Factors Affecting In(III)-hydroxysalt Precipitation and the Equilibria of In(III)-hydroxysalt in Sulfate Aqueous Solution

The present investigation was carried out to examine the chemical composition of precipitates produced during a titration of indium sulfate aqueous solution with sodium hydroxide, to obtain the standard free energy of formation of In(III)-hydroxysalt, and to determine the pH-dependence of its solubility at 333 K. The results obtained are summarized as follows: (1) The hydrolysis proceeds via two steps. The precipitates produced in the first and the second step hydrolysis were identified to be {Nax, (H3O)1−x}In3(SO4)2(OH)6, where the value of x varies with the composition of the solution, and In(OH)3, respectively. These hydrolysis reactions are represented by xNa++3In3++αSO42−+βHSO4−+(7−x)H2O={Nax, (H3O)1−x}In3(SO4)2(OH)6+(5+x+β)H+, where α+β=2, and {Nax, (H3O)1−x}In3(SO4)2(OH)6+(2+x)H2O=3In(OH)3+(x⁄2)Na2SO4+((4−x)/2)H2SO4. (2) The standard free energies of formation of {Nax, (H3O)1−x}In3(SO4)2(OH)6 were obtained for the x-values of 0.26, 0.34, 0.64 and 0.85 at 333 K. (3) The pH-dependence of the solubility of {Nax, (H3O)1−x}In3(SO4)2(OH)6 was determined for various choices of the parameter, which can be calculated from the composition of solutions immediately before the beginning of the first step hydrolysis. Furthermore, several factors affecting the precipitation of In(III)-hydroxysalt were also studied systematically.