Estimation of cyanide with alkaline picrate

Abstract The standard agronomic test for cyanide by reaction with alkaline picrate to give a red complex, both on picrate impregnated paper strips suspended above the cyanide source in a closed system and in free solution, has been examined in detail. Studies on the nature of the standard curves and the effects of temperature, time and pH on absorbance by the red complex at 510 nm suggest that reaction of cyanide with picrate proceeds irreversibly to give the red complex which then undergoes further, reversible reaction to give a compound which absorbs little or not at all at 510 nm. Both reactions are favoured by increased pH and temperature. In the light of these results the validity of procedures described in the literature for quantitative estimation of cyanide with picrate has been critically discussed.