Improved spectrophotometric assay of cyanide with picric acid

An improved spectrophotometric assay of hydrocyanic acid and cyanide ions with picric acid and resorcinol is described. Cyanides at low concentrations in water (0-10 mug mL(-1) CN-) and hydrocyanic acid collected in dilute sodium hydroxide solution react with a solution of 2% sodium carbonate containing 210 mug mL-1 of picric acid and 100 mug mL(-1) of resorcinol to form a novel indophenol compound, 3-hydroxy-4-(2-hydroxy-4-oxo-cyclohexa-2,5-dienylideneamino)-2,6-dinitro-benzonitrile, which is red-brown in color with a maximum absorption at 481 nm. The color system obeys Beer's law at 481 nm in the range of 0.1-10 mug mL(-1) CN-, in which a maximum molar absorptivity of 8.31 x 10(3) 1 mole(-1) cm(-1) has been evaluated. Also, the molar absorptivity at 516 nm of the new dye proved to be several times higher than that of the red-colored compound obtained from cyanide and picric acid only. Spectral and elemental analyses confirmed the structure of the new dye and a reaction mechanism is discussed.