Problems of Analytical Method of Cyanide Ion in Tap Water and Its Improvement

Abstract

It is prescribed by the Waterworks Law of Japan that a cyanide ion should not be detected in tap water and that its detection should be performed by the pyridine-pyrazolone method after distillation of a sample and absorption of the distillate into a sodium hydroxide solution. However, this method is not reasonable because a cyanide ion has been converted to cyanogen chloride by chlorination of tap water and further the cyanogen chloride has been hydrolyzed to a cyanate ion by the sodium hydroxide solution. Therefore, the pyridine-pyrazolone method was applied directly to tap water without distillation, and the influence of possible factors on color development by the method was tested. Combined residual chlorine (NH₂Cl) and metal ions such as Fe³⁺, Mn²⁺, Zn²⁺, Pb²⁺, Cd²⁺ and Ni²⁺ did not interfere with this method. A cupric ion gave a pinkish brown color. Potassium ferrocyanide and ferricyanide were not colored by the method. When it was doubtful whether the color development was due to cyano gen chloride, air was bubbled through a sample, and the gas released was absorbed into the color-developing solution. On the basis of these observations, it was found that the proposed method was applicable to the determination of cyanogen chloride in tap water.