Potentiometric study of mixed cadmium halide complexes in aqueous solution

Abstract

The study of equilibria in aqueous solutions indicates (77) that, in general, mixed complexes tend to be formed when two or more ligands are present in a solution. For a discussion of whether and when mixed complex formation would be preferred over the binary compounds, see ref. 17. Mixed and binary complex formation in cadmium I I halide aqueous solutions has been the subject of several investigations. For a summary of earlier work on the binary compounds, see refs. 3 and 14. A number of studies deal with specific mixed cadmium halides (4, 70, 26, 2 9 ) , but a general study of the CdXY system where X and Y are different halides is still lacking. This is the purpose of the present investigation, and to properly compare the formation of binary and mixed complexes, a study under the same conditions was also made of the corresponding binary system. Previous measurement methods have included spectrophotometry, polarography, ion exchange, solubility, radiotracer, thermochemical, and other analytical methods (73, 27 ) . A potentiometric method using specific ion electrodes was applied to some binary systems under varying conditions (6, 7, 20. 2 1 ) . We have now applied it in a systematic manner to the binary halides and extended the method to mixed halides. For a review of the methods used previously to compute the stability constants, see refs. 75 and 22. Discrepancies have been reported in the stability constants calculated from the same experimental data with different computational methods, and possible uncertainties arising from application of models describing the experimental situation have been discussed ( 5 , 8, 23, 2 5 ) . We have tried to use the present measurements to examine these problems.