Journal of Pharmaceutical and Biomedical Analysis 28 (2002) 527–535 Spectrophotometric determination of metronidazole and tinidazole in pharmaceutical preparations P. Nagaraja *, K.R. Sunitha, R.A. Vasantha, H.S. Yathirajan Department of Studies in Chemistry, Uniersity of Mysore, Manasagangotri, Mysore 570 006, India Received 14 July 2001; received in revised form 3 October 2001; accepted 6 October 2001 Abstract Sensitive and simple spectrophotometric methods for the determination of metronidazole (MNZ) and tinidazole (TNZ) in either pure form or in its pharmaceutical formulations are described. The first method is based on the interaction of 3-methylbenzothiazolin-2-one hydrazone (MBTH) with MNZ/TNZ (reduced drug) in presence of copper sulphate and pyridine in acidic medium. The resulting yellowish orange products have  max of 500 and 490 nm, respectively, for MNZ and TNZ and are stable for about 4 h. The second method describes the reaction between reduced diazotised drugs with N-(1-naphthyl)ethylenediamine dihydrochloride (NEDA) in neutral medium to yield pink products which have  max of 520 and 505 nm, respectively, for MNZ and TNZ, respectively. The products are stable for more than 24 h. Common excipients used as additives in pharmaceutical preparations do not interfere in the proposed method. Both the methods are highly reproducible and have been applied to a wide variety of pharmaceutical preparations and the results compare favourably with those of official methods. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Metronidazole; Tinidazole; MBTH; NEDA; Diazotisation; Spectrophotometry www.elsevier.com/locate/jpba 1. Introduction Metronidazole (MNZ) (2-methyl-5-nitroimida- zole-1-ethanol) and tinidazole (TNZ) (1-[2-(ethyl- sulphonyl)ethyl]-2-methyl-5-nitro-1H-imidazole) are used as antiprotozoal, antiamebic and an- tibacterial drugs. Excellent reviews have been published on the activity and pharmacokinetics of these drugs [1–3]. A survey of literature reveals that there are various methods available for the determination of MNZ and TNZ which includes volumetric [4], gravimetric [5], polarographic [6], gas-chromatographic [7], TLC [8], HPLC and HPTLC [9–14], voltammetric [15], derivative spectrophotometry [16 – 19], flow injection analy- sis [20], official methods [21 – 23] and spectropho- tometry [24 – 55]. Most of the spectrophotometric methods reported suffer from the disadvantages like narrow range of determination, requires heat- ing or extraction, long time for the reaction to complete, use of non-aqueous systems, stability of the coloured product formed, etc. Moreover, ma- * Corresponding author. Tel.: +91-821-541475/412557; fax: +91-821-421263. E-mail address: nagarajap@mailcity.com (P. Nagaraja). 0731-7085/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0731-7085(01)00685-9