Effect of complexing on character of change of partial molar volumes of cadmium chloride and cadmium bromide in aqueous solutions with pressure increase

The effect of pressures up to 10 kbar at 25 deg C on the e.m.f. of the electrochemical chains: Cd/CdCl2(0.5 mol CdCl2 in 5 m NaCl), Hg2Cl2/Hg and Cd/CdBr2 (0.5 mol CdBr2) in 5 m NaBr), Hg2Br2/Hg was studied. At atmospheric pressure the partial molal volumes of cadmium chloride and bromide in solutions, where practically all the cadmium is in the form of complex anions of cadmium tetrachloride and tetrabromide, were calculated. The essentially different nature of the changes in these values with increasing pressure is due to the difference in the structure of cadmium tetrabromide [Cd(H2O)2Cl4]2-, which has a Tsub(d) symmetry and contains no water molecules in the internal coordination sphere, and cadmium tetrachloride [Cd(H2O)2Cl4]2-, which retains two water molecules in the internal coordination sphere. This difference is also confirmed by the fact that at atmospheric pressure the volume effect of complexing ΔVsub(c) of cadmium tetrabromide from cadmium aquo-ions and hydrated bromide ions exceeds by approximately 2 times the ΔVsub(c) of cadmium tetrachloride from the cadmium aquo-ion and the hydrated chloride ions