Direct Microspectrophotometric Determination of Bismuth in Silver with Semi-Xylenol Orange

75–300ng of bismuth in a sample mass of 20mg of silver can be determined spectrophotometrically in a nitric acid medium (0.18molL^–1) with Semi-Xylenol Orange as chromogenic agent. Its selectivity is realized by using less toxic reagents in very small amounts to mask some of the interfering impurity cationic species while the matrix effect is negligible. The conditional formation constant of the colored chelate formed was found to be lgK_f=3.1. It exhibits a maximum at 540nm with a molar absorptivity of 4.2×10⁴Lmo1^–1cm^–1. The linear regression equation for this determination is A=0.022+0.0018C, where C stands for the concentration of Bi (ng per 0.5mL) with a correlation coefficient of 0.9873. The RSD at the level of 200ng of Bi (n=10) was found to be 5%.