Preparation and testing of standard solutions of nickel, selenium, thallium, and zinc

Solutions of Ni, Tl and Zn of concentrations about 1000 mg.l(-1) were prepa red by dissolving the pure metals in HNO3, whereas the Se solution was prep ared by dissolving sodium selenite in water. The solution concentrations we re verified by gravi- metric analysis (using biacetyl dioxime for Ni, hydra zine for Se reduction, potassium chromate for Tl and quinaldinic acid for Z n) and by complexometric titration (Ni and Zn). The results of the two meth ods are close only for zinc. A slight difference between both methods was f ound with nickel. The gravimetric determination can be regarded as more rel iable, the titration being more prone to error due to the inaccurate end po int reading. The uncertainty of the concentrations for all solutions attain ed 1 mg.l(-1), which is half of that typically given for similar solutions by manufacturers. To achieve such precise results, bias must be eliminated to the highest possible extent, particularly by calibrating both the balanc e and volumetric glassware used.