The hydrolysis of cobalt ions in a fairly concentrated solution was st
udied 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 th
e cobalt solution of given initial concentration, pH values were measu
red. 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 i
n this model. It was indicated that, in order to obtain a better agree
ment with the experimental data, a formula derived from a thermodynami
c model should have been modified by adding a correction factor C-app.
Computation was made by using this semi-empirical formula and a satis
factory 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 Els
evier Science B.V.