Abstract
The kinetics of the radiation-induced reduction of hexavalent Cr
VI
to trivalent Cr
III
, through the unstable states Cr
V
and Cr
IV
, in N
2
O-saturated aqueous solution in the presence of formate, has been studied by pulse radiolysis with optical measurements in the pH range
ca.
2-5 and conductometric measurements at pH 3. The absorption spectrum of Cr
V
has been obtained in the pH range
ca.
2-9 by comparing the observed and expected bleaching yields resulting from the one-electron reduction of Cr
VI
by CO
2
&z.rad;
-
or e
aq
-
. The observed second-order rate constant for the disproportionation of Cr
V
, determined from the rate of partial regeneration of Cr
VI
, shows a bell-shaped dependence on pH with a maximum value at pH 3.3. This pH dependence is consistent with a rearrangement of the coordination shell of Cr
V
from tetrahedral, Cr
V
tet
, to octahedral, Cr
V
oct
, in two sequential proton assisted steps followed by disproportionation between Cr
V
tet
and Cr
V
oct
with
k
= 3.4 × 10
8
dm
3
mol
-1
s
-1
. The intermediate form of Cr
V
generated in the first of these steps does not contribute to the disproportionation reaction. Based on the bell-shaped dependence on pH of the observed rate constant for disproportionation of Cr
V
, the forms of Cr
V
in acidic solution are assigned as H
3
CrO
4
(p
K
a
= 2.7), H
2
CrO
4
-
(p
K
a
= 3.8), and HCrO
4
2-
. At pH 6.8 the spectral data indicate the presence of CrO
4
3-
and p
K
a
= 7.0 for HCrO
4
2-
.