Colorimetric determination of isoniazid with 9-chloroacridine

A colorimetric method based on the interaction between isoniazid and 9-chloroacridine was developed. Analytical solutions are shaken for 30 min at 50 degrees, and the absorbance is measured at 500 nm. The procedure is sensitive for isoniazid in the 10(-5) M range. The method was applied to the analysis of isoniazid in pharmaceutical dosage forms and found to be comparable to the USP XVIII assay. Preliminary investigations suggested that the procedure is useful for the determination of free isoniazid in urine and plasma samples containing isoniazid metabolites.