The other redox medium is copper-zinc alloy (brass). It was realized only recently that one reason disinfectant chlorine dissipates so quickly in the plumbing overnight is that the chlorine actively corrodes (oxidizes) the surface of copper pipe and brass fittings, which then liberate ions of Cu+ or Cu++ or Zn++ into the water. (This is also the source of nearly all lead contamination in water-corrosion of brass and solder containing 8% or more lead, used to make the materials machinable and less brittle. Granular brass filter media is made from pure alloy, so there is no lead hazard.) The presence of copper and zinc atoms next to each other in an alloy allows an easy exchange of electrons, and there is a natural tendency for electrons to flow from zinc to copper. If the two were present as separate bars of zinc and copper metal connected by a wire, as in the example in the discussion of Oxidation and Reduction in Section I, the flow of electrons would do work, such as lighting a light bulb. In an alloy no wire is needed, but electrons do not flow spontaneously because there is no place for them to go. But if the alloy is immersed in water containing contaminants like chlorine that can be reduced, the contaminants accept the electrons from the zinc, which then becomes Zn++ ion in solution. Later, further down in the bed of granules, the Zn- ions tend to precipitate out as zinc oxide (ZnO) or hydroxide [Zn(OH)2] and slowly coat the alloy, leaving it less and less effective. This can occur slowly or rapidly, depending on the pH, the amount of chlorine, and the level of dissolved oxygen in the water. Some Cu- ions may be liberated, but this again depends on the levels of chlorine, oxygen, and aciditY. The net result is a low but acceptable concentration of both Zn++ and Cu++ ions in the treated water. The U.S. limits are 5 ppm for zinc (recommended) and 1.3 ppm for copper (mandatory).
The exact electrochemical mechanism is not quite so clear for oxidation of Fe++ or H2S by granular brass filtration. However, the voltages for both of these two oxidations lie between those for copper and zinc in the Electromotive Series (which see, Section I), which means that electrons can go either way (cause either oxidation or reduction) between them. The reduction of hydrogen sulfide is said to proceed by oxidation of copper and zinc, which then precipitate the sulfide as ZnS and CuS for later removal by mechanical fine-filtration.