Sequential injection method for the direct spectrophotometric determination of bismuth in pharmaceutical products

A spectrophotometric method is reported for the determination of bismuth in pharmaceutical products using sequential injection analysis. Methylthymol blue (MTB) was used as a color forming reagent and the absorbance of the Bi(III)–MTB complex was monitored at 548 nm. The various chemical and physical variables that affected the reaction were studied. A linear calibration graph was obtained in the range 0.0–75.0 mg l^-1 Bi(III) at a sampling frequency of 72 h^-1. The reagent consumption was considerably reduced compared to conventional flow injection systems, as only 150 (mu)l of MTB were consumed per run. The precision was very satisfactory (sr=0.5%, at 50.0 mg l^-1 Bi(III), n=12) and the limit of detection, cL, was 0.250 mg l^-1. The developed method was applied successfully to the analysis of various pharmaceutical products containing Bi(III). The relative errors er, were <1.5% in all cases and were evaluated by comparison of the obtained results with those found using atomic absorption spectrometry as the reference method.