CHE230 Environmental Chemistry 2010
Tutorial Problem 6B
The toxicity of chromium depends upon its oxidation state. The speciation of chromium
varies due to redox reactions, complex formation and acid/base dissociation. As part of
their fourth year plant design project, a student group examines the speciation of
chromium in a water sample at 298K. Having developed a deep appreciation for
Environmental Chemistry from second year, they recognize that the speciation of
chromium can be described through the following set of reactions:
R1)
R2)
R3)
R4)
R5)
R6)
R7)
HCrO4- (aq) + 3e- + 4H+ (aq)   [Cr(OH)3](aq) + H2O
HCrO4- (aq)   CrO42- (aq) + H+ (aq)
Cr3+ (aq) + OH-(aq)   [CrOH] 2+ (aq)
[CrOH] 2+ (aq) + OH-(aq)  [Cr(OH)2] + (aq)
[Cr(OH)2] + (aq) + OH- (aq)  [Cr(OH)3](aq)
[Cr(OH)3] (aq) + OH- (aq)  [Cr(OH)4] - (aq)
[Cr(OH)3](s)   Cr3+ (aq) + 3OH-(aq)
ε0= 1.12
pKa = 6.51
Log K = 10.34
Log K = 8.08
Log K = 5.57
Log K = 4.61
Ksp = 6x10-31
a) Give the chemical formula and name of the predominant species of chromium in
water with pH = 8 and pE= 10.
R3 to R6 give the speciation of Cr3+ between different hydroxy complexes. R1 represents
a chemical change (reduction), so we will consider it separately. First, the stepwise
equilibrium constants K3-K6 can be combined by multiplying the constants:
Cr3+ (aq) + 2OH-(aq)   [CrOH2]+ (aq); β = K1K2 = 1010.34+8.08
etc. to derive an expression for Cr3+ species:
So of these species, [Cr(OH)2]+ dominates.
For R1:
So
.
For R2:
So
So there is more
than
. Overall,
, chromate(VI), predominates.
b) Give the equation of the line describing the pE and pH conditions under which the
concentrations of [Cr(OH)2] + (aq) and HCrO4- (aq) are equal.
Combining R1 and R5,
HCrO4- (aq) + 3e- + 4H+ (aq)  [Cr(OH)3](aq) + H2O
[Cr(OH)3](aq)  [Cr(OH)2]+ (aq) + OH- (aq)
HCrO4- (aq) + 3e- + 4H+ (aq)  [Cr(OH)2]+ (aq) + OH- (aq) + H2O
log(Koverall) =
At the line, [Cr(OH)2]+ (aq) = [HCrO4- (aq)]
ε0= 1.12
Log K = -5.57