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

Solutions of Cd, Cu and Pb at concentrations of about 1000 mg/l were prepar ed by dissolving the pure metals in HNO3. Their concentration was verified by complexometric titration and by gravimetric analysis (Cu with salicylald oxime, Cd with quinaldinic acid and Pb with 8-hydroxyquinoline). The result s of the two methods were very similar and the uncertainty values were equa l. 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 sol utions from commercial manufacturers. To achieve such precise results, the bias had to be reduced by the highest possible extent, particularly by cali brating both the balance and the volumetric glassware. In addition to the u ncertainty of the basic operations (volumetric and gravimetric), the uncert ainty of the atomic and molecular weights constituted an appreciable compon ent in the combined standard uncertainty; this manifested itself in the det ermination of Pb, where the expanded uncertainty was 2 mg/l.