In the first stage, this system uses sulfuric acid to lower the pH to approximately 2.5, speeding the reduction reaction and ensuring complete treatment. The most commonly used reducing agents are sulfur dioxide, metabisulfite, and ferrous sulfate, but other reducers can also be used. The reducing agents react and form precipitates as shown in Table 7.42.3.
The following equation describes the reduction reaction with chrome expressed as chromic acid CrO3, which has a +6 charge on the chromium. The reducing agent is expressed as sulfurous acid (H2SO3), generated by sulfites at a low pH. The result is chromium sulfate, Cr2(SO4)3, which has a +3 charge on the chromium, as follows:
Equation 7.42(9) describes the reaction when sulfur dioxide is the reducing agent.
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