Chemistry Reference
In-Depth Information
Cr(V), Cr(V)/Cr(IV), and Cr(IV)/Cr(III) redox pairs are also given in Figure
6.1. In acidic solutions, the one-electron redox potentials increase in the order
Cr(VI) < Cr(V) < Cr(IV). Cr(III) and Cr(VI) show different chemical behav-
iors, toxicities, and bioavailabilities [27]. The following sections first convey the
chemistry of stable oxo species of Cr(III) and Cr(VI) in aqueous solutions,
followed by their reactivity with biologically important molecules to under-
stand their toxicity. The generation and involvement of unstable Cr(III),
Cr(IV), and Cr(V) species are also summarized.
6.1.1 Aqueous Chemistry of Oxo-Cr Compounds
6.1.1.1 Cr(III).
In aqueous solution, Cr(III) exists as a hexaqua ion,
[(Cr(H
2
O)
6
]
3+
, which is acidic (p
K
a
= 4) and is in dynamic equilibrium with its
monomeric hydrolysis product. These species dimerize to form hydroxo-
bridged species, which further polymerize slowly to trimers, teramers, hexam-
ers, and so on [28]. The main aqueous species of Cr(III) are Cr
3+
, Cr(OH)
2+
,
Cr OH
o
)
−
[29]. In an acidic medium, Cr
3+
is the main species,
(
)
3
, and
Cr OH
(
while
Cr OH
o
( )
3
and
Cr O( )
−
predominantly exist in the alkaline medium. The
spectra of Cr(III) were obtained by mixing Cr(III) with either 0.005 M (pH 11.7)
or 0.5 M NaOH (pH 13.7) solutions [30]. The spectra of a monomer and a
dimer had a maxima at 230 nm. The molar absorptivities at 230 nm were deter-
mined as ε
monomer
= 245/M/cm and ε
dimer
= 830/M/cm at pH 11.7, and ε
monomer
=
348/M/cm and ε
dimer
= 965/M/cm at pH 13.7.
The kinetics of the monomer and dimer were studied at 230 nm as a
function of pH. The kinetics curve was explained by the following set of
reactions:
Cr H O OH
III
(
) (
)
H Cr H O OH
+
+
III
(
) (
)
−
p
K
(
calc
.)
=
12 82
.
(6.1)
2
3
3
2
2
4
a
Cr OH
III
(
)
+
Cr OH
III
(
)
[(
Cr
)
III III
,
]
x
(6.2)
3
3
2
Cr OH
III
(
)
+
Cr OH
III
(
)
−
[(
Cr
)
III III
,
]
y
(6.3)
3
4
2
Cr OH
III
(
)
−
+
Cr OH
III
(
)
−
[(
Cr
)
III III
,
] .
(6.4)
4
4
z
2
Cr(III) behaves as a typical “Lewis acid” and forms complexes with inor-
ganic and organic ligands, which is a slow process due to substitution of the
coordinated water molecules with a ligand. Interestingly, the frequency of
dehydration of water from Cr(III) is 10
−6.3
/s while the frequencies for Al(III)
and Fe(III) are 10
−0.8
and 10
−3.5
/s [31].
6.1.1.2 Cr(VI).
Oxo complexes of Cr(VI) have been studied experimentally
and theoretically over the last several years. The chemical bonds in the
CrO
2−
ion are composed of d orbitals of Cr atoms and p orbitals of O atoms. There
are two stable forms of Cr(VI) in the solid state: monomeric,
CrO
2−
, and
dimeric,
Cr O
2
2−
. In aqueous solution, additional species,
HCrO
−
,
H CrO
2−
, and
7
2
4
HCr O
2
−
, are also involved in the following equilibria [32]:
7
Search WWH ::
Custom Search