Reduction can be illustrated by the reaction in which toxic hexavalent chrome (Cr6+) is reduced to the trivalent form (Cr3+). In the latter, toxicity is reduced by a factor of about 100. The Cr3+ is then precipitated as the hydroxide and removed as a sludge. The following equation gives the half-cell reaction in reduced form:
This reaction has a reduction potential of roughly 1.33 V with respect to the standard hydrogen electrode. The conversion time of Cr6+ + Cr3+ is pH dependent, as shown in Figure 7.41.3. To keep the reaction time under 30 min
Cr2O7 + 14H
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