[Ru(edta)(H2O)](-) is strongly adsorbed on a zirconium(IV) oxide-coated silica gel surface, The immobilized complex showed an electrochemical response due to the Ru(II)/Ru(III) redox couple, By substituting the coordinated water molecule in the adsorbed complex, the midpoint potentials shifted in the order (in mV) water, -290; thiocyanate, -200; pyridine,- 180; 4-cyanopyridine, -80; and pyrazine, -50 vs SCE.