The samples (0.1 mL) were diluted with water (1.4 mL) and after dilution 0.6 mL was added to a buffer solution (2.0 mL). The buffer consisted of 6 mM 1,10-phenanthroline (Sigma Aldrich), 0.6 M sodium acetate (Sigma Aldrich) in 0.18 M sulfuric acid, using demineralized water as solvent. The samples were measured using a UV-VIS Spectrophotometer (UV-2501 PC, Shimadzu) in a 1 cm quartz cuvette at the absorption peak of the formed complex found at 510 nm.