In treating industrial wastewater containing metals, it is frequently necessary to pretreat the wastewater to remove substance that will interfere with the precipitation of the metals. Cyanide and ammonia form complexes with may metals that limit the removal that can be achieved by precipitation as shown in Fig. 4.10 for the case of ammonia. Cyanide can be removed by alkaline chlorination or other processes such as catalytic oxidation of carbon. Cyanide wastewaters containing nickel or silver are difficult to treat by alkaline chlorination because of the show reaction rate of these metal complexes. Ferrocyanide [Fe(CN)64-] is oxidizes to ferricyanide [Fe(CN)63-], which resists further oxidation. Ammonia can be removed by stripping, break – point chlorination, or other suitable methods prior to the removal of metals.