A highly selective optical sensor was developed for the Hg2+ determination by chemical immobilization of 2-[(2-sulfanylphenyl)ethanimidoyl]phenol (L), on an agarose membrane. Spectrophotometric studies of complex formation between the Schiff's base ligand L and Hg2+, Sr2+, Mn2+, Cu2+, Al3+, Cd2+, Zn2+, Co2+ and Ag+ metal ions in methanol solution indicated a substantially larger stability constant for the mercury ion complex. Consequently, the Schiff's base L was used as an appropriate ionophore for the preparation of a selective Hg2+ optical sensor, by its immobilization on a transparent agarose film. A distinct color change, from yellow to green-blue, was observed by contacting the sensing membrane with Hg2+ ions at pH 4.5. The effects of pH, ionophore concentration, ionic strength and reaction time on the immobilization of L were studied. A linear relationship was observed between the membrane absorbance at 650 nm and Hg2+ concentrations in a range from 1 x 10(-2) to 1 x 10(-5) mol L-1 with a detection limit (3 sigma) of 1 x 10(-6) mol L-1. No significant interference from 100 times concentrations of a number of potentially interfering ions was detected for the mercury ion determination. The optical sensor was successfully applied to the determination of mercury in amalgam alloy and spiked water samples. (C) 2010 Elsevier B.V. All rights reserved.