Compounds: Na2HPO4, NaH2PO4, NaOH, [Ru(NH3)6]Cl3, H3PO4, quinoline (Q) and 1,2,3,4tetrahydroquinoline (1,2,3,4-Q) of the highest purity were supplied by Merck and were used without further purification. All solutions used in electrochemical measurements were prepared with deionised water (resistivity of 18.2 MΩ.cm at 25°C) passed through a Sartorius purification system. The stock standard solution (25 mM) of the 8AQ was prepared by dissolving accurately weighed amounts of 8AQ in 0.1 M H3PO4 solution. Dilute solutions were prepared by dilution of the stock solution with mixture of 0.1 M NaH2PO4 and 0.1 M Na2HPO4. All solutions were stored in a dark place under an argon atmosphere.