Spectrophotometric Assay of Some Antipsychotropic and Anticholinergic Phenothiazine Drugs Using Ammonium Molybdate

Abstract

A simple, rapid and sensitive spectrophotometric method has been described for the quantitative determination of six phenothiazine-based antipsychotropic and anticholinergic drugs in pure form as well as in formulations. The method is based on the formation of stable phenothiazonium free radical cation by the use of ammonium molybdate as chromogenic agent. The drugs in 3.0 to 5.0M H₂SO₄ medium when heated in a boiling water bath react with molybdate to give intense orange or purple or blue products which possess characteristic absorption maxima at 500–638 nm. Beer's law is obeyed over the concentration range 1–60 μg.ml⁻¹ with apparent molar absorptivities ranging from 4.37x10³-9.61 × 10³ l. mol⁻¹. The proposed method has been applied successfully for the determination of examined drugs both in pure form and representative pharmaceutical formulations. The accuracy and precision of the assay method are comparable with those of the official method.

Key Words:

• Phenothiazines
• Assay
• Spectrophotometry
• ammonium molybdate
• pharmaceutical application