**2.1 Materials**

OCP was obtained from Hopkin & Williams (Dagenham, Essex, UK), Sulfuric acid, K2HPO4, KHphthalate, Borax, NaOH, Hydrochloric acid and NaHCO3 were provided by Merck (Darmstadt, Germany). OHP and MB dye was provided by Aldrich. All chemicals are of analytical pure grade and used as received. All solutions were prepared by using freshly double-distilled water
