that the rate of decoiiiposition was more rapid than For this reason the reactions with phosphoryl the rate of cxchaiige, then only the metal tetra- fluoride were carried out at -40'. S o significant difference in the behavior of chloride and the phosphoryl halide were obtained zirconium and hafnium compounds was noted as decomposition products. .It the melting points of the POF2Cl~I\lC14 except that the hafnium compounds usually and POFs-MCld compounds, however, the metal melted at slightly higher temperatures than the tetrahalide was fluorinated as evidenced by the corresponding zirconium compounds. Acknowledgment.-The authors wish to thank fact that the non-volatile residues resulting from the thermal decomposition were always high in fluorine 11.Woyski of this Laboratory for supplying sonic aiid low in chlorine. Fluorination with phosphoryl of the special reagents used in this work, and for fluoride was also observed when the initial reaction his valuable suggestions. mixture was warmed up to -1.5' under pressure. ~ ~ A D I S O S 1Vrscoxs1s ,
~ ~ . ~ l l X i R l B l ~I'Rirhf l ' l ~ ~I I~l E l ) l C f , A R l M l , ; X ' l ( I F ~ l i I ~ I \ l I S ~ I . RUSIVEF.SITY Y, 01' \ v I S C O N S l N
I
Studies Pertaining to Soluble Silver Iodide Species J ~ YE:un..uw L. KING,HARRY J. KRALL AND X A K YL. PANUOW
Y,
~ ~ E C E I V E FEBRUARY D 1982
T h e iiicreare of the solubility of silver(1) iodide with an increasing concentration of silver(i) ion or iodide ion indicates the existence in solution of cationic and anionic complex ions. Solubility measurements, ion mobility determinations, and zpectral studies have been employed to elucidate the nature of these compiex ions. These studies indicate the existence of :inionic cotnplcx ions which contain varying numbers of silver(1j ions and the existence of cationic complex ions which contain varying nuinhers of iodide ions. It appcars that thc. solutions contain not merely monomeric and dimeric species but a large variety of these polymeric species
In this work solubility iiieasurenients, ion iuobility deterininations and spectra studies have been utilized for the purpose of elucidating thc cyuilibria which exist in solutions containing silver ( I ) and iodide. It will be seen that a wide variety of soluble species contdming silver(1) and iodide iiiust exist. While the methods utilized here are madequate i i i coniplctely describing the systeni, icveral iniportaiit concIusions caii bc tlrawn from the observations. Solubility Studies 'The niaiiner in which the solubility of a slighlly solublc compound is affected by the concentration of a complexing agent provides valuable information regarding the species existing in the solutions. 'The interpretation is particularly straightforward if each of the soluble species contains only one ion or atom of the one kind. I n the case of the soluhility of silver(1) iodide in solutions of sodium iodide, the various possible equilibria woultl bc AgI(s) t AgI,:t and the dependence of the solubility on the iodide ion concentration in media in which the activity coeflicients could be assumed to be constant would allow a calculation of the average value of 1 2 , the charge on the complex. If the value of 12 is known as a function of the iodide concentration, the formulas of the species existing in solution over that concentration range can be established. If several complex species containing different numbers of silver(1) ions' exist, the situation is more complicated. The dependence of the solubility on the iodide ion concentration still gives the average
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(I) I h e bondinz I n tlic ? i l \ e r ( l ) iodidc cvniplcx 1011% ~rndotthtrdl~ 111 t l l i \ I)dpcr t h c ~ r ~ l l s t l t u c l l t s h a s A Ercdt deal of co!diclit LhArALtcr I l V l l 111 i 0 l l i d l l d I ~ K l l d CLoll5 v i lhc c o t n p ~ e xI O I I ~\,111 i l c I c r c r r c d t t
Ilodium ion. Ion Mobility Detorminations.14-The ion mobility determinations were carried out in the same manner as has been tit7scribed elsewhere.6 In the experiment in which the constituent mobility of iodide was determined, silver nitrate solution rather than sodium chloride solution was in contact with the silver-silver( I ) chloride electrodes. The results presented were obtained in single runs a t each set of concentrarion conditions. Pictures of the schlieren pattern were rdien a t several times during each run and the boundary velocities were found to be constant. Spectra Studies.-Spectrophotometric measurenient~ were made using a Beckman DU spectrophotometer equippecl for niaintaining the temperature of the solutions a t approxirnately 25'. Cells of one PITI. and ten cm. path lerigths were Ilsell.
hIost of the sdutions studied were prepared using silver flitrate arid potassium iodide. In some cases the solutiou iri the blarik cell contained potassium iodide but no nitrate
Since nitrate absorbs appreciably at some of the wave Irugths studied, the observed absorption was corrected for this; the absorption by nitrate was determined in experiments in which potassium nitrate-potassium iodide solutions were measured against a potassium iodide solution as :I blank. Some experiments mere performed using silver perchlorate rather than silver nitrate. Since perchlorate ion absorbs much less than nitrate ion, no uncertainty exists regarding corrections for its absorption. In another series of experiments, silver(I ) iodide which had been prepared for the direct solubility determinations and which had stood under water in a darkened cabinet for over a year was dissolved in potassium iodide solution. With certain rxcrptions to be discussed the values of & were the same within the experimental uncertainty regardless of which of rliese several methods of preparation of the solutions was iisecl. This is indicated in Fig. 2 in which different symbols :ire used for the solutions prepared in the different ways. Sodium thiosulfate was present in both sample and blank at approximately 5 to 10 X lo-' molar. At this concentration it contributes negligibly to the absorption and does not compete effectively with iodide ion for complexing of silver(1); however, it does keep iodine in the reduced state. Because of its absorption, IJ-, which would be formed by :iir oxidation, must be absent from the solutions. I n some experiments on solutions containing excess iodide in which silver nitrate or silver perchlorate was used, the absorption was determined a t several different times (e.g., at 1, T and 14 days after preparation). In general, the optical density readings were within a few per cent. of each other but in some cases, it was observed that the absorption a t longer wave lengths (> 330 mp) increased with increasing time by amounts greater than a few per cent. A reasonable explanation of this seems to be that reducing impurities, 4uch as lint, were bringing about the slow formation of colloidal silver. The scattering of light by this material would ?)cinore important relative IO the absorption by the complex ioiis at longer wave lengths. I n the case of the solutions prepared using the aged silver( I ) iodide, it was observed that the absorption at wave lengths greater than 340 mp was markedly greater a t the time of preparation of the solution than was the absorption [Jf a solution prepared using silver nitrate or silver perchlorate. The absorption a t these wave lengths decreased with time. This behavior is consistent with the existence of ii snia11 but significant concentralioii of silver(1i iodide colloiiuii.
. .I.____.I
t l J aggregates in solutions prepared in this way. Thew colloidal aggregates, which must be present in only sniall amounts since the absorption a t shorter wave lengths is approximately the same as expected, slowly react with thr medium to form the equilibrium polymeric species. I t seems reasonable that this would occur in the solutions prepared using silver(1) which was initially present in the very large aggregate units existing in solid silver(1) iodide.
Discussion 'The experimental data presented in this paper provide interesting information regarding the soluble species which exist in solutions containing silver(1) ion and iodide ion. it is not possible, however, to interpret any of these data to yield the elriact cornpositioii o f the several species arid the several equilibrium quotients. ,4n experi~riental technique which will yield such i~iforinatioriis the itieasuremen t of electroiriotive force. While much work has been done on the silver-silver(1) iodide electrode potential, none appears to have been done on the silver-silver(1j potential in iodide solutions which are not saturated with silver(1) iodide. It is such ineasurernents which will yield the average number of silver(1) ions per ion-molecule. -it 25' in solutions of constant iodide concentration, a tenfold change in the silver(1) concentration will result in a change of the silver half-cell voltage of 0.059/n volts, where n is the average number of silver(1) ions per ion molecule. This formation of soluble polymeric species has been observed in a number of metal ion-hydroxide ion (or oxide ion) systems but this type of behavior in the case of metal-halide complex ions does not seem to be as conmon. The large extent ot covalent character in silver-iodide bonds') is undoubtedly one of the factors which stabilize these polymeric species in the case of silver iodide. These covalent forces, which contribute so markedly to the stability of solid silver iodide and lead to its very low solubility in pure water in which it dissolves to give the simple hydrated ions, are also present in these polymeric ion-molecules. The binding in these ion-molecules undoubtedly has a great deal in common with that in the solid. I t would not be surprising to find that species of this sort also exist in solutions of copper(1) iodide and gold(1) iodide in aqueous iodide solutioii. The extent to which such polymeric ion-molecules are present in solutions of the other silver(1) halides is not known. I t would be expected to be less if the suggestion that the covalent nature of the binding in silver(1) iodide is an important
factor
in
stabilizing such species is correct.
(14) The authors wish t o express their appreciation to hIr. E. R. > f A D I \ O A , 11 ISCOSSI\ ._. 1)isrnukes for assistance it1 performing these exprriments arid to ProfesJ W . Williams xiid R. .4. A l k r t v far llrr L I V
