Cyanide ions react with chlorine to form cyanogen chloride, which splits the pyridine ring and forms glutaconedialdehyde. By aldol condensation with barbituric acid a violet polymethine dye is produced.
This test covers free cyanide and cyanide complexes which can be destroyed with chlorine. If interfering substances like heavy metal complexes, thiocyanate, sulphide, dyes or aromatic amines are present, a distillation according to DIN 38 405-D 13–2–2 must precede the cyanide test. For the determination of easily released and total cyanide as well as for the determination of cyanide in stone fruit spirits please contact MACHEREY-NAGEL for special instructions.

Cyanide ions are very toxic because they block the iron of the respiratory enzyme and thus inhibit the oxygen transport. For humans 1 mg cyanide per kg body weight is considered lethal.