Chemistry Reference
In-Depth Information
new information about amounts of chromium being released into the environment.
19
OSHA has recently settled on a more stringent new standard for occupational expo-
sure to hexavalent chromium. The new standard has lowered the permissible expo-
sures from 52
g m
− 3
of air as an 8 hour time-weighted average.
20
These
recent changes in Cr(VI) exposure limits highlight the increasing concern for the
health and safety of occupationally and environmentally exposed human populations.
g m
− 3
to 5
µ
µ
17.1.2 Physical Properties of Chromium that Confer Carcinogenicity
Like most transition metals, chromium can exist in a number of oxidation states.
Known oxidation states for this metal range from
II to +VI.
21
The most common
oxidation states are III and VI but Cr(III) is naturally more abundant than Cr(VI)
due to its higher stability in a reducing environment.
2
Oxidative processing of the
more abundant Cr(III) ores to industrially useful compounds of Cr(VI) have been
extensive in many of the industries listed above. Waste from those processes artifi -
cially increases the amount of Cr(VI) released into the environment, which is of
increasing concern because of the carcinogenic nature of these Cr(VI) compounds
with respect to the more benign Cr(III) form. However, Cr(VI) is not the oxidation
state of chromium that directly interacts with nucleic acids to initiate cancer forma-
tion.
22
Instead, it is the intracellularly reduced oxidation states of Cr(V), Cr(IV) and
Cr(III) that have been observed to cause damage to DNA.
22
Hexavalent chromium
compounds are much more carcinogenic than trivalent chromium compounds
because Cr(VI) and not Cr(III) is able to rapidly cross the cell membrane through
relatively nonselective anion channels
23
that normally regulate the uptake of phos-
phate and sulfate ions. At physiological pH, Cr(VI) exists as a chromate oxyanion,
which is of a similar size and has the same tetrahedral geometry as phosphate and
sulfate ions under those same conditions (Figure 17.1).
3
The octahedral Cr(III) ions
−
O
O
pH > 7
+6
+6
Cr
Cr
HO
-O
pH < 7
O
O
-O
-O
HCrO
4
-
Hydrogen Chromate
CrO
4
2-
Chromate
L
+3
L
L
Cr
L
L
L
CrL
6
Chromium(III)
Figure 17.1
The major forms of physiologically relevant chromium species
