Saturday, April 11, 2009

Cyanide Solution

The basic formulation for all these baths therefore includes the metal cyanide salt and enough sodium cyanide to form the double salt, together with some excess or free sodium cyanide to assist in dissolution of the metal at the anode. Other salts such as sodium carbonate or nitrate are also often present to increase the electrical conductivity of the solution. At one time potassium cyanide was preferred to sodium, cyanide because of the higher conductivity it conferred, but it is much more expensive, and the conductivity can be increased equally well by using as slightly higher temperature. In some cases it is more convenient to prepare to solution from salts other than the heavy metal cyanide, e.g. in the case of zinc and cadmium, the metal oxide is used with even more cyanide. These react in solution to form the metal cyanide and sodium hydroxide. Some indication formulation are given in the table below.

Cyanide solution are of course poisonous if ingested but they are not absorbed through the skin, nor do they fume. The toxic hazard has thus been found been found by long experiment to be remarkably small. Nevertheless, great care must be taken to avoid cyanides being discharged in the effluents from electroplating plants into sewers or streams, as they are so toxic to animal and fish life, and even to sewage bacteria. The reduction of the cyanide content of waste from plants to a very low level is now a statutory obligation and present problem.

Cyanide solutions are alkaline and can be safely contained in steel tanks, which are cheap. They do not require much chemical control, except for ensuring that a sufficient excess of cyanide is maintained (free cyanide). This can be done by a simple titration or even by observation of the state of anodes. On the other hand, cyanide solution absorb carbon dioxide from the air because of their alkalinity, and form a sodium carbonate.

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