3.3. Reagents
The PAR stock solution 5.0 =10 mol l was prepared from the sodium salt monohydrate-C-11. H N Na.H O Merck in an ammonium medium NH 0.05 mol l , where it is stable for about 1 month . The solution was kept in a polyethylene flask covered with aluminum foil. The pH 9.0 buffer solution was prepared by 1:5 dilution of a 0.05 mol borax solution. Analyte stock solutions 1.000 g were prepared by diluting Tritrisol Merck or Fixanal Riedel de Haen ampoules as recommended ¨ by the manufacturer. The standard solutions of each complex and the calibration and test mixtures were prepared by diluting the analyte and PAR stock solutions. The reagents were added in the order metal solution–buffer solution–PAR, and then the volume was completed with the buffer solution. PAR concentration in mol l in all mixtures was set equal to three times the sum of the metal concentrations in the most concentrated calibration mixture, A solution containing the buffer and PAR in the same proportion as the mixtures was used as the blank. Analytical grade reagents . and water purified by a Milli-Q Millipore system were used throughout.