Two simple, sensitive and specific fluorimetric methods have been developed for the determination of some sulphur containing compounds namely, Acetylcysteine (Ac), Carbocisteine (Cc) and Thioctic acid (Th) using terbium Tb+3 and uranium U+3 ions as fluorescent probes. The proposed methods involve the formation of a ternary complex with Tb+3 in presence of Tris-buffer method (I) and a binary complex with aqueous uranyl acetate solution method (II). The fluorescence quenching of Tb+3 at 510, 488 and 540 nm (lambda(ex) 250, 241 and 268 nm) and of uranyl acetate at 512 nm (lambda(ex) 240 nm) due to the complex formation was quantitatively measured for Ac, Cc and Th, respectively. The reaction conditions and the fluorescence spectral properties of the complexes have been investigated. Under the described conditions, the proposed methods were applicable over the concentration range (0.2-2.5 microg ml(-1)), (1-4 microg ml(-1)) and (0.5-3.5 microg ml(-1)) with mean percentage recoveries 99.74+/-0.36, 99.70+/-0.52 and 99.43+/-0.23 for method (I) and (0.5-6 microg ml(-1)), (0.5-5 microg ml(-1)), and (1-6 microg ml(-1)) with mean percentage recoveries 99.38+/-0.20, 99.82+/-0.28 and 99.93+/-0.32 for method (II), for the three cited drugs, respectively. The proposed methods were successfully applied for the determination of the studied compounds in bulk powders and in pharmaceutical formulations, as well as in presence of their related substances. The results obtained were found to be in agree statistically with those obtained by official and reported ones. The two methods were validated according to USP guidelines and also assessed by applying the standard addition technique.

A simple, sensitive and validated kinetic spectrophotometric method for the determination of certain mucolytic drugs namely acetylcysteine (Ac) and carbocisteine (Cc) is described. This method is based on the kinetic investigation of the reaction between the two cited drugs and 4-chloro-7-nitrobenzo-2-oxa 1, 3 -diazole (NBD-Cl) in an alkaline medium. Spectrophotometric measurements were achieved by recording the absorbance at 424 nm at ambient temperature (25 0 C ±5) for a fixed time of 30 minutes for Ac, and at 468 nm for a fixed time of 15 minutes at 70 0 C for Cc. All variables affecting the development of the reaction were investigated and optimized. The method was found to be linear over the range of 2-22 μg ml -1 for Ac, and 5-35 μg ml -1 for Cc with mean percentage recoveries of 99.59 ± 0 .41 and 99.50 ± 0.65, respectively. Molar absorpitivity and sensitivity index were determined and found to be (1.094x10 3 and 1.49x10 -2 , respectively, for Ac, and 2.547x10 3 and 3.05x10 -2 , respectively for Cc. The validity of the method was assessed according to USP guidelines. The proposed method was successfully applied for the determination of the two cited drugs in bulk powder, in pharmaceutical formulations as well as in the presence of their related substances. The results obtained were found to agree statistically with those obtained by official and reported methods. In this study the determination of the two drugs by the fixed time method proved to be more applicable than the fixed absorbance and rate constant methods.