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Studies of difluorophosphoric acid and its alkali metal salts Reed, William 1965

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STUDIES OF DIFLUOROPHOSPHORIC ACID AND ITS ALKALI METAL SALTS  by  WILLIAM REED B.Sc, University of Durham, 1963  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  in the Department of Chemistry  We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA September, 1965  In p r e s e n t i n g  fulfilment  of  the requirements f o r an advanced degree at the U n i v e r s i t y  of  British  Columbia,  available  for  this  thesis  in p a r t i a l  I agree that the L i b r a r y s h a l l  r e f e r e n c e and study,  make i t  I f u r t h e r agree that  m i s s i o n f o r e x t e n s i v e copying o f t h i s  thesis  for  freely per-  scholarly  purposes may be granted by the Head o f my Department o r by his  representatives..  cation of t h i s  thesis  w i t h o u t my w r i t t e n  Department of  It  for financial  q<?  gain shall  permission.  C \ i £ M i S>i jg>/  The U n i v e r s i t y o f B r i t i s h Vancouver 8, Canada Date  i s understood that copying o r p u b l i -  -Kv  Columbia  /Uf  not be allowed  i  ABSTRACT  Difluorophosphoric acid was purified by a double d i s t i l l a t i o n technique.  Alkali metal difluorophosphates were prepared by reaction  of the metal chlorides with purified difluorophosphoric acid; x-ray powder photographs and infra-red spectra of the salts were obtained. Electrical conductivity measurements were made on solutions of the alkali metal difluorophosphates i n difluorophosphoric acid as solvent. The order of mobility of the alkali metal cations in this solvent was found to be Li>Na>K>Rb>Cs. Results indicate that the difluorophosphate ion does not conduct by a proton transfer process i n this solvent. Conductimetric studies on solutions of a number of other compounds in difluorophosphoric acid are also reported.  ii  TABLE OF CONTENTS  INTRODUCTION (a) Historical (b) Purpose of this work EXPERIMENTAL I.  PREPARATION AND PURIFICATION OF MATERIALS (a) Difluorophosphoric acid (b) Difluorophosphates (c) Other materials  II. PHYSICAL METHODS (a) Electrical conductivity (b) Infra-red spectra (c) X-ray powder photography RESULTS AND DISCUSSION (a) Electrical conductivity (b) Infra-red spectra (c) X-ray powder photography  iii  LIST OF FIGURES  FIGURE  PAGE  1  Difluorophosphoric  acid d i s t i l l a t i o n apparatus  10  2.  Fraction collector used on d i s t i l l a t i o n apparatus  11  3. (a)  Dropping funnel used for acid addition for d i s t i l l a t i o n  12  (b) Weight dropper used fox addition of fluorosulphuric acid to the c e l l 4.  Apparatus used for the preparation of alkali metal difluorophosphates  13  5.  Drying-train tester  16  6.  Difluorophosphoric  18  7.  Injector used for solute additions to conductivity c e l l  21  8.  Specific conductances of some difluorophosphates  25  9.  Specific conductances at low concentrations at 25°  28  10.  Specific conductances of some electrolytes at 25°  31  11.  Conductiraetric titration of HSOjF against  12.  Infra-red spectra of CsPo F and NaP0 F  13.  X-ray powder films of some difluorophosphates  acid conductivity c e l l  2  2  2  N H  4  2  P 0  F 2  at 25°  2  d t  2 S  *  3  2  38 40  iv  LIST OP TABLES  TABLE  PAGE  I  Analysis results for P and F i n the difluorophosphates  14  IX  Specific conductances of some difluorophosphates at 25°  24  III  Specific conductances of some electrolytes at 25*  30  IV  Infra-red data of the difluorophosphates  37  V  ACKNOWLEDGEMENTS  The author wishes to express gratitude to Dr. R.C. Thompson who f i r s t suggested the problem, and under whose supervision this work was done. Thanks are due to Messrs. R. Bellamy and S. Rak who constructed the glass apparatus, to Messrs. R. Green and A. Hardin for assistance with the operation of the Perkin-Elmer 421 spectrometer, and to Mr. R. Rao who assisted i n the taking of the x-ray powder photographs. The generous gift of difluorophosphoric acid by Ozark-Mahoning Chemical Company i s also gratefully acknowledged. Finally the author would like to thank Dr. H. Daggett of the Chemistry Department of the University of British Columbia for the use of the conductivity apparatus.  INTRODUCTION  As no previous survey on difluorophosphoric acid and i t s salts has been made i t was f e l t that a review of the literature would prove valuable, (a)  Historical Difluorophosphoric acid was f i r s t prepared by Lange i n 1927 1  phosphoryl trifluoride was hydrolysed in cold water to give difluoro2 3 phosphoric acid, the hydrolysis taking place in three stages: '  F 0»P—F  OH  OH  ^ 0«P—F  OH  * 0=P —OH  ^ F  »r 0=P— OH  ^ F  OH  If the hydrolysis of POFj i s carried out in a weakly basic solution i t i s possible to isolate the compounds HP0 F and O j F in the form of their 4 salts. The fluorine substitution products of meta- and orthophosphoric H  2  2  p  2  c  acids are stable to water in much the same way as fluorosulphuric acid . However, even i n neutral aqueous solution hydrolysis slowly occurs, the rate being much more rapid in the presence of acid or a l k a l i . 6  No further studies were reported on this acid for almost fifteen years. 7 In 1941 Tarbutton, Egan and Frary examined the reaction, CaF • P 0 * POFj • Ca(P0 ) 2  2  5  3  2  and found that when they varied the ratio of the reactants varying amounts of difluorophosphoric acid as a volatile by-product were obtained.  The  presence of the acid was explained by the partial hydrolysis of POF^ by small amounts of water present.  2  The volatile by-products were separated and the boiling point of the acid was given as 108-111°.  This appears to have been the f i r s t isolation  of the acid contrary to the view of Sidgewick that the free acid does not exist . 3  8 Lange  investigated the reaction between phosphoric oxide and hydrogen  fluoride to yield monofluorophosphoric acid. P 0 2  5  •  2HF  •  H0  »  2  2H POjF 2  Difluorophosphoric acid or a mixture of H^PO^F and HPO^were produced depending on the HF concentration. The same products were obtained when 100% orthophosphoric acid was allowed to react with aqueous hydrofluoric acid containing 41% HF.** When liquid, anhydrous hydrogen fluoride was o 1  used  in a 1:1 molar ratio with HjPO^, 67% of the HjP0 was transferred 4  into the fluorinated derivative.  However, the monofluorophosphoric acid,  formed i n the reaction, entered into a new equilibrium reaction with unreacted HF, forming difluorophosphoric acid. H P0 + HF 3 = * H P0 F • H 0 H P0jF • HF 7 * HP0 F + H 0 3  2  4  2  3  2  2  2  2  Analysis of the equilibrium product showed that 33% H^PO^  was unchanged,  60% was transferred to the monofluorophosphoric acid but only 7% to the difluorophosphoric acid. Variations on previous methods were attempted i n order to synthesize the acid.  Anhydrous monofluorophosphoric and difluorophosphoric acids were  prepared** by allowing orthophosphoric acid and phosphoryl trifluoride to react at 70°-80*. obtained  12  However, the usual problem of a mixture of products was  . Further work by Lange and Livingston  13  consisted of a study  3  of the fluorophosphoric phosphoric acid.  acids and a preparation of anhydrous difluoro-  The hydrolysis of phosphoryl trifluoride was investigated  and i t was found that pure acid could be obtained by the reaction: POFj • POP(OH)£  * 2POF (OH) 2  Experimental details are given in the article together with various 25° properties of the acid (d » 1.583, m.p. «* -96.5° * 1* and b.p. • 115.9°). 14 The system H^O-HF-P^O^ was further examined by Shaposhnikova ; difluorophosphoric acid was prepared by adding P 0 to an ice eold solution 2  of HF in fluorosulphuric acid.  5  The product was d i s t i l l e d in vacuo and  fractionated; difluorophosphoric acid was found to decompose at i t s boiling point of 108°, i t s molecular heat of evaporation is given as 9125-360 cals. and Trouton's constant as 23.7-24.6. The acid was found to attack silicates at room temperature, the rate increasing with rise of temperature. Nuclear magnetic resonance studies of solutions in the homogeneous region of the system H 0«HF-P 0 confirm the presence of a mixture of acids. 2  2  5  Ames, Ohashi, Callis and Van Wazer HjPO^ and  HJPOJF  15  detected the presence of HPO^F*,,  H P P  g»  in the system and estimated their relative concentrations 19 31  by examining the F  and P  nuclear magnetic resonance spectra.  An actual  total of nine structural entities were found; in addition to the above acids, free water, end- and middle-phosphate structure units and a new structure unit containing one fluorine per phosphorus atom which i s believed to be a monophosphate end group, were also found. Various other workers have investigated the fluorophosphoric  acids by  nuclear magnetic resonance; multiple magnetic resonance lines have been observed for F  1 9  and P  3 1  nuclei in  HPOJPJ,  PFj, PH , H J P O J F and HPF in the 3  fi  4  liquid state. At a field strength of 6385 gauss a splitting of 0.244 gauss was obtained by Gutowsky, McCall and S l i c h t e r  16  for the P  19  resonance  17 in HPOjFj*  Quinn and Brown  measured the nuclear magnetic resonance  19 splittings for F in HP0 F in weak fields. At a magnetic f i e l d strength of approximately 550 gauss they obtained a splitting of 0.240 gauss, but 19 the doublet structure obseived for the F resonance in HP0 F coalesced 2  2  2  2  as the field was decreased until a single broad resonance was obtained at 18 approximately 200 gauss.  However, Roux and Bene  obtained spectra at  35-15 gauss and found a doublet; the results were in general agreement with experiments at higher fields (at a f i e l d strength of 37 gauss a splitting of 0.245 gauss was observed), but as no singlet was observed i t was concluded that the singlet observed by Quinn and Brown in the field of 200 gauss cannot be explained in terras of the size of the main field only. Further information regarding indirect coupling of nuclear spins in 19 molecules containing P-F and P-H bands has been reported by Frank relative magnitude of J _ p  p  and J  H  p  ; the  are given for HP0 F and other phosphorus 2  2  compounds. For the series (i) FjPO, ( i i ) F P0(0H), ( i i i ) FP0(0H) and (iv) 20 2  PO(OH) Gutowsky and McCall 3  2  found phosphorus resonance in ( i i ) and ( i v ) ,  with phosphorus shielding greater in ( i i ) ; the fluorine shielding increases with F-substitution. 21 Several salts of difluorophosphoric acid are known; the ammonium salt can be prepared by the fusion of phosphoric oxide with ammonium fluoride, the product being extracted with dry alcohol to remove the ammonium difluorophosphate which i s then recrystallised from hot water. Dilute solutions of precipitate nitron-difluorophosphate. a number of salts the ammoniumof salt treated with a soluable Lange salt ofprepared nitron give a crystalline 2  5  by treating hot aqueous solutions of nitron difluorophosphate  with  metallic nitrates, nitron nitrate was filtered off and the solution evaporated to dryness to recover the difluorophosphate.  Most of the salts  were found to be very soluble in water but the potassium and the cesium salts only moderately so.  The difluorophosphates  are stable in neutral  solution, but less so i n alkaline or acid solutions.  Their general behaviour  is similar to that of the perchlorates (and also the fluoroborates and fluorosulphates), the less soluble salts of both acids being the potassium and cesium salts.  The alkali salts are isomorphous with the perchlorates,  fluorosulphates and fluoroborates; this resemblance i s due to these a l l 3  being strong monobasic acids with tetrahedral anions of nearly the same size . 22 Jonas was able to prepare difluorophosphates by allowing salts of hexafluorophosphoric acid to react with oxides such as: S i 0 , B 0j, WO^; CaO 2  2  or sodium metaphosphate. Quantitative yields were obtained.  Ryss and  23 Tul'chinskii  investigated a new method to prepare the sodium salt free  from other fluorophosphates.  NaHF arid P 0g were allowed to react in a 2  2  platinum crucible, the product being extracted with absolute methanol. Rates of hydrolysis of the sodium salt were investigated and i t was found that in neutral aqueous solution, hydrolysis was slow even at 100*.  However, on  heating in 0.1 N NaOH solution at a 3:1 ratio of OH* to NaP0 F for 10 2  2  minutes at 70* quantitative hydrolysis occurred by the reaction: P0 F " 2  2  • 20H" —  POjF " 2  • F" • H 0 2  On heating with excess of 0.1 N. NaOH in a stainless steel ampoule for about 2 hours at 160* complete decomposition occurred by the reaction P0 F " 2  2  + 30H"  HPoJ" + 2F~ • HjO  6  Of the salts, only the potassium and the ammonium salt have been 24 investigated spectroscopically; Corbridge and Lowe  examined the infra-red  spectra of ammonium difluorophosphate in the region 5,000 - 6S0 cm." , and 1  an assignment of the frequencies was made. Buhler and Bues  investigated  the vibration spectra of fluorophosphate melts and crystals; the infra-red and Raman spectra of crystalline KP0 F , KPFg and ^POjF are reported and 2  2  assignments made. The force constants and bond orders in these and in related anions are discussed. The only conductivity studies on difluorophosphoric acid have been made by Barr, Gillespie and Robinson  , who measured the electrical  conductivity of solutions of HC10 , HSOjF, 4  acid. and  HPOJFJ,  and HSOjCl i n sulphuric  They found that HSOjF and HSOjCl behaved as acids, whereas HP0 F 2  2  were bases in the H S0 system. Difluorophosphoric acid has found l i t t l e use in inorganic chemistry*,  CHJSOJH  2  4  27 28 however, Stolzer and Simon  *  have used the acid extensively in organic  29 reactions.  Hood  has shown that the treatment of difluorophosphoric acid  with aliphatic alcohols yields alkyl hydrogen phosphorofluoridates; HP0 F 2  2  •  ROH  >  (RO) (OH) P0F  2  30 & \) The fluorophosphoric acids have been used as polymerisation  ' , condensation  and alkylatlon catalysts, and also as anhydrous acids in the non-oxidising refining -Spoils. The salts of difluorophosphoric acid have been used industrially: 32 Na, K, L i , Ba and Pb salts stabilize chloroethylene polymers ; Zn, Co, Pb, 33 Fe and Cd salts are used as catalysts in the preparation of 8-lactones , and, substituted aluminium chlorides, e.g., A1C1 P0 F serve as alkylatlon 2  34 catalysts  .  2  2  7  00  Purpose of the present work The purpose of this work i s to investigate the properties of solutions  in difluorophosphoric acid and i n so doing extend the range of studied protonic solvent systems.  As no complete study of a l l the alkali metal  difluorophosphates has ever been made by a single author these compounds have been investigated further by infra-red and x-ray powder diffraction methods.  EXPERIMENTAL  I.  Preparation and purification of materials (a)  Difluorophosphoric Acid Commercial difluorophosphoric acid, supplied by Ozark-Mahoning  Chemical Company, was purified by five double distillations at 9.0 cms. of 26 mercury and a temperature range of 45*-49*C, (Gillespie 35 44 _46°) i n the apparatus tt  used 15 mms and  shown in Fig. I. The procedure was as follows.  The apparatus was evacuated and flamed out with a hot bunsen flame. Dry air was allowed to enter through tap H until atmospheric pressure was attained.  A dry, dropping funnel (Fig. 3a) was f i l l e d , in a dry box, with  difluorophosphoric acid and was then fitted into the d i s t i l l a t i o n apparatus at L. The acid was allowed to drain into the flask A. The dropping funnel was removed and the thermometer well replaced.  The system, which was  connected to a vacuum pump at N via liquid nitrogen traps and an acetone-dry ice trap, was evacuated to about 9 cms. of mercury.  The acid was then  refluxed for about 30 minutes to remove hydrogen fluoride which was condensed out i n the liquid nitrogen traps.  The acid was d i s t i l l e d into tube C until  a temperature of 45°C. was reached.  At this temperature the d i s t i l l a t e was  directed into flask B by the fraction collector E (E i s shown in more detail in Fig. 2). Tap M was then closed and the vacuum pump was disconnected from N and attached to 0. The second d i s t i l l a t i o n was carried out under the same conditions, but in this case separation was achieved by rotating F within the B.19 ground glass socket at G so that the acid could be directed into either D or J as 8  9  desired.  Flask D containing the required acid was quickly detached and  capped; i t was then transferred to the dry box and the acid poured into the dropping funnel. repeated.  Clean, dry apparatus was set up and the d i s t i l l a t i o n  In the f i r s t two or three double distillations considerable  attack of glass occurred. In an attempt to reduce this, the acid was f i r s t refluxed under vacuum at temperatures slightly above room temperature for 24 hours in a stainless steel condenser and flask; however, on subsequent d i s t i l l a t i o n in the glass apparatus no improvement was observed. Carrying out the same procedure as above but at room temperature had no effect on the result. Periodic checks of acid purity were made by examination of the fluorine nuclear magnetic resonance spectrum  14 '  (b) Difluorophosphates Ammonium, lithium, sodium, potassium, rubidium and cesium difluorophosphates were prepared by the reaction: M Cl  • H P0 F  where M i s the metal cation.  2  2  > M P0 F 2  2  + H Cl  Fig. 4 shows the apparatus used.  The dry  metal chloride was placed in the reaction vessel D which was then attached to the acid d i s t i l l a t i o n apparatus at K in Fig. 1. The apparatus was evacuated and then flamed out in the normal manner. Difluorophosphoric acid was d i s t i l l e d at the usual pressure and temperature onto the chloride which immediately reacted with the acid resulting in the evolution of HCl. When sufficient  acid had been added to dissolve a l l of the solid the reaction  vessel was removed from the d i s t i l l a t i o n apparatus and a ground glass B 19  FIG. I. DIFLUOROPHOSPHORIC ACID DISTILLATION APPARATUS  12  FIG. 3(a) DROPPING FUNNEL  (b) WEIGHT DROPPER  APPARATUS USED FOR THE PREPARATION OF THE ALKALI METAL DIFLUOROPHOSPHATES  300  14  cap was quickly placed on Q.  Tube R was then removed and the reaction  vessel was attached via S to the vacuum pump. The excess difluorophosphoric acid was removed at a pressure of about 0.5 irons, of mercury, with occasional warming of the vessel D with a bunsen flame. When a l l the acid had been removed the product was washed with ether and then recrystallized from dry methanol (except the sodium salt which was found to be exceedingly soluble and was, therefore, recrystallized from dry ethanol). difluorophosphates  Finally the  were washed with ether, dried and stored over phosphoric  oxide in a vacuum desiccator. Aqueous solutions of the salts showed no precipitation on addition of silver nitrate solution (negative test for chloride) and no precipitation 36 on addition of lead and barium nitrate solutions  (negative tests for  monofluorophosphate and fluoride). Fluorine and phosphorus microanalysis were obtained in the A. Bernhardt Microanalytical Laboratories, Germany, and the results are shown in Table I below. calc.  Li  obt.  TABLE I Na calc. obt.  calc.  K  obt.  %P  28.81  28.7  24.99  24.82  22.12  22.1  %F  35.21  35.5  30.65  30.80  27.14  27.29  obt.  calc.  obt.  calc.  calc.  Rb  Cs  NH. 4  obt.  %P  16.61  16.53  13.24  13.14  26.03  25.83  %F  20.38  20.42  16.25  16.42  31.93  32.23  11.76  11.63  %N -  15  (c)  Other Materials sodium monofluorophosphate:  Commercial Na POjF obtained from Alfa 2  Inorganics Inc. was recrystallized once from water. sodium fluoride: Chemically pure NaF was dried i n a drying pistol at 80° and a pressure of 10rams,of mercury for three days. fluorosulphuric acid:  Commercial HS0 F obtained from the Allied 3  Chemical Co. was double-distilled at 164*. potassium fluorosulphate: by Gillespie et a l . dry  air:  KSO^F was prepared by the method used  3 7  The a i r allowed to flow into the vacuum d i s t i l l a t i o n  system was passed f i r s t through calcium chloride, then magnesium perchlorate and finally through a liquid nitrogen trap followed by an acetone/dry ice trap.  As some compounds, as well as the acid, were handled i n the dry  box, compressed a i r was passed f i r s t through three gas wash-bottles containing concentrated sulphuric acid, then through a tube containing calcium chloride and finally through a tube containing magnesium perchlorate. The air, dried i n this way, was always tested for traces of water by means of the drying-train m,  M  tester, shown i n Fig. 5. The a i r was allowed to  enter the tester at A from the outlet of the dry box, then pass over 30% oleum contained i n B and exit via C.  I f the a i r caused no fuming in the  vessel i t was considered to be dry enough for use.  A i r , dried i n this manner  was passed through the dry box for several hours before use.  Several dishes  of phosphoric oxide were placed at various locations in the dry box to ensure as dry an atmosphere as possible.  17  1 1  •  Physical Methods (a) Electrical Conductivity The design of the c e l l used to measure the conductivities of  solutions in difluorophosphoric acid is shown in Fig. 6.  The c e l l could  be attached to the d i s t i l l a t i o n apparatus at K by means of the B.19 ground glass cone L.  The c e l l has three electrodes and was designed so that the  c e l l constant, when using electrodes B and C, was approximately 5 while the constant, when used electrodes B and A was approximately 15.  Thus,  accurate conductivity measurements could be made on weakly conducting solutions using the electrodes A and B, while measurements on more strongly conducting solutions were made using electrodes B and C.  The capacity of  the cell was about 400 mis. The cell was cleaned with aqua-regia and the electrodes were plated with platinum black by electrolyzing a chloroplatinic acid solution prepared 38 according to Jones and Bollinger  . The solution consisted of a 0.3%  solution of chloroplatinic acid in 0.025N. hydrochloric acid with 0.02%  lead  acetate added, A current of 10 milliaraps. was passed for 15 mins. with a reversal of current every 10 seconds.  The c e l l was steamed out, dried and  then calibrated using aqueous potassium chloride solution according to the 39 method of Lind, Zwolenik and Fuoss  . The cell was replated and recalibrated  after every four or five experiments. A l l measurements were made with the cell immersed in an o i l bath regulated by means of a mercury-thallium  regulator at 25i 0.002°.  The  temperature of the thermostat was measured by Beckmann thermometers which had been calibrated against a platinum resistance thermometer.  FIG. 6.  DIFLUOROPHOSPHORIC ACID CONDUCTIVITY CELL  19  The apparatus used to make s o l i d additions to the c e l l i s shown i n F i g . 7.  It consisted o f a "T"-shaped glass tube with B.19 ground glass  sockets at the ends 0 and P, and a B.24 ground glass cone with an extension at M.  The corks at 0 and P were made o f t e f l o n and they were t i g h t l y f i t t e d  with s t a i n l e s s s t e e l pistons A and B respectively.  The f l a t "runners"  which were also made from t e f l o n interlocked at Q and lay on the bottom o f the tubes.  The compound to be added to the c e l l was weighed into small,  preweighed, dry, glass boats which were inserted through 0 o f the sidearm. Approximately eight boats could be accommodated i n the sidearm.  The loaded  i n j e c t o r was then connected at M t r a rotary vacuum pump v i a l i q u i d nitrogen traps, warmed and evacuated.  This was done to remove any water  absorbed during the weighing-out process.  A f t e r several hours the i n j e c t o r  was detached from the pump and stored u n t i l use i n the dry box. A conductivity run was c a r r i e d out i n the following manner; the conductivity c e l l was attached by means of a B.19 inner ground glass j o i n t d i r e c t l y to the d i s t i l l a t i o n apparatus at K and flushed out with dry a i r . Difluorophosphoric acid was d i s t i l l e d d i r e c t l y into the c e l l ; acid obtained -4 i n t h i s way usually had a conductivity between 2.41 x 10 ohm.  cm. ~ * .  exclude water.  -4 and 2.51 x 10  At a l l times i n handling the acid great care was taken to It was found that there was a gradual increase i n the s p e c i f i c  conductivity with time; over a period of 4-5 hours a 1% increase i n K was observed.  Solutions f o r conductivity measurements were prepared as follows:  difluorophosphoric acid was d i s t i l l e d d i r e c t l y into the c e l l which was weighed before and a f t e r addition o f the a c i d .  The B.24 stopper was removed  and the i n j e c t o r was quickly inserted into the c e l l at F.  Mercury was  poured i n t o the glass tubes holding the platinum electrodes, care being taken  20  to remove a l l the a i r bubbles. The c e l l and the injector were then placed on a support i n the o i l bath.  To make an addition, a glass boat was pushed  by the piston A from the sidearm into the main tube, the boat was then moved by piston B along N and pushed into the acid. After each addition of solute the cell was well shaken to ensure good mixing, and returned to the thermostat. After sufficient time had elapsed to allow for temperature equilibrium (IS to 20 minutes) the resistance measurements were made. The c e l l was then removed from the thermostat, shaken again, and the resistance measurements repeated. In this manner errors due to insufficient mixing were eliminated. Fluorosulphuric acid was added to the c e l l by means of the weight dropper shown i n Fig. 3b.  As both adds hydrolyze i n air, the weighed  sample was added to the c e l l in the dry box.  The cell was returned to the  thermostat and the above procedure for measuring resistance was followed. Resistances of solutions were measured on a precision a-c resistance bridge which has been previously described by Daggett . 40  A 2,000 c/s  oscillator was employed as the source and a telephone headset was used as the null-detector. Throughout this work,specific conductance w i l l be referred to by the symbol K.  (b)  Infra-red spectra A l l spectra were recorded from 4,000 to 250 cm.  on a Perkin-Elraer  421 Double Beam Spectrophotometer under "normal" operating conditions. materials were examined as fine powders spread on cesium iodide plates.  The This  was accomplished by finely grinding the sample and then dissolving i t in dry  21  22  methanol. A thin layer of the resulting solution was obtained on the cesium iodide, which was then placed on a hot plate to drive o f f the methanol.  Specimens were obtained as a thin layer, thicker samples being  used when searching for weak absorption. A decrease in transmittance above about 2,000 cm. "* due to scattered radiation was observed i n many spectra; this was not removed on decreasing the particle size.  By taking spectra  with Nujol mulls in the higher frequency region, sharper absorptions were obtained.  (c)  X-ray powder photographs X-ray powder samples of the difluorophosphates were prepared in 0.3 41  mm. quartz capillaries by the method described by Azaroff and Buerger  .  The x-ray photographs were taken using a General Electric Camera of 14.32 cm. diameter.  This camera employs Straumanis loading. Nickel filtered  (using a 0.089 cm. thick Ni-foil) Cu-Ko radiation (X«- 1.5418 A) was used as the source.  The x-ray tube was operated at 35 kilovolts and 15 milliamps.  The camera employed a s l i t collimator, for which the exposure time required was between 3-6 hours depending on the sample.  RESULTS AND DISCUSSION  (a)  Electrical Conductivity The results of the conductivity measurements on solutions of metal  difluorophosphates in difluorophosphoric acid at 25± 0.002" are given in Table II. As a l l the solutions were made up by weight the concentrations are expressed in molal units (m); due to the lack of accurate density data no attempt was made to express the concentrations in molar units. In each case a plot of ie against molality was made (Fig. 8). with other protonic solvent systems HPO.^  By analogy  would be expected to undergo  autoprotolysis according to the equation: 2HP0 F 2  2  ^=±  H P0 F * 2  2  2  •  P0 F 2  1  2  In this system then, bases may be defined as substances which, when dissolved in difluorophosphoric acid, increase the concentration of the difluorophosphate anion P0 F ~, and acids may be defined as any substance which increases the 2  2  concentration of the difluorophosphoric acidium ion HjPO^P^*/  It i s ,  therefore, expected that the alkali and alkaline earth difluorophosphates w i l l behave as strong bases in this system. M P0 F 2  2  M*  +  P0 F " 2  2  2  In solvents where the mobilities of the autoprotolysis ions are very much greater than the mobilities of other ions (due to a proton transfer mechanism of conduction for the former ions), strong bases exhibit almost identical conductivity curves at low concentrations with small deviations 37 noticeable only in the more concentrated solutions  . As the conductivity  curves for the alkali metal difluorophosphates deviate from each other at even the lowest concentrations measurable, i t must be concluded that P0„F„~ ion does not show abnormal conduction.  24 TABLE II SPECIFIC CONDUCTANCES OF SOME DIFLUOROPHOSPHATES AT 25°C. 9  LiP0 F  10 in  *  o  *  o  4  10\  KPO-F-  2  10 m  10 tc  ohm."* em."" 0.000 0.306 1.020 2.190 4.429 6.773 9.733 13.91 19.69 27.66 37.20 49.84 60.08 70.55  2.482 2.673 3.464 4.528 6.379 8.086 10.15 12.73 16.59 20.46 24.48 29.18 32.21 34.56  NaP0 F 10 m 2  2  0.000 0.222 0.761 2.859 6.008 8.220 11.16 15.13 20.95 25.36 31.37 , 36.88 42.46 47.79  2.503 2.557 2.868 4.500 6.794 8.266 10.12 12.26 15.15 17.96 20.67 22.81 24.75 26.40  RbPO-F, 2  10 m  ohm."* cm."* 0.000 0.1S8 0.566 . 1.336 2.485 4.043 6.054 9.310 13.83 18.53 23.88 29.65 34.37 40.28 48.26  2.410 2.557 2.740 3.153 3.889 4.925 5.939 7.658 10.10 12.79 15.21 17.77 19.82 22.46 24.27  NH P0 F 10"m 10K . -1 -1 ohm. cm. 4  10 ic -1-1 ohm. cm.  4  0.000 1.977 5.864 11.44 18.75 27.15 35.60  2  2  2.473 3.546 5.851 9.110 13.35 18.71 23.91  4  10%  ohm. * cm. * 0.000 0.639 2.051 4.056 7.084 11.22 16.84 23.63 31.88  1Q  2  2.499 2.662 3.318 4.314 5.825 7.829 10.56 13.81 17.72  CsPOJ* *  1Q  4  -1 -1 ohm. cm. 2.443  0.000  J;Jg  3.191 6.155 9.751 13.48 17.92 24.29  2 2  fl 2  3,708 5.041 6.665 8.395 10.51 13.64  NH P0 F,(at low concentration) 7 ,«4 10 m * -1 ohm. cm. 4  0.000 0.380 1.090 1.842 2.724 5.758 4.834 6.718 8.940  7  2.499 2.693 3.088 3.546 4.076 4.691 5.507 6.619 7.740  26  The conductivity of the alkali metal difluorophosphates  at any given  concentration decreases in the order Li>Na>NH ~K>Rb>Cs. As a l l these salts 4 have the common anion P0 F " the difference in conductivity must be due to 2  2  differences in the mobilities of the cations.  This order af cation mobility  is opposite to that found by Gillespie et a l . in their conductance measure37 ments in HSOjF prevailed.  42 and H S0 43  Gillespie  2  4  in which they found the order Cs>Rb>NH~K>Na>Li 4  has suggested that the lighter members have the  larger solvated ion size, therefore, accounting for their lower mobilities. Our results suggest that solvation of cations in HP0 F is weak and, 2  2  therefore, the mobility i s determined by the unsolvated ion size.  As may  be seen the potassium and the ammonium salts give conductance curves which are almost collinear at low concentrations  (Fig. 9).  This agrees with  other workers' findings that the potassium and ammonium ions are of similar 37 42 size and hence have similar mobilities * On extrapolation of the linear portion to zero concentration the curves do not pass through the origin nor through the i n i t i a l point corresponding to the solvent conductivity.  There must, therefore, be some curvature of  the conductivity curve at the lowest concentrations and this appears to be substantiated on close examination (Fig. 9).  The conductivity of the acid  may be attributed at least partly to i t s autoprotolysis. However, i f the conductivity of the pure solvent i s due only to the ions produced in the autoprotolysis reaction the linear portion of the conductivity curve should pass through the origin on extrapolation.  It appears that on extrapolation -4 -1  the conductivity at zero concentration is approximately 2.3x10  ohm.  cm.  This may be attributed to ions other than those produced in reaction (1).  27  These ions presumably arise from impurities such as monofluorophosphoric acid, hydrogen fluoride and water.  Small traces of water would cause  hydrolysis of the acid as given by the following equations: HP0 F  2  •  H0  -^—^ HF  H P0 F  +  H0  HF  2  2  3  2  2  • H POjF 2  +  HjP0  4  Ions would arise from the ionization of HF which would be expected to behave as a weak acid: HP0 F 2  2  •  HF  H  2 2 2* P0  *  F  F  "  and from the ionization of H POjF which would be expected to behave as a 2  weak base: H P0 F 2  3  •  HP0 F 2  2  —  H P0/ 3  •  P0 F ^ 2  2  No evidence of any impurity was obtained on examination of the fluorine nuclear magnetic resonance spectra (a doublet with a splitting of 0.241 gauss (969.5 c.p.s.) was obtained which agrees well with that of Gutowsky* , 6  Quinn  17  and Roux ).  The original acid appeared to contain 50% monofluoro-  18  phosphoric acid and although the final acid showed no monofluorophosphoric acid impurity in the N.M.R. spectrum the concentrations of impurity with which we are concerned in conductivity work are too low to be detected by N.M.R. In order to investigate salts which contain anions other than P0 F " 2  2  solutions of NaF, Na P0 F, KSOjF and HSOjF were studied (Table III). 2  3  Potassium fluorosulphate is insoluble; however, conductance results were obtained for the other solutes and these are given in Fig. 10. At low concentrations the slope of the ic-tn curve for Na PO_F i s more ?  29  than twice that of NaP0 F and this may be explained by assuming that 2  2  Na POjF reacts as i s given by equation below: 2  Na P0 F 2  3  +  2HP0 F 2  >  2  2Na  +  2P0 F~ 2  *  H  P 0 2  3  F  As a result of this NajPOjF should have a conductivity curve with twice the slope of the curve for NaP0 F . 2  2  The fact that i t i s slightly more  than twice can be explained by assuming that monofluorophosphoric  acid is  a base in this solvent and is prctonated by the difluorophosphoric acid: H P0 F 2  3  •  HP0 F 2  2  — V  H P0 F 3  +  3  •  PO^  The slope of the conductivity plot for sodium fluoride i s considerably lower than that of NaP0 F , while this could be due to a much lower mobility 2  2  of the F~ ion compared to that of the P0 F ~ * 2  o n  2  *  t  *  s m o r e  probably due to  incomplete dissociation of NaF through ion pair formation Na* F"  ===^  Na*  F~  This idea i s supported by the fact that at low concentration where ion pair formation i s weakest the slope of the NaF curve i s very similar to the curve for NaP0 F , while at higher concentrations the slope of the NaF 2  2  curve decreases whereas that of NaP0 F remains essentially constant. This 2  2  tendency for ion pair formation i s consistent with the apparent low solvating power of the medium. It i s not immediately obvious why the difluorophosphates themselves show less tendency to form ion pairs than other salts in difluorophosphoric acid. Conductivity data for solutions of fluorosulphuric acid in HP0 F 2  are also given in Fig. 10.  2  Solutions of HSOjF may be neutralized by  addition of the base NH^PO^ giving rise to precipitation of the salt NH^SOjF and a resulting decrease in conductivity (Table III, Fig. 11) of the solution.  These results prove that HSOjF i s an acid in this system,  probably ionizing according to the equation:  30  TABLE III SPECIFIC CONDUCTANCES OF SOME ELECTROLYTES AT 25°C. Na,PO,F ? 10 m  4  3  10\  ohm. 0.0000 0.0754 0.1990 0.5426 1.131 2.064 3.684 6.908 11.83  2 10 m  1  cm.  HSO.F  4 10 K ohm.  1  cm.  -1  2.580 5.02 7.46 9.79 12.19 18.86 24.60  0.000 11.81 17.70 23.62 29.50 41.35 53.10  2.443 2.455 2.567 3.216 4.280 6.368 8.896 14.38 21.30  1  Addition of NH.PO-F, to the 0.531 molal. HSO_F/HPO,F, solution - conductimetric titration* 1  NH P0 F  NaF 10 m 2  4  10 K  ohm. 0.0000 0.4115 1.811 4.166 8.315 13.81 30.15  2  4  1  cm.  2.300 2.441 3.239 4.263 5.685 7.520 11.68  1  lO'm 6.79 22.9 45.6 61.8  2  2  10 K 4  ohm.  1  cm.  19.33 10.25 4.48 8.16  1  33  HSOgF •  HP0 F 2  > 2 2 2 H  2  P 0  F  *  S 0  3 " F  The K versus m curve for HSOjF shows an i n i t i a l flat portion followed by a linear increase in conductivity with concentration.  On extrapolation  of the linear portion to zero concentration the curve does not pass through the i n i t i a l point corresponding to solvent conductivity but appears to pass through the origin.  If the impurity in the solvent is in fact basic  then addition of HSOjF should neutralize this impurity.  In fact the flat  portion of the HSOjF curve may be attributed to titrating the impurity. Since the slope of the linear portion of the HSOjF curve is similar to the slopes of the curves for the difluorophosphates i t may be concluded that fluorosulphuric acid i s undergoing dissociation to roughly the same extent as occurs with the alkali metal difluorophosphates.  Hence fluorosulphuric  acid appears to be a strong acid in this solvent, a result which is consistent 26 with ;the conclusion of Gillespie et a l . that HSOgF i s a very much stronger acid than HPOjFj. 44 Gillespie  has suggested that the strengths of inorganic oxyacids are  determined largely by the number of equivalent oxygen atoms in the anion over which the negative charge may be spread.  The strength of the acid increases  with the number of equivalent oxygen atoms, therefore, HSO^F, HSOgCl and HSOjOH are of the same type, having the anions FSOj", CISOj" and HOSOj" with three equivalent oxygen atoms. C10 ~ 4  should, therefore, be the anion of the  stronger acid HCIO^, which has four equivalent oxygen atoms, and (H0) P0 ~ 2  2  and F P0 ~ anions of weaker acids, having only two equivalent oxygen atoms. 2  2  However, i t seems reasonable to suppose that some of the charge on the anion is accommodated on the halogen atom and i f the sharing of the charge between  34  the oxygen atoms and halogen are equal HSOjF and HSO^Cl would be i n the same class as HCIO^.  A similar effect for  P  O  2  F  w  2 ~  o  u  l  d  P  u t  t  n  e  a  c  *  d  *-  n  t  h  e  s  a  m  e  class as HC10 and i t would be expected to be stronger, rather than w e a k e r ,  t n a n  A  26 HJSOJ.  G i l l e s p i e et a l .  have made comparisons of acid strengths of  d i l u t e solutions o f various acids with that of H^SO^ i n bulk, and obtained the order:  H S 0 >HSO F>HSOjCl>HC10 >HP0 F ; 2  2  7  s  acid strength of H SO 2  A  4  2  2  2  2  they have indicated that the  i n bulk may, because of co-operative hydrogen bonding,  be considerably greater than that o f H S 0 that HP0 F  2  in dilute solution.  4  This suggests  may i n fact be a stronger acid i n bulk but our r e s u l t s do not  substantiate t h i s , since even i n bulk HP0 F 2  2  i s a very much weaker acid than  HSOjF  Conclusions Difluorophosphoric acid i s a solvent of weak solvating a b i l i t y as shown by the low s o l u b i l i t y o f s a l t s such as KSOjF and the order of m o b i l i t i e s of the a l k a l i metal cations. Since one of the requirements f o r proton transfer conduction i n a solvent i s association between the autoprotolysis ions and the solvent molecules through strong hydrogen bonds, then the absence of proton transfer conduction i n t h i s solvent may be due to a large extent to the i n a b i l i t y of the solvent to solvate the ions strongly.  Indeed the r e l a t i v e low b o i l i n g  point o f difluorophosphoric acid compared to the b o i l i n g points o f HSO,F (163*)  and H S 0 ( 2 9 0 - 3 1 7 ° ) f o r example indicates that hydrogen bonding i n 2  4  t h i s solvent i s very weak.  35  (b)  Infra-red spectra Results of measurements of the infra-red spectra of the difluorophosphates  in the region 4,000-300 cm."  are given in Table IV.  1  The position of the  peaks i s In good agreement with the reported spectra for difluorophosphates; the absence of any absorption in the monofluorophosphate region indicates l i t t l e contamination of the compounds used in this work by monofluorophosphate impurity.  The spectrum for the lithium salt differs from that of the other  salts studied; this could be due to a basic difference in the crystal structure.  Spectra of CsPQ F and NaPO* F are given in Fig. 12 as typical 2  2  2  2  spectra. P-0 stretching vibration region: The ionic phosphate vibration has already been connected with absorption .44 .45 .46 at 1040-1000 cm. , 1110-1050 cm. , and 1,170-1,000 cm. . In the series of compounds studied only absorption in the last range of values was observed. The difference in position of the ionic phosphate absorption bands of the difluorophosphates compared with other phosphates has been connected with the presence of highly electronegative F atoms bonded to the 47 24 phosphorus atom . Corbridge and Lowe examined the spectra of ammonium difluorophosphate in the region 5,000-650 cm.** and their values are given 1  in the table.  It can be seen that they obtained an extra peak at 1,005  cm."  1  and i t appears likely that this may be due to monofluorophosphate (which absorbs in the region 1,070-1,000 cm." ) as no peak was detected in any 1  difluorophosphate studied in this work. A difference of 30 cm."  1  was found  for the asymmetric PO stretching frequency from that found by Corbridge. 25 However, the value obtained by Buhler and Bues for the asymmetric P0 stretch  36  in KP0 F agrees well with that found in this work. Their values for 2  KPOJFJ  2  are also given i n the Table;  at 535 cm."  i t i s felt that the peak they report  i s due to monofluorophosphate which absorbs at 530 cm."  1  in  1  48 Robinson  has recently discussed P-0 stretching frequencies in a  number of phosphorus compounds. He found a linear relationship existed between the symmetric and asymmetric stretching frequencies of the P0 group.  In ammonium difluorophosphate 1,125 cm."  1  symmetric PO stretching frequency and 1,262 cm."  1  2  was assigned to the to the asymmetric. However  Robinson used only the ammonium difluorophosphate in his plot of symmetric against asymmetric PO stretches; the straight line correlation i s given by the equation: *sym. '  °*  65  W  •  2 7 0  Making use of the information obtained here on the difluorophosphates a better value for the straight line correlation i s : v  ««« sym. " ° *  70 v ae  «n, asym. •  2 1 0  P-P stretching vibration region. P-F stretch has been assigned to the region 990-840 cm."  1  PFj, P0F and PF,.** and 980-740 cm." 4  1  3  absorption at 835-720 cm."  1  in  i n organophosphorous compounds . 47  The  found in a l l the monofluorophosphates has been 24  assigned by Corbridge and Lowe  as probably due to P-F stretching; they  also found that ammonium difluorophosphate absorbs in this region. However examination of Table  indicates that the P-F stretch of the difluorophosphat  lies in the region 940-818 cm." . I t has been shown that the symmetric PFj deformation occurs around 500 cm." and this agrees well with the values 25 1  5 0  1  obtained.  However Buhler and Bues  have also assigned the symmetric P0  deformation to about the same region, namely 535 cm." . 1  2  37  TABLE IV INFRA-RED DATA OP THE DIFLUOROPHOSPHATES (FREQUENCIES IN cm.* ) 1  LiP0 F 2  NaP0 F  2  2  1273s 1164 s 940 s 890 s ("525 s 1498 s (426 s  K P 0 2  1309s 1152 s (868 s 1844 s [502 s 1458 m 360 w  2 2  RbPOjFj  F  (1332s [1310 s 1148 s j 850 s L832 s [503 s 1495 s  (1330s \l310 s 1145 s (846 m 1827 s (505 ra 1492 s  1415 s-  ("357 s 1342 s  CsPO,F-  NH-P0,F,  MO a 2680 . vb 3,380-2,2680 m,vb. 3  (1321 s U299 s 1137 s  (843 1818 s (503 m 1489 s  Note:  [1443 s (1410 s 1292 s  1138 s  (860 m,sh 1842 s 500 s  NH.PO-F, (Corbridge and Lowe") . 4  2  2 ) 9 0 0  2  KPO-F, (Buhler , and Bues )  2  2  25  2 > S 5 0  (1445 w,sh 11414 s 1262 s  1125 s  1005 ww (870 w,sh. (.832 s  1330 w 1311 s 1145 s  f857^8  834 s 535 w (512 m 1481 s 286 w  Brackets indicate incompleted resolved bands (see Fig.12) s * strong; m '*> medium; w • weak; sh » shoulder  c  FIG. 12. INFRA-RED SPECTRA  1.000  I.S00  1,000  500  WAVE NUMBER (cm."*) 1  (b) C S P 0 F 2  2  2.000  1,500  1,000  WAVE NUMBER (cm. )  500  200  39  The influence of the cations on the spectra do not appear to be very marked, except i n the case of the lithium salt.  A pronounced shift of a l l  peaks to lower frequency was observed, excepting the asymmetric PO stretching frequency which occurs at 1,273 cm. . These shifts are -1  probably bound up with the tendency of lithium salts to exhibit some covalency.  With the remaining alkali metal salts, a tendency for the  shift of certain peaks to higher frequency with increasing mass i s noticeable.  The progressive shift i s most evident with the symmetric PO stretching  frequency, i.e. i n LiP0 F 2  i t i s 1,164 cm." , and drops gradually to 1,137 cm. 1  2  in CsPOjPj.  (c)  X-ray powder photographs Inspection of the x-ray powder films (Fig. 13) indicates that with the  exception of LiP0 F and possibly NaPOjFj the alkali metal and ammonium 2  2  difluorophosphates are isoroorphous. The anomality of the lithium salt may be due to the tendency of the lithium atom to attain only 4 co-ordination in the crystal while the other larger alkali metal cations attain 6 co-ordination. The presence of numerous lines in the NaP0 F powder film indicates 2  2  either that considerable impurity occurred in the sample used or that the NaP0 F, i s i n fact not isomorphous with the other salts. 9  r a y Powder Fi1ms  4;1  REFERENCES 1.  W. Lange, Ber. 60B, 962 (1927).  2.  W. Lange, Ber. 62B, 786 (1929).  3.  N.V. Sidgwick, Chemical Elements and their Compounds, Vol. 1, Oxford (1950).  4. W. Lange, Ber. 61, 799 (1928). 5.  F. Ephraigm, Inorganic Chemistry, Gumey and Jackson (1943).  6. T. Moeller, Inorganic Chemistry, Wiley (1955). 7.  G. Tarbutton, E. Egan and S. 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