SOLUBILITY OF SILVER IODATE IN IODATE SOLUTIONS
May 5, 1956 Now rearranging, one obtains '4x12
1 m(+% - ma(+%, + Bxl = [z) m-"o
+
(13)
where A = - (2.303/2) (al2 a21)and B = +2.303 azl. If A , B and the quantity in brackets, which may be called C, are weak functions of the total molality m ,then x1 is linear with m. A is zero for
[CONTRIBUTION No. 434
FROX THE
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the KC1-KBr system and in general is very small, varying but little with 112. Hence, i t is necessary that a12 and C be independent of m for equation 4 t o hold. It is readily apparent from Table V and plots of the osmotic coefficients of KC1 and KBr against the cube of molality that these conditions are met in the KCl-KBr case, not exactly but sufficiently well that one would not expect the x l m plots to be perceptibly non-linear.
INSTITUTE FOR ATOMICRESEARCH AKD DEPARTMENT OF CHEMISTRY, IOWASTATE COLLEGE]
The Solubility of Silver Iodate in Iodate Solutions. Iodato Complexes of Silver* BY JAMES J. RENIERAND DONS. MARTIN, JR. RECEIVED OCTOBER 20, 1955 Solubilities of silver iodate a t 25.0, 35.0 and 50.0' have been measured by means of a radioassay technique in aqueous solutions with various low concentrations of lithium iodate and with solutions having a constant ionic strength of 1.00 M. and (17.4 =k Solubility product constants obtained for these temperatures were (3.0 i 0.15) X 10-8, (6.3 i 0.3) X 1.0) X lo-*, respectively. In addition t o the normal equilibrium solubility product relationship, two complexing equi1 0 8 - tri AgIOa(aq.), for which AF& = libria were needed t o describe the behavior a t higher iodate concentrations: Ag+ -1.13 =k 0.18 kcal./mole, AH& = 5.1 i 4.5 kcal./mole; Ag+ 2108Ag(IOa)z-, for which AF& = -2.53 0.04 0.79 kcal./mole. kcal./mole, AH& = -5.20
+
Introduction A radiosilver assay procedure has been utilized to determine the solubility of silver iodate in aqueous solutions of lithium iodate. The method has also been applied to solutions for which ionic strengths were adjusted a t 1.00 M by means of lithium perchlorate. This work was undertaken when it,wasnoted that significant deviation from the solubility product existed in moderately dilute solutions of lithium iodate. Possibilities for the existence of iodato-silver complexing phenomena were indicated. Ricci and Aniron' studied the silver iodate-lithium iodate-water system and concluded that the isotherm for this system was of the simplest type with no evidence of compound formation or solid solution, and qualitative tests for silver in the solutions containing silver iodate and the dissolved salt were negative. Shchigol,2 employing a conventional iodometric procedure for analyses of equilibrium solution phases, reported the solubility of silver iodate a t 20-22' in aqueous solutions for three concentrations of potassium iodate. He considered that silver was present in the equilibrium solution phases only in the form of Ag+ and Ag( 1 0 3 ) 2 - ions and reported an instability constant of 2.22 X 10" for the diiodatoargentate species. It was of interest to obtain a more quantitative description by use of an effective radioassay method. Experimental Materials.-All water used in this work was prepared by redistilling distilled water from alkali permanganate solutions. I t s specific conductance was found to be 2.5 X 10-6 mho cm.-'. Lithium perchlorate was obtained in reagent grade from the G. Frederick Smith Chemical Company. Solutions of
* W o r k was performed in t h e Ames Laboratory of the U. S Atomic Energy Commission. (1) J. E. Ricci a n d I. Amron, THIS J O U R S A L , 73, 361.1 (1051). (2) M. B. Shchigol, Zhur. Obshchci K h i m . , 22, 728 (1952).
+
+
+
*
lithium perchlorate were filtered before being used in order to remove any insoluble matter which was present. Lithium iodate was prepared by adding a stoichiometric amount of iodic acid to an aqueous solution of lithium carbonate. The salt was crystallized after carbon dioxide formed in the reaction had been driven off by heating. The product was filtered, recrystallized twice from water, refiltered, washed, dried a t 120" for 24 hours and stored in a desiccator over anhydrous magnesium perchlorate. Analyses of the salt for total iodate content were consistent with the theoretical composition. Platinum used in the plating procedure was 1 mil sheet stock supplied by the American Platinum Works. Silverllom was the tracer employed. I t was obtained from the Oak Ridge National Laboratory in 1.4 ml. of 2 N nitric acid and was reported to have a radiochemical purity in excess of 980/,. Stock solutions containing radioactive silver were prepared by adding aliquots of this solution t o dilute nitric acid solutions containing definite amounts of inactive silver nitrate. These solutions were subsequently scavenged with iron hydroxide and evaporated t o dryness with an excess of nitric acid. The radioactive silver was precipitated a s the chloride from the dilute nitric acid, filtered from the solution and dissolved with concentrated ammonia. The silver was subsequently electroplated on a platinum gauze cathode according t o a method proposed by S10min.~ Following this operation, the silver was dissolved from the electrode with 10 N nitric acid and the entire purification cycle was repeated. The remainder of the procedure involved a gravimetric determination of the silver on the electrode, dissolution in 10 N nitric acid, fuming with perchloric acid and dilution in a volumetric flask. Tagged silver iodate was prepared by adding solutions of lithium iodate to aliquots of the stock solutions of silver tracer after these had been partially neutralized with lithium hydroxide. The wet masses of crystals which formed were washed repeatedly with many small portions of warm water, and finally they were washed, slurried and transferred to the equilibrium flasks with the aid of many small portions of the solution phase with which they were t o be equilibrated. Primary standards and other chemicals used for this work were all of reagent grade according to ACS specifications. Apparatus.-Mixtures were equilibrated in thermostated water baths in which temperatures were constant to =t0.lo. Pyrex crlenmeyer flasks were used to contain the mixtures. Some were of red variety glass and others were wrapped with a black plastic adhesive tape to exclude light. When in use,
~-
(3) G. W .Siomin, "Rapid Quantitative Electrolytic Methods of Analysis," 7th ed., E. H. Sargent Co., Chicago, Ill., 1954.
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JAMES
J. RENIERAND DONS. MARTIN,JR.
their ground glass caps were sealed with paraffin wax and agitation was provided by Burrell Model CC shakers. Separations of samples of the equilibrium solution phases were accomplished by filtration with pressure in the thermostated baths. The filtering medium was “fine” fritted glass having a nominal maximum pore size of 5 p . Before this filtering medium was adopted its effectiveness in removing the solid phase particles was tested. Experiments carried out with equilibrium mixtures and solutions of silver perchlorate in which pore size, pH, iodate and lithium concentrations were varied indicated that “fine” fritted glass discs (nominal pore size, 5 p ) were a suitable medium for use in these experiments if the filter became saturated with silver ion before samples of the equilibrium solution phases were taken for the radioassays. It was observed that
