Spectrophotometric determination of oxyphenbutazone in drug formulations using the indophenol reaction

Abstract

On acid hydrolysis, oxyphenbutazone forms N-4-hydroxyphenyl-N′-phenylhydrazine, which undergoes a rapid benzidine rearrangement to produce 2,4′-diamino-5-hydroxybiphenyl. The latter, owing to the presence of a 4-aminophenol moiety, responds to oxidative coupling with phenol in alkaline medium to produce indophenol. Oxygen in the air has been found to be the oxidant. The molar absorptivity is 1.49 × 10⁴ l mol^–1 cm^–1 at 635 nm. Salicylamide, acetylsalicylic acid and phenylbutazone do not interfere. Paracetamol also gives a positive reaction and dipyrone, being a reductant, interferes with oxidative coupling; both should be removed by extraction with dilute sulphuric acid.