Mechanism for the high bias in the USP colorimetric assay for diazotizable substances in hydrochlorothiazide.
The USP XX colorimetric assay for the determination of diazotizable substances in hydrochlorothiazide was studied. Colorimetric assay results of hydrochlorothiazide bulk powder and captopril-hydrochlorothiazide combination tablets were found to have a high bias when compared with HPLC. A kinetic study of the diazotization step in the colorimetric assay and extrapolation of the free amine content, i.e., 4-amino-6-chloro-1, 3-benzenedisulfonamide, to time zero provided results which correlated favorably with those obtained by HPLC. The high bias of the colorimetric assay was shown to result from the formation of 4-nitroso derivative of hydrochlorothiazide, which is formed by acid hydrolysis during the diazotization step. Bendroflumethiazide and flumethiazide also showed increasing free amine content when the time of diazotization was increased.
D000588:Amines / Q000032:analysis; D002216:Captopril / Q000032:analysis; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003124:Colorimetry / Q000379:methods; D006852:Hydrochlorothiazide / Q000032:analysis*; D007700:Kinetics; D009603:Nitroso Compounds / Q000032:analysis; D011208:Powders
D000588:Amines; D009603:Nitroso Compounds; D011208:Powders; D006852:Hydrochlorothiazide; D002216:Captopril