Abstract
It has been discussed and proposed by the Central Pharmaceutical Affairs Council of Japan that the maximum allowable concentration of cyanide ion (CN-) in plating waste water should be under 2 ppm. The standard value of 2 ppm is not applied to such complex cyanides as ferrocyanide and ferricyanide, but to the free cyanide which is liberated by aeration through the test solution at pH 5.0 and 50∼55°C, because the toxicity of the complex cyanide is less than onethousandth of that of the free cyanide. The determination procedure of free cyanide ion listed in Japanese Industrial Standard (JISK0102-1964) was modified to improve in several points, for example, pH control of the test solution and the adoption of ball filter in aeration apparatus and 1N NaOH solution as absorbing solution for HCN. The methods for removal of HCN, such as alkali-chlorine method and Fe-complex method, were reviewed shortly.
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