a c t i o n  c o u l d be measured by e x p o s i n g a c t i n o m e t e r  s o l u t i o n i n the r e a c t i o n c e l l  o f e x a c t l y the  same absorbance as t h a t o f the c o b a l t i o x a l a t e solution.  T h i s would t h e n take i n t o account  the l i g h t absorbed t h a t i s r e f l e c t e d and s c a t t e r e d from t h e r e a r f a c e o f the r e a c t i o n cell.  (iv)  I n the he K^Co  a n a l y s i s o f Co  c o r r e c t e d by the  approximate amount of  oxj decomposed d u r i n g  so t h a t sample and the  the "blank c o u l d  the  photolysis  blank contain  exactly  same amount of unchanged c o b a l t i o x a l a t e .  At the  same time the  amount o f f e r r i c  i n the  s o l u t i o n s would a l s o have t o  ion  be  equalized. More d e f i n i t e statements about the o f c o b a l t i o x a l a t e c o u l d be made i f the the n a t u r e o f the  intermediates  photochemistry  exact k i n e t i c s  were known.  Using  and  the  t e c h n i q u e o f f l a s h p h o t o l y s i s p r o g r e s s i n t h i s d i r e c t i o n has a l r e a d y been made by P a r k e r and  H a t c h a r d (46)  experiments would p r o b a b l y y i e l d the  desired  and  further  information.  BIBLIOGRAPHY 1.  Winkelblech, C. L e i b . Ann. 1^:166 ( 1 8 3 5 ) .  2.  Kehrmann, F. Ber. 19_:3101 (1886).  3.  Copaux, H. Compt. Rend. 1 2 4 : 1 2 1 5 ( 1 9 0 2 ) .  4.  Sorensen, S. P. L. Z. anorg. Chem. 11:1  5.  Brunner, E. Helv. Chim. Acta 12:212  6.  Benedict, S t . R. J . Am. Chem. Soc. 28:173  7.  Kehrman, F., and P i c k e r s g i l l , IT. Ber. 24:2324 (1891).  8.  Classen, A., and v. Reis, M. A. Ber. 14:1623 (1881).  9.  Marshall, H. J . Chem. Soc. 5_9_:764 (1891).  (1896).  (1929). (1906).  10.  Oberer, E. D i s s e r t a t i o n , Zurich, 1 9 0 .  11.  Jaeger, F. M. Rec. Trav. Chim. 3.8:248 ( 1 9 1 9 ) .  12.  Jaeger, F. M., and Thomas, W. Akad. Amst. V e r s l . 22:680 (1918(19).  13.  Recoder, R. F. 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