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, consistent to the procedure reported. The samples were measured using a UV-VIS spectrophotometer (UV-2501 PC Shimadzu or Horiba Scientific Duetta) in a 1 cm cuvette and the peak absorption of the complex at 510 nm was noted and used to determine the conversion, using calibration curves.