Kinetic Analysis of Sb(III): An Experiment for the Quantitative Analysis Laboratory

Abstract

Sb(III) reduces phosphomolybdic acid to molybdenum blue, and, if phosphomolybdic and hydrochloric acids are in excess, the rate of this reaction is linearly proportional to the concentration of Sb(III). This experiment, developed for the undergraduate quantitative analysis laboratory, employed the pseudo-first-order rate law for the phosphomolybdic acid reduction reaction to determine the concentration of Sb(III) in aqueous samples. The rate of the reaction was determined spectrophotometrically by measuring changes in the absorbance of molybdenum blue over time. Students constructed a calibration curve with Sb(III) standards and then evaluated the concentration of an assigned unknown sample containing Sb(III). The median magnitude of the relative error of the values reported by the students was 11.7%. This experiment illustrates applications of kinetics in analytical chemistry, and, though developed for students in a second-year quantitative analysis laboratory course, the experiment could be adapted for students in general, physical, or environmental chemistry courses.