Sulfate acid

All chemicals and solvents used in the experiment were of analytical reagent grade. A 0.01 mol L⁻¹ standard solution of potassium dichromate was prepared by dissolving the exact sample K₂Cr₂O₇ (Sigma-Aldrich, Germany) in deionized water. A working solution with 0.001–0.000001 mol L⁻¹ concentration of potassium dichromate was prepared daily by dilution of a standard stock solution with deionized water. We used a 5.0 mol L⁻¹ solution sulfuric acid, which was obtained by dilution of concentrated sulfate acid (Sigma-Aldrich, Germany). Solution of potassium iodide was prepared by dissolving the exact sample KI (Sigma-Aldrich, Germany) in deionized water. A 0.001 mol L⁻¹ standard solution of Astrazone Brilliant Red 4G (ABR) was prepared by dissolving the exact sample ABR (Spectrum Chemical MFG Corp., USA), in mixtures ethanol and deionized water.