Preparation and testing of standard solutions of cadmium, copper and lead

Abstract

Solutions of Cd, Cu and Pb at concentrations of about 1000 mg/l were prepared by dissolving the pure metals in HNO3. Their concentration was verified by complexometric titration and by gravimetric analysis (Cu with salicylaldoxime, Cd with quinaldinic acid and Pb with 8-hydroxyquinoline). The results of the two methods were very similar and the uncertainty values were equal. However, the gravimetric determination can be regarded as more reliable, because titrimetric analysis is more prone to error due to inaccurate end-point reading. The expanded uncertainty of the concentrations of Cd and Cu attained 1 mg/l, which is half the value typically obtained for similar solutions from commercial manufacturers. To achieve such precise results, the bias had to be reduced by the highest possible extent, particularly by calibrating both the balance and the volumetric glassware. In addition to the uncertainty of the basic operations (volumetric and gravimetric), the uncertainty of the atomic and molecular weights constituted an appreciable component in the combined standard uncertainty; this manifested itself in the determination of Pb, where the expanded uncertainty was 2 mg/l.