Firststage Reaction

The first-stage reaction equation, assuming a sodium cyanide waste and liquid chlorine oxidizing reagent, is as follows:

The pH level of the reaction must be around 9 to preclude the formation of toxic gases such as cyanogen chloride (COCl) and hydrogen cyanide (HCN). When cyanide is in excess, which occurs when the untreated wastewater leaves the plant, the following reduction potential applies:

CNO— + H2O + 2e ® CN— + 2OH— 7.41(15) oxidized ® reduced

This reaction has a standard potential of —0.97 V with respect to the standard hydrogen electrode. Assuming a Ag-AgCl (4M KCl) reference electrode (see Table 7.41.2) gives the following:

The ratio of oxidants to reductants can be written using Equation 7.41(15) as follows:

Substituting the value of E0 already calculated and Equation 7.41(16) into the Nernst equation gives the following:

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