4oh O2 2h2o 4e

in alkaline solutions.

There are exceptions to this rule. The platinum metals are soluble in hot halogen acids (HF, HCl, HBr) and will dissolve anodically under these conditions. Similarly, oxidizing ligands such as nitrate and nitrite tend to dissolve PGMs, particularly in the presence of halogen acids. Plating solutions based on such systems are highly corrosive, and it is usually necessary to protect the work to be plated by prestriking with gold. Platinum-group metal anodes are also soluble in molten cyanide systems, from which PGMs can be deposited to very heavy thicknesses. Molten cyanide systems operate under an argon atmosphere at temperatures of about 600 °C (1100 °F), and for these reasons are not widely used. They are useful for heavy deposition because the high temperature provides some degree of stress-relief annealing during the plating operation.

Because anodes fabricated from PGMs are inert in most aqueous environments, they are useful not only for the electrodeposition of PGMs but also for plating of other metals, such as gold. Platinum is the metal of choice for such applications and is available in the form of wire mesh, or plated onto anodizable metals such as titanium, or clad onto passive-prone metals such as niobium or tantalum. In the plated and clad configurations, the required mechanical strength is provided by the substrate, and the actual amount of platinum used is quite small.

Reference 39 is a good general resource of information about anode selection and general plating practices.

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