Abstract
In the presence of cyanide ion, copper (II) ion is quantitatively reduced to stable cyano complexes of copper (I) in aqueous solution. If ο-cresolphthalin is present in the solution, it is oxidized to ο-cresolphthalein, and the color turns purplish red in alkaline solution. This color change has been used by Nicholson to determine hydrocyanic acid in cyanogenetic plants. However, the conditions necessary for stability and reproducibility of the color have not been established. Therefore, the optimum conditions for the operation at each stage of the procedure were investigated, and the following procedure was established.
A neutral sample solution containing 05μg of cyanide ion is taken in a 25 ml volumetric flask, and 1 ml of 0.2 per cent ο-cresolphthalin in 0.1 N sodium hydroxide solution, 1 ml of ethanol and 1 ml of 0.039 per cent copper (II) sulfate pentahydrate solution are added. It is made up to the mark with water, and allowed to stand at 30°C for 30 minutes. The absorbance of the colored solution is measured at 568 nm with a 1-cm cell against the reagent blank.
The apparent molar extinction coefficient based on cyanide ion is about 6.5×104 l/mol cm, and the sensitivity of this method is inferior to the pyridinepyrazolone method, i.e., 90 per cent of the latter. This procedure is applied easily for the determination of cyanide ion in the samples free from interfering ions such as Co2+, Ni2+ which form complexes with cyanide ion.
