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Radiochemical studies on graphite ferric chloride Lazo, Robert Martin 1950

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L (5 5 ft 7 RADIOCHEMICAL STUDIES' on GRAPHITE FERRIC CHLORIDE BY ROBERT MARTIN LAZO A Thesis Submitted i n P a r t i a l Fulfilment of the Requirements fo r the Degree of MASTER of ARTS i n the DEPARTMENT of CHEMISTRY The University of B r i t i s h Columbia. A p r i l 1950. ABSTRACT Graphite f e r r i c chloride, C j ^ F e C ^ , w a s prepared by heating anhydrous f e r r i c chloride with graphite at 3 0 5°C o The p u r i f i e d compound containing 53$ f e r r i c chloride was not attacked by hot 6N hydrochloric a c i d or 6N sodium hydroxide solutions. X-ray d i f f r a c t i o n measurements gave a powder pat-tern which was d i s t i n c t l y d i f f e r e n t from that of e i t h e r gra-phite or f e r r i c chloride and from which i t was apparent that no free f e r r i c chloride existed i n the compound. ,The separa-t i o n of the graphite layer-planes was increased from 3 . 3 6 to 9 . 4 A by the i n t e r c a l a t i o n of the f e r r i c chloride molecules. Tests f o r exchange between Ci2FeCl3 and F e + + + ion were made using radioactive F e 5 9 . i n no instance was any measurable exchange observed. The compound was subjected to neutron i r r a d i a t i o n and the Szilard-Chalmers y i e l d of sepa-rable a c t i v i t y calculated and i d e n t i f i e d . The separated por-t i o n contained l e s s than 1% of the t o t a l a c t i v i t y and consis-ted of Fe$9t P^ 2 and S^, free of detectable radiation decom-posi t i o n products. The active Fe portion was separated by ether extraction. The lack of exchange and the low Szilard-Chalmers y i e l d are both attributed to the formidable s t e r i c hindrance e f f e c t s which result from the configuration of the "stacked layers" structure f o r graphite f e r r i c chloride. ACKNOWLEDGEMENTS I wish to express my gratitude to Dr. J.G. Hooley who so ably directed t h i s research project. Appreciation i s also expressed to the National! Research Council of Canada f o r the summer research scholarship awarded the author during the course of th i s study, and to the B r i t i s h Columbia Research Council who so kindly permitted the use of th e i r X-ray spectrometer. TABLE of CONTENTS Introduction 1. Graphite and Graphite Compounds 1 A. ) Graphite Ferric, Chloride 9 2* Radiochemical Theory 16 A*) Isotopic Exchange Reactions ....18 B. ) The Szilard-Chalmers Reaction i n the Chain Reacting P i l e 26 3» Radiochemical Techniques 33 Experimental Results 1» Preparation of Graphite F e r r i c Chloride [ c 1 2 F e C l 3 ] n . 36 2. Tests f o r Exchange between Graphite F e r r i c Chloride and F e r r i c Ion, using en Radioactive Fe^ 7 45 3« The S z i l a r d Chalmers Reaction with Graphite F e r r i c Chloride 56 Discussion of Results 63 Suggestions f o r Future Research. 66 Bibliography 67 Radiochemical Studies on Graphite F e r r i c Chloride. INTRODUCTION Carbon I t s e l f Is a refractory, non-volatile, insoluble s o l i d , and gives r i s e to lamellar compounds which are i n s o l -uble, non-volatile and often unstable s o l i d s about which c l a s s i c a l chemical methods give very l i t t l e information. X-ray studies have proved p a r t i c u l a r l y f r u i t f u l , and most of the information concerning the structure of s o l i d carbon, and i t s lamellar compounds has been determined from X-ray d i f f r a c t i o n measurements• Carbon, i n the form of diamond, belonging to the cubic system, was one of the e a r l i e s t c r y s t a l s to be investigated by X-rays(2). The e s s e n t i a l point of the space l a t t i c e structure f o r diamond, i s that every atom of carbon i s surr-ounded by four other atoms situated at the corners of a regular tetrahedron. The distance between the centers of two adjoining atoms i s 1.54& , which corresponds very close-l y to the distance between two carbon atoms attached to each other by a single covalent bond i n a l i p h a t i c organic comp-ounds. This agreement, together with the fact that each carbon atom i n diamond has four others situated round i t at the corners of a regular tetrahedron, suggests that every atom i s joined to four others by covalent linkages. A diamond c r y s t a l may then, be regarded as a macromolecule of carbon. Its hardness can be ascribed to the strength of the chemical bonding of the atoms and i t s uniformity i n a l l directions throughout the c r y s t a l . (1) The physical properties of the other a l l o t r o p i c form of carbon v i z . , graphite, of the hexagonal system, are quite d i f f e r e n t from those of diamond. These differences are found to correspond to important changes i n the int e r n a l structure of the c r y s t a l * Although graphite i s so obviously c r y s t a l l i n e , i t s opaque cha rac t e r and the r a r i t y of well developed c r y s t a l s r e s t r i c t considera b l y the crystallographlc information which can be obtained by optica 1 methods. The l a t t i c e s tructure which f i r s t received wide acceptance was deduced i n 1924 by J.D. Bernal (1) and confirmed by C. Mauguin (1926). (12) and by H. Ott (1928) (16). The carbon atoms are arranged i n f l a t layers each having aa hexagonal honeycomb-like structure. These are stacked p a r a l l e l to each other i n such a way that hal f the atoms In one layer H e normally above h a l f the a toms i n the layer beneath. Alternate layers H e , atom f o r atom mormally above each other* The carbon atoms i n the layers are spaced center to center st a distance of 1.415 A compared with a spacing of 1.54 A In the diamond l a t t i c e . The spacing between adjacent layer-planes of graphite i s 3«36 A' • The carbon atoms within the layer-planes therefore are bonded together by powerful covalent valency forces, more powerful tha n those In the diamond. The four valences of each carbon atom are used to form bonds with i t s three neighbors, aand the giant layser molecule resonates among many va lence-bond structures i n such a; way that each carbon-carbon bond achieves one-third double-bond character. This arrangement corresponds very c l o s e l y to the s i x - memb-ered rings formed i n benaene, naphthalene and other aromatic hydrocarbons* Indeed, i n his review of recent work on the s i m l l i a r i t y of the graphite electron structure with that of aromatic compounds (6), Hofmann has stated, "No sharp bound-ary i s found. The intermediary stages of graphite nuclei i n coal amd i n various charcoa Is constitute ar. gradual cha nge from aromatic compound to c r y s t a l l i n e graphite". However the resonance system i n the graphite layer-planes i s so deg-enerate that odd electrons behave more l i k e metallic electrons and account f o r the electronic conductivity of graphite, which with certa i n pure specimens i s twice that of mercury. The e l e c t r i c a l conductivity i n a d i r e c t i o n p a r a l l e l to the layer-planes i s much greater than i n a d i r e c t i o n at r i g h t angles to i t . The hexagona 1 layers of carbon molecules are separated by a distance- so large (3.36 A ) that there can be no covalent bonds between them, and the superimposed layer molecules are held together only by weak van der Waals forces (17). The l a y e r - l a t t i c e set up f o r graphite accounts f o r the cleavage which occurs so e a s i l y between the separate layer-planes i n the c r y s t a l , and the use of graphite as a lubricant depends on t h i s a b i l i t y of one plane of atoms to s l i d e e a s i l y over another. 14; Figure 1. The arrangement, of carbon at i n the graphite c r y s t a l * (5) The structure of the graphite c r y s t a l l a t t i c e appeared to be d e f i n i t e l y established u n t i l D.S. La i d l e r and A. Taylpr (10) i n 194-0 pointed out that t h i s structure does not account for some f a i n t l i n e s which occur i n the X-ray powder photographs of many specimens of graphite. Another form of graphite present to about 10% i s suggested. Instead of the alternate planes being normally above one another, at t h i r d plane i s Inserted which Is symmetrically r e l a t e d to the planes above and below. The graphite l a t t i c e structure with i t s powerful covalent bonds within the layer-planes and the r e l a t i v e l y weak bonding forces between the layer-planes offers an explanation of Figure 2. The two ways of stacking the hexagon layer-planes i n the graphite l a t t i c e . the i n t e r e s t i n g anisotropic properties both physical and chemical, associated with graphite c r y s t a l s . Observations on the swelling of graphite electrodes and the intumescence of graphite, led to the discovery that c e r t a i n molecules and/or ions could penetrate between the layer-planes of the graphite l a t t i c e to form more or less stable structures. This swelling, as pointed out by H. Thiele (33), occurs with highly c r y s t a l l i n e forms of graphite and occurs ex c l -usively along the C axis of the hexagonal f l a k e . Graphite S a l t s : The action of concentrated acids i n the presence of a suitable oxidizing agent, has been shown to bring about the formation (6f> blue graphite s a l t s (7) (23) • Each hexagon layer-plane becomes a macro positive ion with m charge equal and opposite to the number of negative ions bonded to I t . When graphite i s treated with b o i l i n g s u l -f u r i c acid i n the presence of a few drops of n i t r i c acid, the iridescent blue graphite b i s u l f a t e r e s u l t s . Graphite + nHgSO+ + n/20 -» Graphite™" (HSO^ "") n + £H a0 The compound containing the maximum amount of inte r c a l a t e d b i s u l f a te ion and s u l f u r i c acid corresponds to the i d e a l -ized stiochiometric composition Cg 4HS0 4'2 HgSO^ . The addition of a small quantity of water to the s u l f u r i c acid, even from exposure of the acid to the atmos-phere brings about the decomposition of the blue graphite into the o r i g i n a l graphite. X-ray examination of a specimen of graphite b i s u l f a t e suspended i n pyrophosphoric acid showed that the Intercalation of the s u l f u r i c a c i d had increased the inter layer-plane spacing of the graphite from 3 . 3 6 A to 7.98 A (19). compounds. H.L. Ri l e y , i n his review of "Lamellar compounds of carbon" (19) describes the graphite s a l t s n i t r a t e , per-chlorate, biselenate, phosphate, pyrophospha te and arsenate, a l l prepared and characterized by Rttdorff and Hofmann ( 2 3 ) . Graphite Monofluoridet (27) Graphite combines with f l u o r i n e at one atmosphere pressure and a temperature of 420°C. to form a grey hydrophobic s o l i d the composition of which i s (CP)n • The constant composition, the grey color and the low e l e c t r i c a l conductivity, which i s only one-hundred-thousandth that of graphite, a l l suggest that graphite mono- flu o r i d e i s a chemical campound. The fl u o r i n e atoms are arranged i n s i x p a r a l l e l planes between each pair of carbon layer-p l a nes which a re spaced at 8.17 A . Rttdorff has recently announced the preparation of a new graphite-fluorine com-pound (24) tetracarbon monofluoride (C^F)^ • The compound i s not a ttacked by d i l u t e acids or a l k a l i even on heating although concentrated s u l f u r i c acid above 100°C slowly dec-omposes i t . Brom-graphite: A pecular bromine adsorption complex of graphite has been produced (21) by shaking a suspension of graphite i n cold concentrated s u l f u r i c acid with bromine. The graphite samples w i l l take up roughly one atom of Besides the b i s u l f a t e , graphite forms other s i m i l a r bromine to each eight atoms of carbon and give i t up again completely on standing i n the a i r . The X-ray analysis of brom-graphite showed a. carbon layer-plane spacing of 7.05 A • A l k a l i - g r a p h i t e ; When graphite i s heated with potassium, rubidium or cesium i n an evacuated tube, a blue-black alkali-graphite compound i s produced* Shaking the compound i n mercury converts i t to graphite, i t was also found that the adsorption of potassium vapor occured In a stepwise manner, in d i c a t i n g the formation of more than one compound. Schleede and Wellmann (28) determined from X-ray studies that f o r the C 8K compound, each potassium atom lay normally over the center of every second carbon atom hexagon of the basal planes. C/«K has a layer of potassium atoms i n every second Inter-layer-plane space i n the graphite l a t t i c e . These substances were shown to be quite d i f f e r e n t from ordinary a l k a l i metal carbides because only hydrogen and no trace of hydrocarbon was evolved when they reacted with water. The heat of formation of CgK or C,6K using excess potassium, was determined to be about 1500 cal./gm. atom of carbon (4)» Graphitic oxide: Samples of graphitic oxide have been pr-epared by treating graphite that had been previously washed with hydrochloric acid, with concentrated s u l f u r i c acid and n i t r i c acid i n the presence of potassium chlorate (26). After the product has been dried under vacuum over potassium pentoxide the acid groups can be determined by methylation (81 and acetylation. However, i n no case has a fixed s t i o -chiometrical r e l a t i o n s h i p been found, as analyses for C : 0 varies between 6 j l and 6:2.5* Further studies of the c r y s t a l structure have indicated that the carbon layers display a hydroaromatlc character. It i s in t e r e s t i n g to note the close p a r a l l e l i s m between the chemistry of the graphite compounds that have been men-tioned and that of the triarylmethyls (18). This i s indicated by: the existence of the graphite s a l t s , graphite monofluoride, the a l k a l i graphites and graphite oxide, a l l of which appear to: be d e f i n i t e compounds. The triarylmethyls also form a l k a l i s a l t s , halides and peroxides and i n general show the same amphoteric properties as do the hexagon layer-planes of the graphite c r y s t a l l a t t i c e . This Is not surprising, f o r considering one carbon atom i n a layer-plane, It w i l l be seen that i t s three valency bonds are connected each to an aromatic grouping of carbon atoms and that these three bonds are co-planar, l i k e those i n triphenylmethyl. The s t a b i l i t y of the triphenylmethyl free r a d i c a l has been expl-a ined by assuming that the odd electron on the central carbon a torn resonates among many of the aromatic carbon atoms i n the molecule. This i s a very s i m i l a r type of resonance to that occuring i n graphite. A* Graphite f e r r i c c h l o r ide: Perhaps the most in t e r e s t i n g of the graphite compounds reported to date, i s the s u r p r i s i n g l y sta ble graphite f e r r i c chloride, which appif^s to f a l l into a (10) different category, as no similar compound is formed by the triarylmethyls• Graphite f e r r i c chloride was prepared and characterized by W. RUdorff and H. Schulz (25) in 1940 and is reviewed in the U. S. publication F.I.A.T. (1946*) (22). When graphite i s heated to 200°C. and above with two or three times i t s own weight of anhydrous f e r r i c chloride in a sealed tube, the amount of free f e r r i c chloride decreases. The ferric chloride which does not react can be sublimed off or extracted in dilute acid solutions, leaving an apparently homogeneous green to matt black reaction product which has increased in weight up to 200$ of the weight of the original graphite. Specimens prepared between 180° and 300°C. contain between 72$ and 60$ of fe r r i c chloride, which i s equivalent to 1 FeCl : 5.5-9 C atoms. Using reaction temperatures between 325° and 400°C, the f e r r i c chloride content f a l l s to between 37$ and 31$ i.e. 1 FeCl : 23-29 C atoms, and between 400° and 500°Co the product contains only 5$ f e r r i c chloride. This i s completely expelled only at a temperature above 500°C. At 309°C. the graphite f e r r i c chloride complex showed a pronounced intumescence accompanied by the evolution of FeCl 5 vapour. This phenomenon was repeated again at 409°C. and there then remained a grey pulverulent substance which s t i l l contained about 5$ f e r r i c chloride. RUdorff and Schulz found that part of the f e r r i c chloride could be extracted from the complex with water or dilute acids or with alcohol, ether, benzene, etc. When extracted in (11) t h i s way the graphite f e r r i c chloride prepared below 309°C. l e f t a product containing about 56% f e r r i c chloride no matter what solvent was employed and independently of the o r i g i n a l graphite c r y s t a l form. This value of 56% f e r r i c chloride cor-responds to about one molecule of FeCl-j to ten G atoms. Treated i n the same way a specimen prepared between 310 and 400 degrees centigrade gave a product containing 31% f e r r i c chloride or one FeCl3 molecule to t h i r t y C atoms'. The f e r r i c chloride remaining i n the complex a f t e r the extraction was found to be extraordi-n a r i l y unreactive and specimens of the compound were scarcely attacked by hot d i l u t e acid or a l k a l i . Reducing agents such as hydrazine and sulfurous acid were found to be without appreci-able action. Oxidizing agents such as concentrated n i t r i c a c i d or not concentrated s u l f u r i c acid however, decomposed the complex. The properties of graphite f e r r i c chloride and the stepwise decrease i n f e r r i c chloride content, suggested the existence of two compounds and t h i s was confirmed by X-ray d i f f r a c t i o n measurements. The powder photographs of specimens containing 56-72% f e r r i c chloride showed pronounced d i f f e r -ences from those of specimens containing 30-37$ of the metallic s a l t . The d i f f r a c t i o n patterns were a l l d i s t i n c t l y d i f f e r e n t from those of eith e r graphite or f e r r i c c h l o r ide. RUdorff confirmed from X-ray d i f f r a c t i o n measurements, that the spacing between the layer-planes i n the graphite had been increased from 3.36 to 9.4 A by the penetration of f e r r i c chloride molecules. This agreed with the increase i n volume of a single c r y s t a l to about 2.5 times observed microscopically and from density measurements. It i s known that anhydrous f e r r i c cHriLoride c r y s t a l l i z e s i n a layer l a t t i c e . The iron ions form a regular hexagonal network above and below which there i s a p a r a l l e l triangular net plane of chloride ions. The dimensional requirements of these layer-planes along the C axis i s about 5.8?A . The question arises as to how far the l i m i t s of the f e r r i c chloride content found by chemical analysis v i z . 1 PeCl3: 5»5G to 1 PeCl3: 9C can be correlated with the suggested c r y s t a l structure for graphite f e r r i c chloride. RUdorff states that t h i s requires an upper l i m i t of 1:6.02 for tBoe F e C l ^ C r a t i o , and that somewhat higher value i s due poss-i b l y to c a p i l l a r y condensation. The compound with the high-est f e r r i c chloride content probably has a hexagonal packing of the f e r r i c Ions, and when the excess of f e r r i c chloride i s removed by washing the f e r r i c ions take up a triangular packing I.e. one i n which alternate f e r r i c ions have been eliminated from the ' hexagonal arrangement.. In the l i m i t i n g case with the removal - of half the f e r r i c ions to form the triangular arrangement, the r a t i o of FeCl^tC would become 1:12.04. In the work done by RUdorff and Schulz i n 1940, the r a t i o of 1:10.4 was reported for washed specimens of graphite f e r r i c chloride. One sample a f t e r being boiled with 10$ H 2S0^ for twenty-four hours gave the r a t i o 11.9. Vapour pressure measurements indicated that the f e r r i c chloride taken up by the graphite i n excess of about 56$ was loosely bound and probably only served to f i l l holes In the large mesh network of the' more fi r m l y bound f e r r i c chloride molecules. (13) 10 10 ZOO 500 4-00 SOO TEMPERATURE °C Figure 3-The e f f e c t of temperature on Graphite f e r r i c c h l o r i d e . Figure 3 * shows the change i n the amount of i n t e r -c a l a t e d f e r r i c c h l o r i d e w i t h temperature, as i l l u s t r a t e d by Rttdorff. In the compound prepared below .309° 0. the f e r r i c c h l o r i d e has penetrated between a l l the layer-planes of the graphite l a t t i c e . When t h i s compound i s heated to 309°Oe and above there i s a sudden e v o l u t i o n of f e r r i c c h l o r i d e as these molecules are for c e d from between every a l t e r n a t e pair of layer-planes of g r a p h i t e . The dotted l i n e s represent the p u r i f i e d compounds, l ) w i t h f e r r i c c h l o r i d e between a 11 the graphite l a y e r - p l a n e s , and 2)with the i n t e r c a l a t i o n of f e r r i c c h l o r i d e only between every second p a i r .of l a y e r - p l a n e s . I t w i l l be noted that between 2 0 0 and 3 0 9 ° C temperature has no e f f e c t on the percentage of f e r r i c c h l o r i d e held by the graphite i n the p u r i f i e d f u l l y I n t e r c a l a t e d compound. The same i s true f o r the second compound between the temperature of 3 0 9 and 4 0 9 * 0 . Above 409 the percentage of f e r r i c c h l o r i d e drops again l e a v i n g only about % « Figure 4» shows the layer-plane sequence of the two compounds. c • a Fe-ci -.. c — c ct Fe a c C 5 0 f e Q 3 Figure 4° i f n C -Cl Fe -a c -« -Fe <U • c ct fe a c C.2 Fe Cl 5 The layer-plane sequence of Graphite f e r r i c c h l o r i d e . The p a r t i c u l a r s t a b i l i t y of the graphite f e r r i c c h l o r i d e compounds has n e c e s s i t a t e d some explanation of the type of bonding they e x h i b i t . As has been mentioned p r e v i o u s l y , the s t r o n g l y bonded carbon atom layer-planes of the graphite l a t t i c e are held together only by weak van der Waals f o r c e s . The extreme i n a c t i v i t y e x h i b i t e d by graphite f e r r i c c h l o r i d e , i n which the graphite l a y e r -planes have been separated from 3.36 to 9 .4 A , makes i t seem quite u n l i k e l y that the bonding of the f e r r i c c h l o r i d e i n the compound Is of the same type. Rttdorff has found that the magnetic moment of the *e ions i n the compound i s +++• the same as that of pe i n FeCl^ and concludes that co-valent bonding of the Fe-C type i s out of the question ( 2 2 ) . He suggests i n s t e a d , that there i s a p o l a r i z a t i o n of the c o n d u c t i v i t y e l e c t r o n s of the carbon layer-planes toward the c e n t r a l l a y e r of i r o n atoms but that these remain e s s e n t i a l l y i o n i c . One can then imagine some degree of e l e c t r o v a l e n t bonding of the c h l o r i n e ions to the macro p o s i t i v e i o n l a y e r - p l a n e s . In view of the nature of the bonding of the f e r r i c ions and the c o n f i g u r a t i o n of atoms about them, i n the graphite f e r r i c c h l o r i d e s t r u c t u r e , i t seemed that an i n -v e s t i g a t i o n of the p o s s i b i l i t y of exchanging these ions or of e j e c t i n g them from the l a t t i c e by S z i l a r d Chalmers r e a c t i o n , would be of i n t e r e s t and perhaps f u r t h e r e l u c -idate the s t r u c t u r e of the compound. 2. RADIOCHEMICAL THEORY Nuclear chemistry i s a f i e l d of a c t i v i t y as old as nuclear physics. The workers In the late nineties made studies of the possible influence of chemical binding on radioactive decay and searched among the elements for those displaying radioactiveproperties, with the r e s u l t that a number of new elements were discovered, existing only i n radioactive forms. Nuclear science remained pre-dominantly i n the hands of p h y s i c i s t s , however, i n spite of the vast chemical implications of developments i n nuclear transmutation and a r t i f i c i a l r a d i o a c t i v i t y that had made available radioactive forms of a l l of the chemical elements.-With the developments i n "modern alchemy", born with the emergence of the uranium chain-reacting p i l e , chemists of the world began to play a more prominent r o l e i n nuclear research. Today nuclear chemistry has achieved greater importance as a part of chemical science, rather than as an operational appendage of nuclear physics. The basic p r i n c i p l e s of radiochemistry have been well, worked out on the na t u r a l l y occuring radioactive elements by such pioneers as Heresy, Paneth, Fajans, Hahn and others ( 5 ) . With the extension of radiochemical tech-niques to a l l the elements, the f i e l d of radiochemistry expanded tremendously as chemists hastened to make use of t h i s powerful new t o o l f o r research. The voluminous l i t -erature that has been compiled on radiochemical work necessitates that this discussion be confined to only the p a r t i c u l a r l y relevant topics of exchange reactions and the Szilard-Chalmers reaction. (lb) A. Iso t o p i o Exchange Reactions* Since a r t i f i c i a l l y produced r a d i o a c t i v e isotopes have become r e a d i l y a v a i l a b l e to s c i e n t i f i c workers, a, great d e a l of informat i o n concerning the nature of chemical l i n k a g e s and the mechanism of r e a c t i o n s has been obtained from a study of the exchange of i s o t o p i c atoms. The l i t -e rature of such researches has become quite voluminous and i t i s not w i t h i n the realm of t h i s paper to more than mention some general c o n s i d e r a t i o n s . The k i n e t i c mechanisms operating i n systems at e q u i l i b r i u m or i n the steady s t a t e can be s t u d i e d d i r e c t l y by the use of i s o t o p i c l a b e l i n g techniques. In t y p i c a l experiments on i s o t o p i c exchange a l l p a r t i c i p a t i n g molec-u l a r species are i n elementic e q u i l i b r i u m and the o v e r a l l chemical composition of the system i s maintained i n v a r i a n t throughout the experimental p e r i o d . One or more of the r e a c t i n g molecules i s l a b e l l e d by i n c o r p o r a t i o n of a r a d i o -a c t i v e isotope of a. c o n s t i t u e n t atom and observations are made on the r a t e at which i s o t o p i c e q u i l i b r a t i o n i s a t t a i n e d . The r a t e law f o r i s o t o p i c exchange r e a c t i o n s i s f i r s t - o r d e r i n the concent r a t i o n of l s o t o p i c - l a b e l l e d mol-ecules (15)* This g e n e r a l i s a t i o n i s a consequence of the requirement that the o v e r a l l chemical composition of the r e a c t i n g system, be maintained i n v a r i a n t . Thus, suppose the exchange r e a c t i o n to be w r i t t e n i n the form AX + BX' *=* AX* + BX ^ where the prime symbol r e f e r s to the i s o t o p i c a l l y l a b e l l e d (19) atom. In such a r e a c t i o n , there must e x i s t some r e v e r s i b l e mechanism whereby the atomic partners are enabled to exch-ange p o s i t i o n s i n the molecules AX and BX. Suppose t h i s mechanism d i c t a t e s a r a t e of exchange R which i s any f u n c t i o n of the thermodynamic concentrations and a system v a r i a b l e such as temperature. Then, the f i r s t - o r d e r law f o r the exchange of isotope i s dx = d / y * _ ] (2) dt l b a- / where a i s the concent r a t i o n i n moles per l i t r e of molec-\\lea(AX)+(AX') , b the concent r a t i o n of molecules (6x)+(s\'J , x the conc e n t r a t i o n of i s o t o p i c molecules (AX') and y, the conc e n t r a t i o n of i s o t o p i c molecules (BX')» The g e n e r a l i z e d equation I n v o l v i n g polyatomic types AXn w i l l be amenable to the same treatment i f the co n c e n t r a t i o n of the r e a c t i n g species are expressed i n gram atoms per l i t r e . This expr-e s s i o n i m p l i e s t h a t , no matter what the o v e r a l l rate of exchange r e a c t i o n may be, the rate of appearance of l a b e l l e d Isotopes i n the molecules i n i t i a l l y u n l a b e l l e d f o l l o w s a f i r s t - o r d e r law. Prom equation (2) i t i s easy to show that R t = - ( a ) ( b ) JU (\ - x/xJ where t Is the time and Xoo i s the co n c e n t r a t i o n of isotope i n AX at e q u i l i b r i u m . The requirements f o r an I s o t o p i c exchange then are, ( l ) t h e existence of some r e v e r s i b l e e q u i l i b r i u m whereby the r e a c t i n g molecular species may consummate the exchange, (20) and (2) that no I s o t o p i c d i f f e r e n t i a t i o n can occur. I t i s important a l s o t o note that side r e a c t i o n s due to r a d i a t i o n e f f e c t s should he taken care of i n c o n t r o l experiments at d i f f e r e n t r a d i a t i o n l e v e l s . A great number of experiments have been c a r r i e d out by various workers seeking correspondences between types of chemical bonding and the k i n e t i c s of i s o t o p i c exchange. I t i s of i n t e r e s t to mention some of these experiments here. The great mass of inform a t i o n gathered about the s t r u c t u r e of molecules using methods based on X-ray d i f f r a c t i o n e l e c t r o n - d i f f r a c t i o n , magnetic a n a l y s i s , measurement of e l e c t r i c moments, band spectrum a n a l y s i s , e t c . , has been remarkably w e l l c o r r e l a t e d and systematized i n terms of the c l a s s i f i c a t i o n of chemica 1 bonds i n t o c o v a l e n t , e l e c t r o -v a l e n t and m e t a l l i c types. P a u l i n g has extended t h i s c l a s s -i f i c a t i o n of chemical bond type (17) using a quantum mech-a n i c a l approach based on the a d d i t i v i t y of bond energies c a l c u l a t e d from atomic o r b i t a l theory. To evaluate c o r r e l a t i o n s between I s o t o p i c exchange data and bond type, one should neglect systems i n which there e x i s t obvious routes f o r exchange through i o n i c mech-anisms. Systems i n v o l v i n g couples such as Br2 / Br" , E g * / HgjT*', Pe (CN)g"/ Pe (CN)g 3" e t c . , have been observed to e x h i b i t r a p i d exchange through e l e c t r o n t r a n s f e r , e i t h e r through an i o n i z a t i o n mechanism or through an intermediate complex. However, u s e f u l i n f o r m a t i o n about such bonds i s i n f r e q u e n t l y obtained from observations of exchange r a t e s . (21) Other i n v e s t i g a t i o n s i n v o l v e systems f o r which r e a c t i o n mechanisms i n v o l v i n g bond s p l i t t i n g and d i s s o c i a t i o n e q u i l -i b r i a other than those of i o n i z a t i o n are r e q u i s i t e i n pr-oducing exchange. Bonds between atoms i n molecules p a r t i c -i p a t i n g i n such exchanges were c l a s s i f i e d , as "pure" covalent or e l e c t r o v a l e n t or as a mixture of these two extreme types, on the ba s i s of non-isotopic techniques. Then the . c r i t e r i o n based on r a p i d i t y of exchange was ap p l i e d to a s c e r t a i n wheth-er any correspondence e x i s t e d between degree of covalency and r a t e of exchange. In c r y s t a l l i n e diphenyliodonium iodide i t has been shown by X-ray a n l y s i s (13) that two iodine bonds are co-va l e n t and one i o n i c . The X-ray data are c o n s t i t u e n t w i t h a scheme i n v o l v i n g the two covalent bonds i n the iodonium i o n w i t h the i o n i c bond between the iodonium i o n and the iod i d e v i z . (CgH^gl*".- I~ • When t h i s substance i s brought i n t o aqueous s o l u t i o n i n the presence of l a b e l l e d i " and then r e c r y s t a l l i z e d from s o l u t i o n , l a b e l l e d iodine i s found i n the s o l i d . This l a b e l l e d iodine can be removed w i t h s i l v e r i o n (as the hydroxide), the r e s u l t i n g (CgH^^IOH co n t a i n i n g no l a b e l l e d i o d i n e . Furthermore no exchange can be detected between the two io d i n e atoms of the d i p h e n y l -iodonium iodide i t s e l f (9)« I t has been showm by J.F. Flagg, that i n cobaltous c o b a l t i c y a n i d e , there i s no exchange between the cobalt atoms (3)« Here the cobaltous atoms are obvi o u s l y i o n i c i whereas the c o b a l t i c atoms are assumed to be held mainly by covalent bonds by analogy w i t h other c o b a l t i c complexes ( 2 2 ) f o r which magnetic s u s c e p t i b i l i t y data are a v a i l a b l e . Another experiment i l l u s t r a t i n g good c o r r e l a t i o n between covalenay and no exchange and between i o n i c b i n d i n g and r a p i d exchange was c a r r i e d out by P.A.Long i n 1941. (11)• The research reported on the exchange between fr e e oxalate ions and complex oxalates of i r o n and c o b a l t . Prom measure-ments of ma g n e t i c s u s c e p t i b i l i t y i t appears that the bonds between the c e n t r a l atom and the oxalate ions i n the c o b a l t complex are covalent whereas i n the i r o n complex they are mainly e l e c t r o v a l e n t . When these complexes are brought i n t o aqueous s o l u t i o n as the a l k a I I s a l t s i n the presence of f r e e oxalate ions l a b e l l e d w i t h C" i t i s found that f e r r i c t r i o x a l a t e exchanges r a p i d l y whereas the c o b a l t complex shows no exchange. S. Ruben states t h a t , "many examples may be c i t e d to support the notion that e l e c t r o v a l e n t l i n k a g e s lend them-selves to r a p i d exchange and covalent linkages do not. However the cogeny of examples such as those c i t e d i s weak-ened by the f a c t that i n p r a c t i c a l l y a l l cases the exchange i s observed i n a p o l a r solvent i n which d i s s o c i a t i o n e q u i l -i b r i a operate and what i s u s u a l l y being measured i s merely the tendency of i o n i c l i n k a g e s to I o n i z e . I t i s not d i f f -i c u l t to f i n d systems i n which the readiness of atoms to exchange bears l i t t l e r e l a t i o n to the assigned bond type". (20). There i s no exchange between magnesium ions and the Aacetone magnesium porphyrin, c h l o r o p h y l l , d i s s o l v e d i n an &0%Asol-u t i o n . One may assume the c e n t r a l magnesium atom to be held (23) p r i m a r i l y by e l e c t r o v a l e n t linkage because of the high degree of e l e c t r o p o s i t i v e character of magnesium compared to the c o o r d i n a t i n g n i t r o g e n atoms of the p y r r o l e c o n s t i t -uents of c h l o r p h y l l . Nor does exchange occur between p e + + + and ferriheme, ferrihemoglobin, f e r r i c pheophytin and f e r r i c t e t r aphenylporphin, or between Cu + +and c u p r i c pheophytin using various mixed sovents to e f f e c t a homogeneous system ~ (20). In ferriheme or f e r r i h e m o g l o b i n , and presumably i n the other porphins c i t e d , the magnetic data i n d i c a t e mainly e l e c t r o v a l e n t bonding. Yet no exchange occurs. Another i n t e r e s t i n g experiment was c a r r i e d out using the compound ferrous<*,ct.' - d i p y r i d y l s u l f a t e (CiQHg^)^-FeSO^. The complex s a l t i s known to be diamagnetic and P a u l i n g has concluded that Jbfee Fe-N bonds are therefore mainly covalent. Nevertheless an exchange of about 25$ w i t h Fe i n aqueous s o l u t i o n was noted a f t e r two hours (20). From a c o n s i d e r a t i o n of the data a v a i l a b l e at pres-ent i t appears that there need be no systematic c o r r e l a t i o n between covalency or e l e c t r o v a l e n c y and r a t e of i s o t o p i c exchange. The f a c t o r s which i n f l u e n c e such exchange, i . e . s t r e n g t h of bond, s t e r i c hindrance, solvent i n t e r a c t i o n s , equivalent s t a t e s a v a i l a b l e , e t c . , are not yet c l e a r l y def-ined or separable f o r e i t h e r extreme type of bonding, l e t alone f o r mixed types. Therefore i t i s p e r m i s s i b l e only t o make the r a t h e r attenuated statement that any atom bound i n a molecule whether by a covalent or e l e c t r o v a l e n t bond, w i l l not exchange w i t h s i m i l i a r atoms i n another molecular species unless a. mechanism e x i s t s f o r b r i n g i n g such atoms (24) r e v e r s i b l y Into equivalent s t a t e s . I t i s becoming more apparent to i n v e s t i g a t o r s i n these researches, that s t r u c t u r a l r e l a t i o n s are very important and perhaps predominate over bond type In determining exchange r a t e . In the porphin type s t r u c t u r e s already mentioned i t i s necessary to break a "fused" r i n g , i . e . four bonds must be broken simultaneously. Hence one may expect l i t t l e or no exchange because the "fused" r i n g s t r u c t u r e does not permit the escape of the c e n t r a l atom and the e q u i l i b r i u m i n v o l v e d i s p r a c t i c a l l y i r r e v e r s i b l e i n favor of the bound atom. In exchangeable compounds w i t h a r i n g c o n s i s t i n g of separated molecules instead of a "fused" r i n g type, there e x i s t s the p o s s i b i l i t y of stepwise d i s s o c i a t i o n w i t h e q u i l -i b r i a i n v o l v i n g molecular species i n which the metal i o n i s held by two or l e s s bonds.. I t has seemed of i n t e r e s t to extend these I n v e s t i g -a t i o n s to another "bound atom" type s t r u c t u r e , namely, graphite f e r r i c c h l o r i d e ( C ^ F e C l ^ ) . x " r a y s t u d i e s have shown that the F e + + + i o n s between the l a r g e layer-planes of t i g h t l y bonded carbon atoms of the g r a p h i t e l a t t i c e , form a t r i a n g u l a r planar network above and below which there i s a p a r a l l e l net plane of CI i o n s . Rttdorff (22) has found th a t the magnetic moment of the Pe ions i n the compound i s unchanged w i t h that of Pe ions i n PeCl^ and suggests t h a t the bonding of the f e r r i c c h l o r i d e In the l a t t i c e i s the r e s u l t of a p o l a r i z a t i o n of the c o n d u c t i v i t y e l e c t r o n s of the carbon layer-planes toward the c e n t r a l i r o n i o n s . (25) This explanation seems v a l i d , for i t would be d i f f i c u l t to account for the extreme i n a c t i v i t y exhibited by graphite f e r r i c chloride i f one was to assume that the metallic chloride was held by no more than the van der Waals forces between the l a t t i c e layer-planes. From a consideration of the configuration of atoms around the Fe+4~^" ions i n this "sandwich" structure for (Ci^FeCl^) , i t seemed u n l i k e l y that the iron ions i n the compound would exhibit any rapid exchange with F e + + + ions i n a queous solu t i o n . The series of experiments to be described herein was done to determine whether such exch-ange took pla ce under any of a variety of conditions. (26) B. The Szilard-Chalmers Reaction i n the Chain Reacting P i l e * A w e l l known f i e l d of nuclear chemistry based on the s p e c i a l p r o p e r t i e s of r e c o i l atoms i s that of the S z i l a r d -Chalmers r e a c t i o n , wherein the r a d i o a c t i v e atoms from neutron a c t i v a t i o n separate themselves from the bulk of m a t e r i a l being i r r a d i a t e d ( 3 6 ) ( 3 0 ) . The r a d i a t i v e capture of a neutron by a s t a b l e nucleus i s an important nuclear r e a c t i o n , which f r e q u e n t l y gives r i s e to a u s e f u l r a d i o i s o t o p e . However, the chemical i d e n t i t y of the a c t i v e isotope w i t h the unchanged targe t element places serious l i m i t a t i o n s on the s p e c i f i c a c t i v i t i e s obtained by t h i s r e a c t i o n . The Szilard-Chalmers r e a c t i o n , which e f f e c t s s e p a r a t i o n of the a c t i v a t e d atoms from the t a r g e t m a t e r i a l by v i r t u e of the gamma-ray r e c o i l , can be used to enhance the s p e c i f i c a c t i v i t y of the a c t i v e m a t e r i a l under favorable circumstances. Three c o n d i t i o n s have to be f u l f i l l e d to make a Szilard-Chalmers chemical s e p a r a t i o n p o s s i b l e , ( l ) T h e r a d i o -a c t i v e atom In the process of i t s formation must be broken loose from i t s molecule and i t must not recombine w i t h the molecular fragment from which i t separated. (2) The element must be capable of existence i n at l e a s t two mutually s t a b l e and separable forms. (3) At l e a s t two of these forms must show l a c k of r a p i d i s o t o p i c exchange. Most chemical bond energies are i n the range of 1 t o 5 ev. ( 2 0 - 1 0 0 k . c a l . per mole). In any nuclear r e a c t i o n i n v o l v i n g heavy p a r t i c l e s e i t h e r e n t e r i n g or l e a v i n g the nuc-leus w i t h energies i n excess of 10 or 100 kev. the k i n e t i c (27) energy imparted to the r e s i d u a l nucleus f a r exceeds the mag-nitude of bond energies. In the case of thermal-neutron capture, where the Szilard-Chalmers method has i t s most imp-ortant a p p l i c a t i o n s , the i n c i d e n t neutron does not impart n e a r l y enough energy to the nucleus to cause any bond rupt u r e . But neutron capture by a nucleus i s accompanied by the r e l -ease of 8 or 9 m.e.v. of evergy i n the form of s e v e r a l ener-g e t i c gamma quanta. The r e c o i l energy thus imparted to the capt u r i n g atom may be as much as one hundred times as great as the energies of the chemical bonds In which i t p a r t i c i p a t e s . Thus i n most n, y processes the p r o b a b i l i t y of bond rupture i s very high. The t h i r d c o n d i t i o n f o r the operation of the S z i l a r d -Chalmers method re q u i r e s at l e a s t that thermal exchange be slow between the r a d i o a c t i v e atoms i n t h e i r new chemical state? and the i n a c t i v e atoms i n the t a r g e t compound. However, the energetic r e c o i l atoms may undergo exchange more r e a d i l y than atoms w i t h ordinary thermal energies. I t i s these ex-change r e a c t i o n s and other r e a c t i o n s of the high-energy r e c o i l atoms ("hot atoms") that determine t o a* la r g e extent the sep-a r a t i o n e f f i c i e n c i e s obtainable i n Szilard-Chalmers processes (36). Recent attempts to e n r i c h a c t i v i t i e s produced i n the high f l u x of the c h a i n - r e a c t i n g p i l e have shown the high gamma and neutron r a d i a t i o n f i e l d s cause marked chemical changes i n the bombarded^compounds aside from the e f f e c t s of accompanying a c t i v a t i o n . That such r e a c t i o n s may y i e l d products s i m i l i a r to those obtained i n a c t i v a t i o n r e a c t i o n s i s to be expected, since both types are e s s e n t i a l l y a. decomp— (28) o s i t i o n by e x c i t a t i o n . But r a d i a t i o n decomposition can y i e l d small amounts of the chemical form i n which the a c t -i v i t y i s found, thus d i l u t i n g the a c t i v e i s o t o p e . I t i s a l s o p o s s i b l e that the r a d i a t i o n f i e l d w i l l cause f u r t h e r chemical r e a c t i o n s of the separable a c t i v e isotope which may change i t , ; to a form which i s no longer separable, or cause some of the i n i t i a l l y separated a c t i v i t y to be l o s t by a r a d i a t i o n - i n d u c e d back r e a c t i o n . As suggested by R.R. W i l l i a m s , the r a t e of decomposition i s undoubtedly r e l a t e d to d i f f e r e n t f l u x comp-onents from those r e s p o n s i b l e f o r a c t i v a t i o n (36). V a r i a t i o n s among these components must be e l i m i n a t e d or measured before a q u a n t i t a t i v e t e s t of the proposed r a t e equations w i l l be p o s s i b l e . Many workers are engaged i n a study of these proc-esses and i t i s l i k e l y that much more than the present q u a l i t -a t i v e data w i l l soon be a v a i l a b l e . The r e a c t i o n s which the "hot" atom or fragment w i l l " undergo depend to some extent on the nature of I t s environm-ent and i n t h i s connection the present research on the e f f e c t of p i l e I r r a d i a t i o n on graphite f e r r i c c h l o r i d e (CijjFeCl-^) was c a r r i e d out. The l a r g e s t amount of work i n the f i e l d of S z i l a r d -Chalmers sepa r a t i o n s has been done on halogen compounds (29) (32). Many d i f f e r e n t organic h a l i d e s ( i n c l u d i n g C 2H^I, CH^I, CCl^, C2H/C1 2, C 2H 5Br, C 2 H 2 B r 2 , CgELBr) have been i r r a d i a t e d * 128 38 and the products of neutron capture r e a c t i o n s ( I ,01 , 80 82 Br , Br ) removed by various techniques. S e p a r a t i o n s of ha logens w i t h 70 to 100 per cent y i e l d s have a l s o been (29) obtained i n neutron i r r a d i a t i o n s of s o l i d or d i s s o l v e d c h l o r -a t e s , bromates, i o d a t e s , p e r c h l o r a t e s and pe r i o d a t e s . The bombardment of metal-organic compounds and complex s a l t s i s o f t e n u s e f u l f o r Szilard'-Chalmers separat-ions i f the fre e metal i o n does not exchange w i t h the compound and i f the two are separable. Some of the compounds which have been used s u c c e s s f u l l y are: c a c o d y l i c a c i d (CH^J^AsOOH, 76 ^ from which As can be separated as s i l v e r a r s e n i t e i n 95% y i e l d j copper s a l i c y l a l d e h y d e o-phenylene diamine, from which as much as 97% of the Cu a c t i v i t y can be removed as Cu i o n ; u r a n y l benzoylacetonate* VO2(0qE^GOGEGOGE^)2, from which TJ^ 29 a c t i v i t y has been e x t r a c t e d i n about 10% y i e l d . I t has been suggested that metal i o n complexes which e x i s t i n opt-i c a l l y a c t i v e forms and do not racemize r a p i d l y may be gener-a l l y s u i t a b l e f o r Szilard-Chalmers processes because the metal i o n i n such a>. complex i s not expected to exchange r a p i d l y w i t h f r e e metal i o n i n s o l u t i o n . Some complexes of t h i s type have been used s u c c e s s f u l l y , e.g. the tr i e t h y l e n e d i a m i n e n i t r a t e s of i r i d i u m , platinum, rhodium, and c o b a l t . Recent studies of the i r r a d i a t i o n of these cobalt complexes have shown the dependence of a s u c c e s s f u l Szilard-Chalmers' r e a c t i o n on-the c o n f i g u r a t i o n of the compl-exing molecules surrounding the c e n t r a l m e t a l l i c atom. P. Stte and G-. Kayas i n 1948, I r r a d i a t e d i n a neutron f l u x , the three cobalticomplexes , hexamine cobalt I I I n i t r a t e Co(lTH^) 6 (N03)^, t r i e t h y l e n e d i a m i n e c o b a l t I I I n i t r a t e Co(en)3 (NO3)., and d i e t h y l e n e t r i a m i n e cobalt I I I n i t r a t e (30) C o ( t r i ) 2 (N0^)^ (31). They found f o r a l l three compounds, that i s o t o p i c exchange, w i t h i n experimental e r r o r s , was n i l . The percent y i e l d s of separated a c t i v i t y , however, v a r i e d g r e a t l y f o r the three compounds. Prom s a l t no. I , hexamine coba l t I I I n i t r a t e , they were able to e x t r a c t 86$ of the t o t a l a c t i v i t y as the hydroxide. Prom s a l t n o . I I , where the ethy-lenediamine molecules each take up two p o s i t i o n s i n the o c t -ahedral c o n f i g u r a t i o n of the c a t i o n , the amount of the a c t -i v i t y separated dropped to 75$ of the t o t a l In the case of s a l t n o . I l l , d i e t h y l e n e t r i a m l n e cobalt I I I n i t r a t e , the ethylenetriamine molecules each cover three p o s i t i o n s of the octahedral arrangement around the c e n t r a l cobalt atom. Here the Szilard-Chalmers separation was s u c c e s s f u l to the extent of only 10$. \ \ C o ( N H 5 ) 6 ( N 0 3 ) 3 C o ( e n ) 3 ( N 0 3 ) 3 C o ( t r i ) 2 ( N 0 3 ) 3 Figure 5 - S t e r i c hindrance i n cobalt-complexes. These somewhat conclusive r e s u l t s are explained by the authors who s t a t e that when an a c t i v a t e d r e c o i l i n g atom of t h i s type i s surrounded by a number of large i n t e r f e r i n g molecules, ( 3 1 ) that i t i s more or l e s s improbable that there w i l l f o l l o w any bond rupture a l l o w i n g the e j e c t i o n of the "hot" atom from the compound. I t i s assumed t h a t , under the e f f e c t of the shock, the c o o r d i n a t i n g chain enters i n t o v i b r a t i o n and absorbs s u f f i c i e n t energy to prevent the r e c o i l atom from l e a v i n g the molecule. This s t e r i c e f f e c t r e s u l t s i n the production of an a c t i v e compound w i t h most of the A+1 i s o t -opic atoms s t i l l bonded i n the same p o s i t i o n as the neutron c a p t u r i n g atom they r e p l a c e d . I t i s suggested t h a t , i n the case of a coplanar mol-ec u l e , t h i s s t e r i c e f f e c t would not be as pronounced, l e a d i n g to a f a i r l y h i g h y i e l d of separated a c t i v i t y . I t may be of i n t e r e s t to mention that Szilard-Chalmers r e a c t i o n on uranium by means of uranyls&licylaldehyde o-phenylenediimine gives a y i e l d of about 80% ( 1 4 ) . The c e n t r a l atom of t h i s compound i s enclosed i n a l a r g e and coherent organic molecule and the high y i e l d i s l i k e l y due t o the probable c o p l a n a r i t y of the molecule which enables the uranium atom to escape. Consider now the case of the very s t a b l e graphite f e r r -i c c h l o r i d e ( C ^ F e C l ^ ) n w i t h the f e r r i c c h l o r i d e molecules bound between the layer-planes of carbon atoms i n the g r a p h i t e l a t t i c e . As has been suggested, the p r o b a b i l i t y of any r a p i d i s o t o p i c exchange between f e r r i c ions i n s o l u t i o n and the i r o n e x i s t i n g as f e r r i c c h l o r i d e bound i n the graphite l a t t i c e , seems somewhat remote. I t i s therefore l i k e l y that any i r o n a c t i v i t y separated by a Szilard-Chalmers process on t h i s compound could be obtained i n high s p e c i f i c a c t i v i t y . (32) However the c o n f i g u r a t i o n of atoms surrounding each i r o n atom i n t h i s "sandwich" s t r u c t u r e , would very defi-nliaLy be expected t o c o n t r i b u t e to a l a r g e r e t e n t i o n of the a c t i v -i t y by recombination r e a c t i o n s , and r e s u l t i n a low y i e l d of separated a c t i v i t y f o l l o w i n g neutron i r r a d i a t i o n . The success of a Szilard-Chalmers se p a r a t i o n on ^12^ e^^3^n W a S c 3 - e " t e r m ^ n e < ^ f o l l o w i n g i r r a d i a t i o n of a dry p u r i f i e d sample of the compound i n Canada's N a t i o n a l Research C o u n c i l atomic p i l e . The r e s u l t s and d i s c u s s i o n of these experiments f o l l o w i n the experimental s e c t i o n of t h i s paper. (33) 3* RADIOCHEMICAL TECHNIQUES. In r a d i o a c t i v e t r a c e r experiments, a r a d i o a c t i v e isotope i s used as an i n d i c a t o r to t r a c e or f o l l o w the path taken by the i n a c t i v e isotope* The r a d i o a c t i v e isotope decays w i t h the emission of alpha, beta, or gamma r a y s , or p o s s i b l y p o s i t r o n s and these r a d i a t i o n s can be measured w i t h a s u i t a b l e instrument such as an i o n i z a t i o n chamber or a Gelger counter. By t a k i n g s u i t a b l e precautions, the measurements can be made q u a n t i t a t i v e . The decay of a r a d i o a c t i v e substance f o l l o w s the exponential law - K t N « N.e where N i s the number of unchanged atoms at time t , No i s the number present at t = 0, and A. i s the decay constant. The h a l f - l i f e t-|- of a r a d i o a c t i v e species i s g i v e n by In p r a c t i c a l work the number of atoms N i s not d i r e c t l y evaluated, a nd even the r a t e of change d^/d.t ±s u s u a l l y not measured a b s o l u t e l y . The usual procedure i s to determine the a c t i v i t y A, w i t h A-CA.N . The d e t e c t i o n c o e f f -i c i e n t C, depends upon the nature and e f f i c i e n c y of the r e c o r d i n g instrument and the geometrical arrangement of sample and d e t e c t o r . In cases where the decay of the r a d i o a c t i v e species i s appreciable during the course of the experiment i t i s necessary to c o r r e c t f o r the r e s u l t i n g decrease i n act-i v i t y before comparative determinations can be obtained* The l i m i t of s e n s i t i v i t y of a Geiger-Mttller counter i s set by the background counting r a t e which must be subtrac-( 3 4 ) t e d to f i n d the a c t i v i t y of the sample. By sheathing the enclosed counter w i t h a few centimeters thickness of l e a d most of the i o n i z a t i o n from small amounts of a c t i v i t y present as i m p u r i t i e s i n c o n s t r u c t i o n m a t e r i a l s , and from the apprec-i a b l e and v a r i a b l e amount of radon, thoron, and t h e i r decay products contained i n the a i r , can be e l i m i n a t e d . However the cosmic-ray e f f e c t w i l l s t i l l be s i g n i f i c a n t and r e s u l t s i n a. background counting r a t e of from 20-25 counts per min-u t e . Because r a d i o a c t i v e decay i s a random process, u l t i m a t e accuracy i n assay of r a d i o a c t i v i t y i s l i m i t e d by s t a t i s t i c a l f l u c t u a t i o n s inherent i n counting data. This random phenom-enon i s subject to e s t a b l i s h e d methods of s t a t i s t i c a l a n a l y s i s which have been v e r i f i e d abundantly by experiment. The term "standard d e v i a t i o n " as used i n the experimental s e c t i o n of t h i s t h e s i s i s equal to the square root of the number of counts observed. The probable e r r o r , d e f i n e d as the e r r o r which i s as l i k e l y to be exceeded as not, i s 0.6745 times the standard d e v i a t i o n . In general, the counting i s s u f f -i c i e n t i n d u r a t i o n to make the standard d e v i a t i o n l e s s than e r r o r s from n o n - s t a t i s t i c a l sources such as sampling uncert-a i n t i e s and u n c o n t r o l l a b l e chemical l o s s e s . In the researc h t o be de s c r i b e d , only r e l a t i v e counts from a c t i v i t i e s were r e q u i r e d and many of the c o r r e c t i o n s necessary to o b t a i n an absolute count, such as e x t e r n a l ab-s o r p t i o n l o s s , b a c k s c a t t e r i n g and sample geometry, were e l i m i n a t e d by s t a n d a r d i z a t i o n of counting techniques. A com-p l e t e d e s c r i p t i o n of the experimental methods f o l l o w e d i n ( 3 5 ) t h i s research has been included i n the experimental s e c t i o n of the t h e s i s . I t has seemed more convenient to leave any d i s c u s s i o n concerning the v a l i d i t y of counting r a t e r e s u l t s and the j u s t i f i c a t i o n of the manner of handling the r e q u i r e d c o r r e c t i o n problems u n t i l that more appropriate time. EXPERIMENTAL• 1 « Preparation of Graphite F e r r i c Chloride, [c^FeCl^j n • In view of-the nature of the experiments to be carr-ied out, i t was decided that the f u l l y i n t e r c a l a t e d graphite f e r r i c chloride [c^FeCl^j n would be better suited to our purpose than the second compound containing less f e r r i c chlor-ide. Accordingly, It was the compound containing about 5 6 $ f e r r i c chloride which was prepared i n a l l cases by keeping the reaction temperature below 3 0 9*C. a. ) Anhydrous F e r r i c Chloride: Dry C I 2 was passed over C.P. iron powder contained i n an e l e c t r i c a l l y heated Pyrex tube at 350°C. Beautiful hexagonal p l a t e l e t s of a deep red wine color formed throughout the cool portion of the tube. Dry a i r was admitted during the cooling and the cr y s t a l s were removed and handled only i n a dry box. To prepare the active chloride, radioactive i r o n wire was used together with the powder. b. ) C 1 2 F 9 C I 3 : Acid p u r i f i e d , 2 0 0 mesh graphite, (G-66 Graphite powder-FisherScientific Co.) was mixed with three times i t s weight of anhydrous f e r r i c chloride, sealed i n a Carius bomb tube, and heated for twelve hours at 3 0 5 * 0 . The approximately ^7% of the reaction product which i s excess f e r r i c chloride was removed by successive refluxings with hot 6 N H C 1 . A f t e r a t o t a l washing time of 3 6 hours a further r e f l u x l n g at. 1 0 0 °C with a one gram sample gave no test f o r F e + + + w i t h NHyCNS or less than 4 . 3 u g m . Fe as determined radiometrically. 6k NaOH at 80°C showed no action on the (56) (37) g r a p h i t e - f e r r i c c h l o r i d e a f t e r p u r i f i c a t i o n . The compound was s t a b l e i n a i r and decomposed at temperatures only above 309°C (with e v o l u t i o n of FeCl^ vapour). From the i n i t i a l weight of graphite and f i n a l weight of compound, the percentage composition was determined and found always to correspond to C ^ F e C l ^ * a compound c o n t a i n i n g 53$ f e r r i c c h l o r i d e * According to RUdorff ( 2 2 ) t h i s same composition r e s u l t s throughout the p r e p a r a t i o n range 2 0 0 to 309°G. and independently of the o r i g i n a l g raphite c r y s t a l form ( 8 ) . X-ray d i f f r a c t i o n measurements on the G-^FsCl^ powder, made w i t h a P h i l l i p s - G e i g e r Counter X-ray Spectrometer and using the 1 . 5 3 9 A Cu l i n e , showed three peaks at d= 9 » 4 0 , 4 . 6 6 and 3 . 1 3 A . The f i r s t of these has been Interpreted by Rttdorff as the s e p a r a t i o n of the planes of carbon atoms. © His value was 9 4 A . In the three t a b l e s which f o l l o w , are set down the numerical r e s u l t s from X-ray d i f f r a c t i o n measurements on g r a p h i t e , anhydrous f e r r i c c h l o r i d e and C-^pFeCl^* In "the column headed "degrees" i s l i s t e d the g l a n c i n g angle, equal to twice the & i n the Bragg equation n A = 2,^ s i n e . The second column contains the values of d, the distances between r e f l e c t i n g atomic planes. The values I/Io l i s t e d i n the t h i r d column, are the r a t i o s of the i n t e n s i t y of the r e f l e c t e d beam of X-rays to the greatest i n t e n s i t y observed. The values l i s t e d f o r anhydrous f e r r i c c h l o r i d e In Table I I . have been obtained from the l i t e r a t u r e . Tables I and I I I c o n t a i n the (38) values obtained f o r graphite and C 1 2 F e C l 3 from d i f f r a c t i o n measurements made i n the l a b o r a t o r i e s of the B r i t i s h Colum-b i a Research C o u n c i l , who so k i n d l y permitted the use of t h e i r X-ray Spectrometer. (39) TABLE I X-ray D i f f r a c t i o n Measurements on Graphite Graphite 3-36 1.68 2 . 0 2 Degrees ( 2 0 ) 0 dA i / l o 2 4 . 0 3.701 0.13 26 . 5 3 . 3 6 1.0 41.2 2.187 0 . 1 42.2 2.137 0.13 43 . 0 2.100 0.16 4 4 . 8 2.02 0 . 3 46 . 0 1.969 0 . 1 2 50.2 1.82 0 . 1 5 4 . 5 1 . 6 8 0.7 56.2 I . 6 3 0.09 60 . 0 1.539 0 . 1 61.6 1.503 0.06 77 . 0 1.236 0.21 83.7 1.154 0.19 85.0 1.139 0.06 8 6 . 4 1 . 1 2 2 0 . 1 3 ( 4 0 ) TABLE I I X-ray D i f f r a c t i o n Measurements on Anhydrous x F e r r i c C h l o r i d e * FeClo 2.68 2*08 5 . 9 Decrees ( 2 a ) dA 1 5 . 0 5 . 9 0 0 .32 * 17*4 5 . 1 0 0 . 0 5 18 .5 4 - 7 9 0.06 1 9 . 8 4 . 5 0 0.03 29 .4 3 . 0 3 0 . 0 3 3 3 . 4 2 . 6 8 1 . 0 0 * 3 7 * 4 2 . 4 0 0 . 0 2 4 3 . 5 2^.08 0 . 4 0 4 6 . 2 1 . 9 6 0 . 0 3 52 .1 1 . 7 5 0 * 3 0 55.0 1 .67 0 . 06 5 6 . 5 1 . 6 3 0 . 16 ' 6 3 . 5 I . 4 6 0 . 06 70.1 1 . 3 4 0 . 0 5 80.5 1 .19 0 . 0 3 8 7 . 2 1 . 1 2 0 . 0 5 x A.S.T.M.Philadelphia,Pa.,X-ray d i f f r a c t i o n powder patterns J.D.Hanawalt and co-workers,card index. ( 4 1 ) TABLE III X-ray D i f f r a c t i o n Measurements on Graphite F e r r i c C h l o r i d e C 1 : )FeCl . -L/C 3 C 1 2 P e C l 3 9.4 4*66 3.13 Degrees (2a) dA i / l o 8..1 10.90 0 . 3 0 9.4 9 . 4 0 O .64 11.8 7 . 4 9 0.25 19.0 4 . 6 6 1.00 26.8 3.32 0.21 28.5 3.126 0.81 * 35 . 5 2.524 0.09 45.9 1.973 0.07 52.2 1 . 4 2 0.11 55.1 1.663 0.09 58.8 1.567 0 . 1 0 63.6 1.464 0.08 68.1 1.374 0.12 80 . 2 1.195 0.05 80.. 8 1.187 0.05 81,8 1.175 0.09 10 io i° *o ,ro co 7o ao yo GLANCING ANGLE (m;uui.s) • — . _ : ; . Plate I X - R a y d i f f r a c t i o n patterns of I - Graphite I I - Anhydrous F e r r i c Chloride I I I - Graphite F e r r i c Chloride. (42) P l a t e I shows the comparison between the three X-ray;'*? d i f f r a c t i o n diagrams. I - g r a p h i t e , I I - anhydrous f e r r i c c h l o r i d e and I I I - graphite f e r r i c c h l o r i d e ( C ^ F e C l ^ ) . I t w i l l be seen that the p a t t e r n f o r g r a p h i t e f e r r i c c h l o r -ide i s d i s t i n c t l y d i f f e r e n t from that of e i t h e r g r a p h i t e or f e r r i c c h l o r i d e a nd i t i s apparent that no f r e e f e r r i c c h l o r i d e e x i s t s i n the compound. The value obtained f o r the percentage f e r r i c c h l o r i d e i n the compound compares very w e l l w i t h the l i m i t i n g value of FeCl^tC equal to 1:12 .04 r e q u i r e d by the c r y s t a l s t r u c -t u r e as determined by RUdorff. R i g o r o u s l y p u r i f i e d spec-imens gave the c o n s i s t e n t f i g u r e 53% f e r r i c c h l o r i d e , which corresponds to a F e C ^ t C r a t i o of 1:12.02. This f i g u r e which i s somewhat b e t t e r than the value 1:11 .9 r e p o r t e d by RUdorff (22) i s probably due to the longer p e r i o d of a c i d - l e a c h p u r i f i c a t i o n employed i n t h i s work. In view of the experiments t o f o l l o w , i t was necessary t o determine to what degree of c e r t a i n t y the l a s t t races of f r e e f e r r i c c h l o r i d e had been removed from the p u r i f i e d c 1 2 F e C l 3 ' 1 ) . 5 gms. of C^FeClcj were p u r i f i e d by successive r e f l u x i n g s In 6N. HC1 f o r t h i r t y - s i x hours. This specimen was then t r e a t e d to a f u r t h e r r e f l u x i n g i n lOOmls. of 6N. HCl f o r s i x hours, and the a c i d e x t r a c t t e s t e d f o r Fe*++ w i t h 1% NH^CNS s o l u t i o n . The NH^CNS r e a c t i o n w i t h Fe+++ which i s w e l l adapted to spot t e s t s has a s e n s i t i v i t y of 0.25 u.g.m. Fe^ + +and a conc e n t r a t i o n l i m i t of 1 i n 200,000. ( 4 3 ) The a c i d e x t r a c t s o l u t i o n was evaporated to 10 mis. before t e s t i n g . 1 ml. po r t i o n s gave no c o l o r a t i o n w i t h the reagent and were ther e f o r e assumed to co n t a i n l e s s than 5 u. gnu F e + + + . Therefore the 100 ml. a c i d .extract from the 5 gm. sample of C l 2 P e C l 3 could not have contained more than 50 u. gm. F e + + + , or l e s s than 10 u.gm. F e + + + p e r gram of graphite f e r r i c c h l o r i d e t e s t e d . 2 ) . A second t e s t f o r completeness of removal of free Fe^^was made using r a d i o a c t i v e C-^FeCl-^. The a c t i v e * 3 <-'12Fe ^ 3 w a s measured at 229*10 counts per minute per 3 gm. which was equivalent to 229 x10 c.p.m. from 0.1820 gm. Pe • The sample was t r e a t e d by the 36 hour washing procedure and then examined as f o l l o w s : 5 gms. C 1 2 F e * C l 3 r e f l u x e d w i t h lOOmls. 6N HC1 f o r 4 hours. The 100ml. a c i d s o l u t i o n was evaporated to 10ml. and t e s t e d f o r a c t i v i t y . l m l . a c i d e x t r a c t s o l n . gave 4 4 0*21 counts/10 min. =44*2 c.p.m. Background count = 44*2 c.p.m. Probable e r r o r 0.6745* 2= ±1 . 3 5 c.p.m. Maximum a c t i v i t y due to F e + + + i n e x t r a c t = 2x1 . 3 5 c.p.m./ml.= 2.7*10 c.p.m. i n t o t a l volume of e x t r a c t . 3 229*10 c.p.m. are recorded from 0.1820 gm. Pe. /.Max, Pe i n a c i d extracts 27 3 x 0.1820 =21 .5 u.gm. from 229*1& 5 gm. ( C 1 2 F e C 1 3 ) .-.less than 4 . 3 u.gm. P© extractable/gm. C l pPeGlo (U) From a c o n s i d e r a t i o n of the above r e s u l t s i t appeared reasonably c e r t a i n that we werB d e a l i n g w i t h C ] ^ 9 ^ ! ^ * and of s u f f i c i e n t p u r i t y to be s u i t a b l e f o r use i n the f o l l o w i n g experiments on exchange and the Szilard-Chalm-ers r e a c t i o n . (45) 2. Te3ts f o r Exchange between Graphite F e r r i c C h l o r i d e 59 G 1 2 F e C l 3 a n d F e r r i c i o n , using Radioactive Fe . The exchange experiments were performed by shaking p o r t i o n s of f i n e l y powdered r a d i o - a c t i v e graphite Iron (59) I I I c h l o r i d e i n s o l u t i o n s of i n a c t i v e f e r r i c i o n of various P.H.'s, temperatures, and s o l v e n t s . The reverse procedure, using r a d i o a c t i v e i r o n (59)111 c h l o r i d e s o l u t i o n s , was a l s o c a r r i e d out i n some cases. These mixtures were mechanically shaken f o r various lengths of time, then separated by cent-r i f u g i n g . A l i q u o t s were withdrawn and the s p e c i f i c a c t i v i t y of the a c t i v e components determined from the counting r a t e , c o r r e c t e d f o r decay. The c o n d i t i o n s of the experiments were v a r i e d consider-ably but the same general procedure was f o l l o w e d i n every case. Tests f o r exchange were c a r r i e d out i n a c i d media. As w e l l as f e r r i c c h l o r i d e , aqueous s o l u t i o n s of f e r r i c n i t r a t e and f e r r i c s u l f a t e were mixed w i t h the a c t i v e compou-nd. To t e s t the e f f e c t of temperature on the r a t e of exchange a l l the mixtures i n aqueous media were shaken at temperatures of both 20 and 80*C. The i n f l u e n c e of solvent i n t e r a c t i o n on the exchange was observed by c a r r y i n g out experiments using f e r r i c c h l o r i d e dissolved i n ethanol, d i e t h y l ether, i s o - p r o p y l ether, acetone and benzyl a l c o h o l . I n one exper-iment u s i n g benzyl a l c o h o l as the media, t e s t s f o r exchange between graphite f e r r i c c h l o r i d e and f e r r i c Ions were made 0 at a temperature of 190 C. (46) R a d i o a c t i v i t y Measurements: Pure i r o n wire was a c t i v a t e d i n the neutron f l u x of the Canadian N.R.C. p i l e at Chalk 59 R i v e r , Ontario. A mixture of the 47 day Fe e m i t t i n g 0.26 and O .46 Mev. beta's and 1 . 1 0 and 1 . 3 0 Mev. gamma's and the 55 4 year Fe which decays by K capture e m i t t i n g 0.07 Mev X-59 r a y s , was re c e i v e d w i t h an a c t i v i t y of 0 . 0 5 9 mc. Fe /gm. 55 , plus about 0.29 mc. Fe /gm. This was mixed w i t h Fe powder and converted t o f e r r i c c h l o r i d e as already d e s c r i b e d . The decay curve ( F i g . V I ) , over a f i v e month p e r i o d i s l i n e a r w i t h a Tir of 4 7 days, i n d i c a t i n g no need f o r a 55 c o r r e c t i o n f o r the weak Fe r a d i a t i o n . Measurements were made w i t h an "end on" G.M. s e l f quenching counter made by the U.B.C. Physics Department. I t had a 3mg./cm window 2 . 0 c m . i n diameter, and a 1 5 0 v o l t p l a t e a u which was f l a t w i t h i n 1% s t a t i s t i c a l e r r o r and over which the count increased 8$. ( F i g . VII) The counter was mounted on a l u c i t e base and s h i e l d e d to a background count of 4 2 * 2 counts per minute. The pulses were counted by a Nuclear Instrument Co. Model 163 s c a l i n g u n i t . Response was l i n e a r to 2% up to 3 3 0 0 counts/minute. Samples were counted on l " diameter watch g l a s s e s , placed i n an e a s i l y r e p r o d u c i b l e p o s i t i o n about 3 c m . below the counter window. The samples of graphite f e r r i c c h l o r i d e 2 compared were always l e s s than 0 . 2 mg. per cm. and hence no s e l f a b sorption c o r r e c t i o n was made i n c a l c u l a t i n g spec-i f i c a c t i v i t y . The I n i t i a l a c t i v i t y was 0 . 1 uc. per gm. of compound, c o r r e c t e d only f o r background. The evaporated f e r r i c c h l o r i d e s o l u t i o n samples however, wer© ofte n t h i c k T I M E I N P A Y S Figure VI - Rate of decay of radioactive Fe COUNTS PER MINUTE (47) enough to r e q u i r e a s e l f absorption correction.. This was handled i n the usual f a s h i o n . A l i q u o t s of 0.2, 0.4, 0.6 and 0.8 ml, of s o l u t i o n were p i p e t t e d onto the watch g l a s s e s , evaporated to dryness u s i n g an i n f r a red lamp, and counted. The measured a c t i v i t y per u n i t volume of sample i n each case was c a l c u l a t e d (apparent s p e c i f i c a c t i v i t y ) , and p l o t t e d against the a c t u a l volume of the sample used. Prom t h i s curve, e x t r a p o l a t e d to zero volume ( i . e . zero thickness of sample) was obtained the true s p e c i f i c a c t i v i t y . The s o l u t -ions of r a d i o a c t i v e i r o n (59) I I I c h l o r i d e used were prep-ared from a standard 0.4M s o l u t i o n of P.H. l.Q. The a c t i v -i t y of t h i s s o l u t i o n corrected as above and f o r background was 580 counts per minute per mg. of i r o n . Later measure-ments of both s o l i d and s o l u t i o n were c o r r e c t e d f o r decay back to t h i s time. Exchange Measurements! A 0.01 t o 0.15 gm. sample of C]2FeCl3 was mechanically shaken w i t h 5ml. of a s o l u t i o n c o n t a i n i n g f e r r i c i o n and c e n t r i f u g e d . The s p e c i f i c a c t i v i t y of the washed s o l i d and of the s o l u t i o n were determined ( and c o r r -ected f o r background and decay, and i n the case of the evap-orated s o l u t i o n a l i q u o t , f o r s e l f absorption) as d e s c r i b e d above. , The v a r i a t i o n s made i n the c o n d i t i o n s were: 1) P.H. : 0.2 to 3.0 2.) Temperature : 20 t o 80°C. f o r water and to 190°for benzyl a l c o h o l . 3£) Solvent f o r PeGl^ : water, ethanol, acetone, d i e t h y l ether, i s o p r o p y l ether, benzyl a l c o h o l . (48) 4.) Anion : c h l o r i d e , n i t r a t e , s u l f a t e . 5) A c t i v e i r o n i n the compound only or i n the s o l u t i o n o nly. 6) Time of shaking. 7) Concentration of f e r r i c i o n and weight of compound. (-49) RESULTS AND DISCUSSION. The data obtained on the exchange of graphite f e r r i c c h l o r i d e and f e r r i c ions i n a c i d s o l u t i o n , are presented i n Table I. The q u a n t i t y r e p r e s e n t s the s p e c i f i c a c t i v i t y expressed i n a r b i t r a r y u n i t s . I i s the counting r a t e i n counts per minute c o r r e c t e d f o r decay, and C i s the number of m i l l i g r a m s of iron.present i n the sample of the graphite f e r r i c c h l o r i d e being counted, i n column 7 i s recorded the s p e c i f i c a c t i v i t y of the o r i g i n a l g raphite i r o n (59)111 c h l o r i d e , i n column 8 i t s s p e c i f i c a c t i v i t y a f t e r remaining f o r the recorded l e n g t h of time i n the s o l u t i o n d e s c r i b e d , and i n the l a s t column the s p e c i f i c a c t i v i t y c a l c u l a t e d f o r complete exchange. • The r e s u l t s of the experiments reported i n Table I show that no appreciable exchange takes place between C-^Fe ~ C l ^ and F e + + + i n a c i d s o l u t i o n w i t h i n periods of time up to ./ twelve hours. Further observations on some of these r e a c t i o n mixtures were made a f t e r they had been shaken f o r n i n e t y -s i x hours, and again a f t e r standing i n the l a b o r a t o r y f o r a p e r i o d of four months. In a l l cases the exchange was n e g l i g i b l e . (Table I I ) . Reaction temperatures above room temperature were obtained by connecting a hot water c i r c u l a t o r y system to the mechanical shaker. Where t e s t s f o r exchange were made using an aqueous medium a maximum temperature of 80 was used. Table I I I shows the r e s u l t s obtained from comp-arison s at d i f f e r e n t temperatures, of t e s t s f o r exchange and F e ^ i n Aqueous Acid Solution at 20^2 C, No. c 1 2 F e * c l 3 gms •, FeClo soln. mis. Cone, of FeCl^ M. P.H. Time of Shaking i / c (original) I/O (obs. at time 1 /o t) (*>ealc 1. 0.025 5 0.01 1.1 4 min. 1259*9 1257*8 780 2. 0.025 5 0.01 1.1. 2 hrs. 1259±9 1251+10 780 3. 0.010 5 0.05 3.0 1 nr. 1260±10 1 2 4 9 + 1 4 145 4. 0.010 5 0.05 3.0 8 hrs • 1260*10 1249±12 145 5. 0.025 5 0.10 1.2 4 min. 1260*10 1253 + H 176 6. • 0.025 5 0.10 1.2 2 hrs. . 1260±10 1262*10 176 > 7. 0.030 5 0.167 1.0 4 min. 1257*11 1254±10 131 1 8. 0.030 5 0.167 1.0 6 hrs. 1257*11 ' 1251+11 131 H 9. 0.025 5 0.25 1.0 4 min. 1255±8 1261±10 78 10. .0.025 5 0.25 1.0 12 hrs. 1255±8 1260±9 78 11. . 0.025 5 0.10 2.5 8 hrs. 1257*11 1259±10 176 12. 0.050 5 0.5 0.5 1 hr. 1250*10 1254*10 78 13. 0.050 5 0.5 0.5 4 hrs. 1250±10 1246±10 78 14. 0.100 5 0.78 0.2 4 min. 1258±8. 1252*10 100 15. 0.100 5 0.78 0.2 4 hr. 1258+8 1254+10 100 16. 0.100 5 0.78 0.2 10 hrs. 1258*8 1246±12 100 17. 0.150 5 1.0 0.5 4 min. 1252±10 1260*10 123 18. 0.150 5 1.0 0.5 8 hrs. 1252*10 1248*10 123 >J1 o No. Composition P.H. I/C (original) I /0 (after 4min. shaking) I /C • I /C I/O (after 96hrs. (after 4mo. («ecalc.) shaking) contact) 1 . O.Olgm. C 1 2FeClo, 3»0 1260±10 5ml. 0.051 PeCl|. 1262*10 1248±10 1252*10 145 2. 0.025gm. C 1 ?PeClq 1.2 1260±9 5ml. 0.1M P5Cl3.^ 1253±H 1255±10 1259±10 176 3 . 0.025gm. C 1 2 P e C l 3 1.0 1255±8 5ml. 0.25M FeCl^. 1261±10 1260±10 1248±10 78 4. 0.05ga. C 1 2 F e C l 3 0.5 1250±10 5ml. 0.5M F e C ^ . 1259±10 1251±10 1242±10 78 5. O.lOgm. C 1 2Fe*Cl 0.2 1258*8 1252±10 1239±10 1247±10 100 5ml. 0.780M F e c L . b3 6. 0.15gm. C-ipFeCl, 0.! 5ml. 1.0M PeCl|. 1252*10 1260±10 1247^10 1245±10 123 Tests for Exchange between C 1 2 F e * C l ^ and Fe*in Acid Solution at 20 ± 2 ° C After 4 minutes, 96 hours and 4 months. TABLE II (52) TABLE I I I E f f e c t of Temperature on Tests for Exchange between  C 1 2 F ® C 1 3 a n d F e + ^ i n Various Acid Solutions. o * r-i H <S O CD o B -P H a) ct! o a O CD B o O O O O O O O O O O O O O O O o O H H r-i r-i rH rH rH rH rH rH rH rH rH rH rH rH r-i •H -H •H •H -H •H -H -H -H •H * •H •H •H -H •H a -<t O O i r \ O CO CV ITv CO O CV rH ir> O t> CV Vf\ lf\ i r \ IT\ i r \ CO IT\ -* -4" -<t i r \ -«* i r \ i r \ m c v CM CV CV ( V CV c v CV CV CV CV CV c v c v c v CV c v CV rH rH H rH iH rH r-i rH H rH rH rH H rH rH rH r-i rH EH S o En — <M CD O bO G as a3 •H o • 03 O O ITv O • ^ — O rH T> « ° ? *CD fc CV r-i O O 125 in IA IA r-i r-i r-i r-i H r-i l f \ I f t l f \ •sf r-i r-i rH -4- l f \ Vf\ rH rH H o o cv cv o o o cv cv cv o o O o o o o cv rH rH rH rH rH rH rH r-i rH r-i r-i rH rH r-i rH rH rH rH 44 +l +i •H +t M •H H a •h •H -h •M -H +t -h o o O i r \ CO to o CV cv rH c\ O o fr- rH C- CO CO i r \ <o to to l f \ i r \ ies ITS IT\ i r \ cv cv cv CV cv CV CV cv cv CM CV cv cv cv cv CV CV CV H r-i rH H H rH rH rH rH r-i r-i r-i H r-i r-i r-i r-i rH Ji £ Jl £ U Xi a h i ! i * D3 h CO to co to to to CO to CO CO 3 o CO o CO o CO o -* o o> o o o o o O O O o o o O o O o O O CV CO CV c o c v CO c v c v c v CV <D a w o o o •P c v e c c E - p 1 e • ^ CD w fe O OA <J —• c * \ ^—* f— o o o O r-i »— r-i 00 r-i O ^— c v O *— O CD CD e CD <D ' CD e ' CD c ' CD Em fe fe fe fe fe o cv o c v o CV o CO rH O rH CD O . CD O >?-d OJ3H o Si -P PH 43 >»rH •P CD p4 CD ta odi O C 10 CD H CQ CD O o O o o o o o o o o O O O O O O O O rH rH rH H CV cv H r-i H rH CV CV rH rH rH rH H rH O O O O o o o o o O O O O O O O O O O O O O o o o o o O O O O O O O O O • • • • • . ' . . . O H CV i r \ to O CO H CV to o CO o rH rH H rH r-i r-i rH rH H (53) between graphite f e r r i c c h l o r i d e and f e r r i c iron i n s o l u t i o n s of water, e t h y l a l c o h o l , acetone, d i e t h y l ether, i s o p r o p y l ether and benzyl a l c o h o l . The i n f l u e n c e of both solvent I n t e r a c t i o n and temperature appear to be n i l . Even•at*temp-o erature of about 190 C w i t h b o i l i n g benzyl a l c o h o l no exch-ange was observed a f t e r four hours r e f l u x i n g . To complete the r a t h e r strong evidence that no exch-ange takes place between C 1 2 P e C l 3 and F e + + + , one f u r t h e r set of experiments was performed. In the experiments set f o r t h i n t a b l e s I , I I and I I I , r a d i o a c t i v e graphite i r o n (59)111 c h l o r i d e was brought Into contact w i t h i n a c t i v e f e r r i c i r o n In s o l u t i o n and a f t e r s u i t a b l e periods of time the components were separated and t h e i r s p e c i f i c a c t i v i t y a s c e r t a i n e d . In no instance was any appreciable a c t i v i t y detected i n the i n a c t i v e i r o n c o n t a i n i n g species nor was there any t r a n s f e r of a c t i v i t y from the s o l i d compound. In the above experiments however, no account was taken of the p o s s i b i l i t y of p r e f e r e n t i a l adsorption of the mi g r a t i n g a c t i v e ions by the a c t i v e compound. I f exchanging a c t i v e i o n s , trapped i n the complex by c a p i l l a r y condensat-i o n were not removed by washing the compound before counting, there would of course be no decrease i n the s p e c i f i c a c t i v i t y of the s o l i d . Likewise no increase i n the a c t i v i t y of the f e r r i c i o n s o l u t i o n would be observed. Therefore t e s t s f o r exchange between r a d i o a c t i v e i r o n (59)111 c h l o r i d e and i n -a c t i v e graphite f e r r i c c h l o r i d e were c a r r i e d out. The r e s u l t s of these experiments are shown i n t a b l e IV. * r/c r/c i/c i/c i/c No. PeClo PH C]_2PeGl3 ( o r i g i n a l ) ( a f t e r 4 h r s . ) ( a f t e r 9 6 h r s . ) ( a f t e r 4 m o . ) (~calc.) 5 mis. (j»"s) (M,) . , 1 . 0 . 4 1 . 0 0 . 1 5 0 582*6 5 8 4 * 6 580 t 6 570±6 4 6 8 2 . 0.25 1 . 0 0 . 1 5 0 5 8 0 * 5 581*5 5 8 4 * 5 5 7 8 ± 5 417 3 . 0 . 1 0 1 . 0 0.150 5 8 1 * 6 580±6 587±6 585*6 2 9 2 4» 0.08 1 . 0 0.150 5 8 4 * 6 575*6 5 7 8 * 6 570*6 262 5 . 0.06 1 . 0 0 . 1 5 0 582*6 580±6 582*6 5 7 6 ± 6 2 2 0 6 . 0 . 0 4 1 . 0 0 . 1 5 0 580±6 572*5 572*5 569*6 170 7 . 0 . 0 1 1 . 0 0.150 579*6 575*6 579*6 5 7 4 * 6 54 8 . 0 . 0 0 5 1 . 0 0.150 580*6 581*6 580*6 5 7 8 ± 6 28 Tests f o r Exchange Between Fe +* +and C^gFeCl^ i n Aqueous Aci d S o l u t i o n at 20 ±2°C. TABLE IV. ( 5 5 ) As before, no raeasureable exchange was obser-ved even a f t e r long periods of time. The samples of i n a c t -i v e graphite f e r r i c c h l o r i d e were e a s i l y washed c l e a r of any a c t i v i t y f o l l o w i n g s e p a r a t i o n from the r a d i o a c t i v e f e r r i c c h l o r i d e s o l u t i o n . ( 5 6 ) 3» The S z i l a r d Chalmers Reaction w i t h Graphite F e r r i c  C h l o r i d e . I t has been shown that there i s no measurable exchange between C 1 2 F 9 Cli^ and f e r r i c i o n i n surrounding s o l u t i o n . I t i s therefore l i k e l y that any i r o n a c t i v i t y separated by a Szilard-Chalmers process on the compound could be obtained i n high s p e c i f i c a c t i v i t y . I t i s of i n t -e r e s t to note the success of a Szilard-Chalmers s e p a r a t i o n , w i t h regard t o the s t r u c t u r e of t h i s compound. Pure C 1 2 P e C l 3 w a s i r r a d i a t e d i n the neutron f l u x of the Canadian N.R.C. p i l e at Chalk R i v e r , Ontario. . The a c t i v i t y of the compound was measured, the separated p o r t i o n of a c t i v i t y removed i n hot h y d r o c h l o r i c a c i d s o l u t i o n , and the y i e l d c a l c u l a t e d . 1 . 5 0 g m s . of C^gFe. C l ^ powder ( c o n t a i n i n g 0.27gm. Fe) were a c t i v a t e d b y 4 8 hours i r r a d i a t i o n at about 3 » 9 12 2 59 10 n./cm. /sec. A mixture of the 47 day Fe and 4 year 55 Fe was obtained, but as w e l l , the r e a c t i o n s occuring on neutron i r r a d i a t i o n of c h l o r i n e r e s u l t e d i n the production 3 5 of f o u r other a c t i v e i s o t o p e s . C l undergoes an (n,)r) 6 36 r e a c t i o n to produce the 10 year C l e m i t t i n g 0 . 6 6 Mev. 35 beta's, an (n,p) r e a c t i o n producing 87 day S which emits 0.17 Mev. beta's, and a l s o an (n,«t) r e a c t i o n to 1 4 . 7 32 37 day P e m i t t i n g 1.7 Mev. beta's. C l a l s o undergoes an 3 8 (n,fc) r e a c t i o n to C l which has a 37 minute h a l f l i f e and decays w i t h the emission of 1.1, 218 and 5 . 0 Mev. beta's and 1 . 6 5 and 2 . 1 5 Mev. gamma's. ( 5 7 ) 3 ^  Because of i t s long half l i f e , the C l r a d i o a c t i v i t y developed is. n e g l i g i b l e when r e l a t i v e l y short i r r a d i a t i o n periods are used. Short-lived C l decays i n a day or so. 32 Contamination by P i s of the order of 1% of the t o t a l S , i n terms of beta p a r t i c l e s emitted (35)• The i n i t i a l a c t i v i t y of the i r r a d i a t e d compound was estimated at 0.008 mc. Fe^/gra», 0.027 mc. F e ^ V s m » a n d 1*27 mc. S ^ V s m « The separated portion of the a c t i v i t y from the Szilard-Chalmers reaction would then be expected to be composed mainly of these three radioactive isotopes. The i r o n a c t i v i t i e s c a n be separated by a very e f f i c i e n t and simple extraction procedure based on the solvent extraction of f e r r i c chloride (34) • In the experiments reported here, d i e t h y l ether was used as solvent. The p a r t i t i o n c o e f f i c i e n t for extraction of f e r r i c chloride by d i e t h y l ether from aqueous 6N hydr-c h l o r i c acid i s about 100. 0.100 gm. of p i l e i r r a d i a t e d graphite f e r r i c chloride was refluxed with constant b o i l i n g (~6>N) hydrochloric acid, f o r s i x hours and the aqueous solution separated by c e n t r i f -uging« A few drops of 30$ hydrogen peroxide were added, to ensure that a l l the i r o n was present as f e r r i c ion. The colorless aqueous solution was shaken with successive 1/4 volumes of d i e t h y l ether saturated with hydrochloric acid, and f i n a l l y washed with an equall volume of solvent. The ether extracts were then c o l l e c t i v e l y stripped with d i s t i l l e d water. The r a d i o a c t i v i t y of the resultant solutions was mea-sured and the components d i f f e r e n t i a t e d by adsorption meas-(58) urements. . Figures VIII, IX and X give the absorption curves for the aqueous,residue solution, the stripped ether, and the water extract r e s p e c t i v e l y . It i s evident that no detect-able amount of radioactive iron remains'in the aqueous 35 residue, nor can the a c t i v i t y of S be detected i n the 32 . extract. Apparently the P i s present in a form which has a small but measurable p a r t i t i o n c o e f f i c i e n t , as It i s ext-ractable to some extent. The small amount of S^ -5 taken up by the solvent (possibly as sulfur chloride) i s not re-extr-acted 'by d i s t i l l l e d water. The chemical form of the i n the aqueous residue Is presumably as F e 2 (S-^o^) ^ . In a s i m i l l a r type of solvent extraction of ir r a d i a t e d FeCl^, 35 M.B. Wil« (35) has reported 99.5$ of the o r i g i n a l S and 32 83$ of the P to be contained i n the aqueous residue s o l -59 ution which i s free of detectable Fe • The stripped ether . 3 5 portion contained only the remaining 0.5% of the S and 32 59 0.01% of the P • A l l the o r i g i n a l Fe and the small amount of remaining P^ 2 was found i n the water extract which was • 35 free of detectable S a c t i v i t y . The t o t a l a c t i v i t y of the hydrochloric acid solution, which had been refluxed for six hours with the 0.100 gm. sample of i r r a d i a t e d ^2.2^e^3' w a s m e a s i ; i r e c * a * 0.682$ of the a c t i v i t y of the untreated compound. No detectable act-i v i t y was obtained i n a further treatment with hot HCl soluti o n . 35 The very low energy of the beta p a r t i c l e s from S 1000 Figure IX - A c t i v i t y i n stripped ether. 10 ooo too zoo 500 400 soo A B S O R B E R T H I C K N E S S M ^ / ^ (59) poses special problems as regards t h e i r quantitative detect-ion. The low energy of the radiation makes the self-weaken-ing e f f e c t a major one. Experiments have shown that samples of thickness 3 mgm./cm2 w i l l be weakened to the extent of 1+0% of the t o t a l a c t i v i t y ( 3 5 ) . In a c t i v i t y measurements of the various solutions involved, the s e l f absorption corrections were obtained i n the same manner as already discussed under exchange experiments. The graphite f e r r i c chloride samples compared were always le s s than 0.2 mgm./cm2. The a c t i v i t y of the i r r a d i a t e d Ci2FeCl3 was compared with the a c t i v i t y of hydrochloric acid solution containing the separated isotopes, by absorption measurements on the two. Both samples measured were l e s s than 0.2 mgm./cm2. Figure XI gives the absorption curves comparing I - i r r a d -iated C-L2F e Cl3 snd II - evaporated acid extract s o l u t i o n . It w i l l be seen that the two absorption curves are ident-i c a l . ^ From t h i s i t seems apparent that the component radio-active isotopes exist i n the a c t i v i t y separated by the S z i l a r d Chalmers reaction i n the same r a t i o as they are present i n the i r r a d i a t e d compound, A t h i r d series of absorption measurments on the i r r a d i a t e d compound from which the separated portion of a c t i v i t y had been removed gave a curve i d e n t i c a l with the other two. It i s apparent that some of the r e c o i l i n g atoms of both chlorine and iron have achieved bond rupture and ejection from the C ^ F e C ^ l a t t i c e . However, the very low f r a c t i o n of the t o t a l a c t i v i t y separated i n hot hydrochloric A B S O R B & K T H I C K N E S S M&n./cwz flL (60) acid, indicates that almost a l l the neutron capturing atoms are s t i l l f i r m l y bound i n the graphite compound. A series of experiments, designed to see whether or not changing the method of recovering the separated a c t i v i t y would r e s u l t i n a larger Szilard-Chalmers y i e l d , was carried out. Samples of the i r r a d i a t e d compound that were refluxed with HCl f o r four, eight and sixteen hours, relinquished no more of t h e i r t o t a l a c t i v i t y than did a sample which was washed for 15 minutes i n a solu t i o n of 4N HCl at 80 C. The valuetffor the percentage of t o t a l a c t i v i t y separated, which were obtained from these experiments, were a l l within the range 0.681 to 0.688$, It was noted, however, that i f the i r r a d -i ated graphite f e r r i c chloride was subjected to a f i n e g r i n -ding previous to the acid-leach treatment, the f r a c t i o n of a c t i v i t y separated increased s l i g h t l y . The percentage y i e l d was raised, from about 0 . 6 8 4 $ to an average value of 0 . 7 2 7 $ by grinding the samples by hand i n a mortar or between .ground-glass plates. The use of a fine aluminum oxide grinding powder raised t h i s value to 0 . 7 3 7 $ . The r e s u l t s from these above experiments are l i s t e d i n Table V. In a l l cases the aqueous acid solutions containing the separated a c t i v i t y , were colorless and gave no test for Pe+4'*' with N H 4 C N S . It is therefore apparent that l i t t l e or no r a d i a t i o n decomposition takes place as a r e s u l t of the i r r a d i a t i o n of the compound. It i s unfortunate that the small amount of i r o n separated i s not detectable by other than radiochemical methods, as f a c i l i t i e s for handling the large (61) TABLE V. The E f f e c t of Different Extraction Methods on  the jo S z i l a r d Chalmers Separation. Exp. Conditions for acid I n i t i a l Separated $ S z i l a r d No. extraction of separ- a c t i v i t y a c t i v i t y i n Chalmers able a c t i v i t y from of C i 2 P e 0 l 3 HCl soln. separat-Cl2 F QCl3» (c.p.m.) (c.p.m.) ion. 5 3 1. 0.0827gm. i n 4-N HCl 58.11*10 39.6*10 0.681 at 80°C. S t i r r e d i n blender for 15 min. 2. 0.200gm. in~6NHCl at 134.32*I0 5 92.4*103 0.688 100*C. Refluxed f o r 4 hours. 5 ? • 3. 0.200gm. i n ~ 6 N HCl 120x1.0) 82*KT 0.683 at 100 °C. Refluxed for 8 hours. 5 3 4'i. 0.1523gm. i n ^ 6N HCl 87.06*10 59.4-10 0.682: at 1100° C. Refluxed f o r 16 hours. 5 3 5» 0.1094gm. i n ~6N HCl 38.56*10^ 28.08*10 0.728 at 25°C. Ground i n a mortar(approx. 1 hr.) 5 3 6. 0i0482gm. i n ~ 6 N H C l 11.99*10^ 8.7>KT 0.7255 at 100 C. Sample ground between two ground-glass plates before acid extraction. 5 3 7. 0.0530gm. i n -6N HCl 13.187-10 9.72A10 0.737 at 100"C. Sample ground as 1A #6, hut with fine AI2O3 grinding powder. (62) , a c t i v i t i e s that would be necessary to make a quantitative determination of the s p e c i f i c a c t i v i t i e s of iron and s u l f u r , were not a v a i l a b l e . (63) DISCUSSION of RESULTS. It has been shown that the Szilard-Chalmers separ-ation on C-^FeCl^ i s successful to less than 1$. As has been already suggested, t h i s r e s u l t does not appear too surprising when one considers the structure of the compound under discus-sion. Above and below the planar f e r r i c ion network there i s a p a r a l l e l triangular net plane of chloride ions, and above and below these planes of chloride ions there i s a t i g h t l y bonded hexagonal plane of carbon atoms. This arrangement repeated through the l a t t i c e , serves to produce formidable s t e r i c hindrance effects f o r an atom of either iron or chlor-ine which might leave the l a t t i c e . This has been shown i n tes-ts for exchange between the Fe + + + ions bound i n the. graphite l a t t i c e and Fe"^ ions i n surrounding s o l u t i o n . No measurable exchange was observed even after a four month period. The retention by the C-^FeC^, of over 99$ of the a c t i v i t y developed by neutron i r r a d i a t i o n may possibly also be explained by the pronounced s t e r i c e f f e c t . Recombination of "hot" fragments (in a so-called reaction cage) would appear to be quite probable i n a s o l i d material of t h i s type. The separation of a c t i v i t y which does r e s u l t from the S z i l a r d -Chalmers reaction (approx. 0.68$),. cannot be explained on the basis of impurities i n the C 12 P e C l l3 ,> J t h a 3 heen shown that the p u r i f i e d compound contained less than 4 u.gm. Fe/gm. as free f e r r i c c hloride. This amount could only account for 0.3$ of the f r a c t i o n of a c t i v i t y that was separated, emen i f i t were a l l extractable following i r r a d i a t i o n . ( 6 4 ) One theory which might explain the small separation of a c t i v i t y , i s that i t i s only the edge atoms i n this "stacked layers" structure which are able to break away following neutron capture* This idea finds some support i n the experiments.which showed.that grinding the powdered compound to a f i n e r size inc-reased to some; extent the f r a c t i o n of a c t i v i t y separated. Thus the percentage y i e l d was raised from 0 . 6 8 4 to 0 . 7 3 7 $ presumably due to the breaking of the c r y s t a l s and exposure of new edges from which i r r a d i a t i o n freed active atoms were removed by the hydrochloric acid solution* It would be of interest to subject samples of G^^sO-j of d i f f e r e n t p a r t i c l e size to neutron i r r a d i a t i o n and compare the Szilard-Chalmers y i e l d . Also there i s the p o s s i b i l i t y that the i r r a d i a t i o n of the compound for a shorter period or i n a lower neutron f l u x might lead to a more successful separation o f . a c t i v i t y . Radiation experiments have shown that the estab-lishment of a successful Szilard-Chalmers enrichment reaction, i n experiments of low f l u x or short bombardment, does not. ensure i t s success when longer or more intense bombardments are employed. It may be that i n the case of C^PsCl^* some of the i n i t i a l l y separated a c t i v i t y i s lost by a r a d i a t i o n -induced back reaction. This s i t u a t i o n does not seem too l i k e l y , since the separable form represents a less complicated breakdown product of the s t a r t i n g material. The S z i l a r d -Chalmers enrichment experiments with antimony pentafluoride (R.R. Williams ( 3 6 ) ) however, show thi s loss of a c t i v i t y :: (65) without apparent decomposition. The a c t i v i t y i n the separ-able form dropped from 60 to 5% of the t o t a l a f t e r s e v e r a l hours bdmbardment at constant p i l e power. Nevertheless, . i t seems apparent that even under more s u i t a b l e c o n d i t i o n s , a compound of the graphite f e r r i c c h l o r i d e type could not be expected to give any appreciable s e p a r a t i o n of a c t i v i t y by a S z i l a r d -Chalmers r e a c t i o n . (66) SUGGESTIONS for FURTHER RESEARCH. 1. It i s probable that graphite f e r r i c . c h l o r i d e and the other graphite compounds described represent only a few of the"stacked layer" structures which are capable of existance. Rttdorff has attempted the preparation of graphite compounds of A s l j , SDI3, BH3, AsCl^, SbCl^, BiCl-j, AICI3, C0CI3 and CrCl^, without r e s u l t . (22) There i s l i t t l e doubt however, that further research w i l l bring to l i g h t a dditional examples of molecules penetrating between the layer planes of the graphite l a t t i c e ' t o form more or less stable structures. 2. The idea that i t i s only a few exposed edge atoms that are able to achieve bond rupture and subsequent e j e c t i o n from the c r y s t a l l a t t i c e of C-^FeCl^* could be investigated. The neutron i r r a d i a t i o n of samples of d i f f e r e n t p a r t i c l e size (different r a t i o of edge atoms to t o t a l atoms) might possibly lead to greater percentage yie l d s of separated a c t i v i t y . 3>» A further research concerning a study of r a d i a t i o n effects on graphite f e r r i c chloride would be of i n t e r e s t . The c o r r e l -ation between f l u x i n t e n s i t y and period of bombardment and f r a c t i o n of a c t i v i t y separable, could be ascertained. (67) BIBLIOGRAPHY. (1) Bernal, J*D.» Proc. Roy. Soc. Acvi 749 (1924) (2) Bragg, W.L., Proc. Roy. Soc. A89 277 (1913) (3) Flagg, J.P., J . Aim-Chem. Soc. 63 577 ( 1 9 4 D (4) Fredenhagen, Cadenbach, and Suck, Z. anorg. a l l g . Chem. • 158 178" 249 353 (1926) (1929) (5)' Heyesy, G.> and Paneth, P., Manual of Radioactivity, Oxford University Press, (1938) Hahn, 0., Applied Radiocbemistry, Cornell University Press, (1936) (6) Hofman, U., Naturwissenshaften, 32 260 (1944) (7) Hofman, U«, and prenzel, A., Z. Elektrochem. 4^0 511 '(1934) (8) H u l l , A.W.,- Phy. Rev., X 661 (1917) (9) Juliusberger, P., Topley, B., and Weiss J . Chem. S o c , p.1295 , J • > (1935) (10) L a i d l e r , D.S., and Taylor, A., Nature, 146 130 (1940) (11) Long, P.Ao, J.Am. Chem. S o c , 63 1353 (194D (12) Manguin, C.» B u l l . S o c Franc. Min., XLIX 32 (1926) (13) Medlin, W.V., J . Am. Chem. S o c , 57 1026 (1935) (14) .Melander, L., Acta Chem. Scand., 1 169 (1947) (15) McKay, H.A.C., Nature, 142 997 (1938) (16) Ott. H.. Ann. Physik., LXXXV(IV) 81 (1928) (17) Pauling, L., The Nature of the Chemical Bond Cornell University Press,-2nd E d i t i o n , . (1948) (18) Riley, H.L., Journ. Inst. Fuel, X 1 4 9 (1937) (68) (19) R i l e y , H.L., Fuel 8 (1945) (20) Ruben, S ., Kamen, M.D., Al l e n , M.B., J . Am. Chem. S o c , and Nahinsky, P. 6£ 2297 (1942) (21) RUdorff, W., Z. anorg. Chem. Ml 383 (1941) (22) RUdorff, W., F i e l d Information Agency, Technical(F.I.A. Review of German Science 1939-4 6 Inorganic chemistry, Part I 239 (1948) (23) RUdorff, W. and Eofmann, H., Z. anorg. Chem., 238 1 (1938) (24) RUdorff, W., and RUdorff, G., Chem. Ber., 80 413 (1947) (25) RUdorff, W., and Schulz, H., Z. anorg. Chem., 245 121 (1940) (26) Ruess, G •L»> Monatsheften, 76 381 (1947) (27) Ruff, 0. * and Brettschneider, 0., Z. anorg. Chem., 217 1 (1934) (28) Schleede and Wellman, Z. Physik, Chem. XVIII 1 (1932) (29) Seaborg, G.T., Chem. Rev., 27 199 (1940) (30) Stelgraan , J . , Phy. Rev., 59 498 (194D (3D Stte, P., and Kayas, G., Jour. Chem. Phys., 188 (1948) (32) S z l l a r d , L., and Chalmers, T.A., Nature, m 462 (1934 (33) Thie l e , H., Kolloid-Z., 56 129 (1931) (34) Welcher, F.S., Organic A n a l y t i c a l Reagents,Vol.1 (1947) (35) Wilk, M.B., Canadian Journal of Research, 27 475 (1949) (36) Williams , R.R., J . Phy. C o l l . Chem. » 12 603 (1948) 

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