Gas chromatographic determination of technical pyrazon (5-amino-4-chloro-2-phenyl-3(2H)pyridazinone)

5-Amino-4-chloro-2-phenyl-3(2H)pyridazinone (common name pyrazon) is the active ingredient of herbicide formulations known under the trade names Burex, Chlorazine, Pyramin, Phenazon. Pyrazon content was evaluated using the internal standard 4-chloro-5-methylamino-2-phenyl-3(2H)pyridazinone. FID detector and glass column (50 × 0.25 cm) packed with 2% cyclohexane-di-methanol succinate on silanised Chromosorb G (80-100 mesh) were used. Technical products can contain two interesting impurities, the non-active pyrazon isomer 4-amino-5-chloro-2-phenyI-3(2H)pyridazinone and the unreacted 4,5-dichloro-2-phenyl-3(2H)pyridazinone, the later of which issignificant from the point of view of toxicology. Determination of these two compounds under the conditions used for pyrazon is practically impossible. They were therefore determined on a separate glass column (70 × 0.24 cm) packed with 5% cyclohexane-di-methanol succinate on Chromosorb G (60-80 mesh). A standard addition method for small concentrations of 4,5-dichloro-2-phenyl-3(2H) pyridazinone in technical pyrazon products is also described.