Abstract
The rates of oxidation of chromium(III) to chromium(VI) with H
2O
2 have been measured in NaCl and the major sea salts at pH 8 and 25°C. The effect of aging the chromium(III) solutions before the addition of H
2O
2 has also been investigated for the same solutions. Borate was the only major component of seawater found to affect the oxidation in seawater. Like OH
−, an increase in the B(OH)
4
− concentration causes the rates of oxidation to increase. The rate-determining steps in seawater involve two chromium (III) species
Cr(H
2O)
5OH
2++H
2O
2
→
KOH
products
Cr(H
2O)
4(OH) [B(OH)
4]
+ + H
2O
2
→
KB
products
and the rate equation is given by
dCr(III)/dt
= −k
3[H
2O
2] [OH
−] [B(OH)
4
−]
0.3
where
k
3=
k
1/[H
2O
2][OH
−][B(OH)
4
−]
0.3 (
k
1 is the pseudo-first-order rate constant).
Aging of the solutions causes the rates of oxidation to decrease and is affected by the concentration of borate, carbonate and magnesium ions. Borate causes a reduction of the aging effect, while carbonate and magnesium produce an increase of the aging effect. The Mg
2+ influence is much stronger than CO
3
2− when these ions are present at their seawater levels in individual Na-Mg-Cl and Na-Cl-CO
3 solutions. When CO
3
2− and Mg
2+ are present at the same time, the combined aging effect is less than that found in simple Na-Mg-Cl solutions. Since the aging effect is related to the precipitation of chromium(III), magnesium and carbonate ions will probably affect the solubility of chromium in natural waters.
