Solutions of Ni, Tl and Zn of concentrations about 1 000 mg/l were prepared by dissolving the pure metals in HNO3, whereas the Se solution was prepared by dissolving sodium selenite in water. The solution concentrations were verified by gravimetric analysis (using biacetyl dioxime for Ni, hydrazine for Se reduction, potassium chromate for Tl and quinaldinic acid for Zn) and by complexometric titration (Ni and Zn). The results of the two methods are close only for zinc. A slight difference between both methods was found with nickel. The gravimetric determination can be regarded as more reliable, the titration being more prone to error due to the inaccurate end point reading. The uncertainty of the concentrations for all solutions attained 1 mg/l, 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 balance and volumetric glassware used.