Chemistry Reference
In-Depth Information
6.1 CHROMIUM
The oxidation states of chromium range from −2 to +6, which vary in colors
and geometries of their compounds [23]. The electronic configuration of ele-
mental chromium is 3d
5
4s
1
. The most common and stable oxidation states are
+3 and +6 with electronic configurations of 3d
3
4s
0
and 3d
0
4s
0
, respectively.
Chromium(III) is amphoteric, capable of forming complexes with both acids
and bases, which are, to some extent, kinetically substitution-inert [24]. Com-
pounds of Cr(III) are mainly octahedral [25]. Oxo compounds of Cr(VI) in
aqueous solutions are tetrahedral in their geometries. Compounds of other
oxidation states, particularly +2, +4, and +5, are unstable and are easily con-
verted to stable oxidation states of Cr.
Chromium is considered an essential micronutrient for humans, but a recent
review on this topic disputes this conception [26]. The results of the nutritional
biochemistry of Cr(III) within the last two decades are considered, concluding
there is still a lack of understanding of the identification of the molecular-level
mechanism(s) by which administration of Cr to humans occurs, and further
research is necessary to confirm if Cr is an essential element.
The potentials of redox pairs of chromium are presented in Figure 6.1,
which suggests the +2 oxidation state is a good reductant, while the +6 oxida-
tion state is a powerful oxidant [24]. In acidic solutions, Cr(VI) is unstable and
proceeds to the most stable state, Cr(III). The estimated potentials of Cr(VI)/
E
o
(acidic)
E
o
(basic)
CrO
4
2-
Cr(
V
I)
0.55
Cr(
V
)
1.34
-0.11
Cr(
I
V)
2.10
Cr
3+
Cr(OH)
3
-0.424
Cr
2+
-1.33
-0.90
Cr
Cr
Figure 6.1.
Potential diagrams of chromium.
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