Study on the hydrolysis of cobalt ions in aqueous solution

The hydrolysis of cobalt ions in a fairly concentrated solution was studied based on a thermodynamic model. Cobalt solutions of 0.02, 0.05, 0.1 and 0.2 mol/dm(3) were prepared and, by adding NaOH solution to the cobalt solution of given initial concentration, pH values were measured. Measurements were made under different experimental conditions as to the control of gas surrounding the apparatus and the water used in this experiment. These data were then analyzed using a thermodynamic model, in which it was assumed that only one kind of complex ion, CoOH+, contributes to the hydrolysis reaction and an apparent equilibrium constant of this hydrolysis reaction K-app is an important parameter in this model. It was indicated that, in order to obtain a better agreement with the experimental data, a formula derived from a thermodynamic model should have been modified by adding a correction factor C-app. Computation was made by using this semi-empirical formula and a satisfactory agreement was obtained with the experimental data for various cobalt concentrations and for different experimental conditions as to the control of gas and the water used in this experiment. (C) 1998 Elsevier Science B.V.