The solubility of AgCl, AgBr and AgI in a dimethylsulfoxide solution of ammonium nitrate has been determined by a potentiometric method. At 25°C the solubility products in terms of molarities are 2 × 10−10, 9 × 10−11 and 3 × 10−12 resp. in a 0.1 M NH4NO3 solution. The stability constants of the complexes AgCl2− AgBr2− and Ag2I3−, which are formed with excess halide ions are 4 × 1010, 4 × 1010 and 9 × 1023 resp., under the same conditions of temperature and inert salt concentration. Extension of the measurements to other temperatures supports the view of a “structural change” of the solvent at higher temperatures, affecting the solution equilibria.

The temperature dependence of the solubility product of AgCl in pure dimethyl sulfoxide in the range 25 to 70 "C has been investigated. Thermodynamic values at zero ionic strength are estimated. Formation constants for the AgCI2-complex, existing in excess chloride are also tabulated as a function of temperature. The entropy and enthalpy data thus obtained are discussed in terms of ionic solvation. 1NTRODUCTlONIn a previous paper [l], a preliminary study of the solubility and complex formation equilibria of silver halides in a DMSO-NH,NO, solution was reported. It has meanwhile been recognized that in NH,NO, media ion pairing could have considerable influence on the ionic equilibria present [2]. Therefore, in the present work, we obtained our potentiometric data without addition of any supporting electrolyte, the slight variations in ionic strength being compensated for by Debye-Huckel corrections. Moreover, accuracy was gained by a modified titration device and by increasing the number of experiments. EXPERI MENTAL PARTReagent grade DMSO was dried during several days upon molecular sieves (Linde 4A), and then distilled twice under reduced pressure in a nitrogen stream. This gas, of high purity, was carefully dried over sulfuric acid and molecular sieves.The titration cell was water-jacketed and thermostatted to 0.05 "C. The silver electrode was a platinum strip, electrolytically plated with silver metal. The reference electrode was also a silver plated platinum strip, submerged in a 0.01 M AgN0,-DMSO solution. A special model of salt bridge was employed, so as to eliminate virtually any AgNO, diffusion into the titration vessel. It was filled with 0.01 M AgN0,-DMSO solution. All parts of the system, except the microburet (5 ml, 0.002 ml precision) were thermostatted.Potentials were measured by means of a compensation device composed of a Leeds and Northrup potentiometer and a pH meter Radiometer PHM 22p, which was used as a zero indicator. The accuracy of the measured potentials was 0.1 mV.Silver nitrate of high purity was crystallized twice from water by addition of methanol, dried in vacuo at 50°C and stored under nitrogen.

The stability constants of the Ca, Sr and Ba complexes with citrate ion have been determined as a function of the temperature, in the presence of 0.2 M NH,CI. There is also evidence for 1 : 2 complexes, the constants of which are reported. The stability constants for the Sr and Ba monocomplexes show a minimum at 30'C. Above this temperature, there exists a linear relationship between the enthalpy and the entropy of complex formation, the isokinetic temperature being 104 ' C . EXPERIMENTALThe radiochemical method of Schubert [ I ] was employed because for these complexes the method gives optimal accuracy and reliability, not attainable by other known methods.Reagent grade chemicals, not otherwise purified, were used throughout the work. The tracers were checked for radiochemical purity by gamma ray spectroscopy, except for which is a pure 1 3 emitter. They were furnished by the Belgian Nuclear Centre, Mol.The cation exchanger, Dowex 50, X12, 200-400 mesh, was converted t o the ammonium form, washed and dried in air. Assay solutions were prepared by equilibrating 0.2000+0.0005 g resin with 50.00 ml aqueous solution, containing tracer and ligand, during about five hours in a constant temperature bath. Longer periods did