Buffer solution (pH = 5.4). A mixture of potassium dihydrogenphosphate (0.03 mol/dm3) and disodium hydrogenphosphate (8.99x10-4 mol/dm3) was prepared by dissolving 4.08 g of KH2PO4 (Fluka) and 0.16 g of Na2HPO4·2H2O (Merck) in 1000 cm3 of distilled water.