Kinetic-spectrophotometric determination of low cyanide concentrations in the presence of large amounts of thiocyanate
A kinetic-spectrophotometric method to determine cyanide based on the reaction between CN– and Mn(CN)5NO2– ions with measurement of the absorbance at 384 nm is proposed. The optimum pH is 10.0. The effect of temperature is not critical, the activation energy of the reaction being 12.9 kcal mol–1. Using the method of tangents the range of linearity was 0.1–1.3 mg l–1 of CN– with a coefficient of variation of 1.8% for n= 7 and 0.8 mg l–1 of CN– and a detection limit of 0.06 mg l–1 of CN–. By applying the fixed time method at 15 min, the coefficient of variation and detection limit were 1.0% and 0.04 mg l–1, respectively. The thiocyanate ion does not interfere. Other important interferents in Aldridge's method, such as bromide, iodide and nitrite ions, are minimised. Good results were obtained for the determination of cyanide impurities in analytical-reagent grade potassium thiocyanate reagent, the mean recovery being about 98%.