CHAPTER 15 • Redox Processes of Chromium in Sea Water
291
log k = 11.93 + 0.95 pH - 4260 I T - 1.06 1°·5
which are valid forthe pH ranges 1.5-4.5 (<1 == 0.2) and 5-8.7 (<1= 0.2), respectively, from
5 to 40°C and 0.01 to 2 M ionic strength. The logarithm of the overall constant (log k)
shows a typical parabolic dependence on pH, decreasing from pH 1.5 to 4.5 and increasing from pH 5 to 8.7 (Fig. 15.7). The effect of pH, temperature and ionic strength
on the reaction led to the reactions at low pH being due to
2+
kH2A-Fe
H 2 Cr0 4 + Fe ----''---~) products
while the reactions at high pH are due to
HCr0 4 + FeOW kHA-FeOH ) products
and
kHA-Fe(OH>2
HCr0 4 + Fe(OH}z -----'---"'~) products
The overall rate expression over the entire pH range can be determined from
(H2A = H2Cr04)
where kH2A-Fe = 5 X 10 6 , kHA-FeOH = 1 X 10 6 , kHA-Fe(OHh = 5 X lOll and a(i) are the molar
fractions of Cr(VI) and Fe(II) which can be determined from the dissociation constants of chromic acid (Shen-Yang and Ke-An 1986) and the hydrolysis constants of
Fig. 15.7. Linear fits of log k for
the reduction of 0.95 f1M Cr(VI)
with Fe(II) as a function of pH
in 0.1 M NaCl at 10°C (Pettine
et al.1998b)
6 , - - - - - - - - - - - - - - - - - - ,
5
4
3
2
o
2
•
[Fe(lI)) = 39.2 iJM
o [Fe(ll)] = 9.8 iJM
4
6
pH
8
10