Determination of Copper and Zinc in Brass: Two Basic Methods
Résumé
In this experiment, the concentrations of copper and zinc in brass are obtained by two methods. This experiment does not require advanced instrumentation, uses inexpensive chemicals, and can be easily carried out during a 3-h upper-level undergraduate laboratory. Pedagogically, the basic concepts of analytical chemistry in solutions, such as pH, solubility, complexation, and redox reactions, are illustrated. In addition, this experiment can be used to introduce the science of alloys. Oxidation of brass pieces by concentrated nitric acid yields a solution of Cu2+ and Zn2+ cations. The first method consists in an acid−base titration of Cu2+ and Zn2+ cations by sodium hydroxide. The successive precipitations of the corresponding hydroxide complexes are observed in a pH titration curve that allows the calculation of the concentrations of Cu2+ and Zn2+ and thus the brass composition. The second method is based on complexometric titrations with and without the competition between complexing agents, steps 1 and 2, respectively. In step 1, the copper and zinc together are titrated by EDTA. In step 2, the addition of sodium thiosulfate masks the Cu2+ and the titration by EDTA leads to the determination of Zn2+ concentration. Cu2+ concentration and brass composition are readily obtained.
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