Three simple, accurate, and sensitive spectrophotometric methods (A, B and C) have been described for the indirect assay of diltiazem hydrochloride (DIL.HCl), either in pure form or in pharmaceutical formulations. The first method (A) is based on the oxidation of DIL.HCl by N-bromosuccinimide (NBS) and determination of unconsumed NBS by measuring the decrease in absorbance of amaranth dye (AM) at a suitable λmax=521 nm. Other methods (B) and (C) involve the addition of excess cerric ammonium sulfate (CAS) and subsequent determination of the unconsumed oxidant by a decrease in the red color of chromotrope 2R (C2R) at a suitable λmax=528 nm or a decrease in the orange-pink color of rhodamine 6G (Rh6G) at λmax=525 nm, respectively. Regression analysis of Beer-Lambert plots showed good correlation in the concentration ranges 3.0–9.0, 3.5–7.0 and 3.5–6.3 μg ml−1 for methods A, B and C, respectively. The apparent molar absorptivity, Sandell's sensitivity, detection and quantification limits were calculated. The proposed methods have been applied successfully for the analysis of the drug in its pure form and its dosage form. No interference was observed from a common pharmaceutical adjuvant. Statistical comparison of the results with the reference method shows excellent agreement and indicates no significant difference in accuracy and precision.
 C.A. Fisher, M.F. Josephson, Y.T. Wacht-Fogel, R.W. Coleman and V.P. Addonizio: “Verapamil and diltiazem as antiplatelet agents, spectrum and mechanisms of activity”, Circulation, Vol. 68, (1983), pp. 318.
 T. Funyu, K. Nigawara, K. Ohno, W. Hamada and Y. Yagihashi: “Effects of benzothiazepine derivative on blood pressure, excretion of electrolytes and plasma rennin activities”, Clin. Ther., Vol. 3, (1981), pp. 456–466.
 British Pharmacopoeia, Vol. I, Her Majestys Stationary Office, London, 1998.
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