Ion optical sensors are one of the most common sensors in chemistry. They are used for the determination of various ions in aqueous and organic solutions. We report a new ion optical sensor for the determination of La(III) and Cd(II) based on covalent immobilization of H 2 (4-hydroxy salophen) at a traarent cellulose acetate film. Cellulose acetate film had previously been subjected to a base hydrolysis and then H 2 (4-hydroxy salophen) was immobilized on it with or without thiourea. In the frist case, thiourea was used as interface between H 2 (4-hydroxy salophen) and cellulose film and in the other one H 2 (4-hydroxy salophen) is immobilized directly to the cellulose film. The absorption spectra of the optode membrane in the presence of different concentrations of La(III), Cd(II) showed a pronounced changes of absorbance at 434 nm, 431 nm and this property was used as an analytical criteria. The effects of different parameters such as hydrolysis time, immobilization pH, complexation pH, immobilization time and contact time were investigated. Under optimal conditions, this sensor has wide liner response rang of 1.0 × 10 -2 -1.0 × 10 -5 M for Lanthanum(III) and 5.0 × 10 -3 -1.0 × 10 -6 M for Cadmium(II) ions. The response time of the optode thin film was 5 min to reach 98% of final signal. The relative standard deviation for determination of La(III) in the first case were 1.9 %, 4.0 % and 2.0 % for 5.0 × 10 -3 , 8.0 × 10 -4 and 5.0 × 10 -5 M (n=7) respectively and in the second case for Cd(II) were 1.8 %, 3.0 % and 2.4 % for 5.0 × 10 -3 , 5.0 × 10 -4 and 1.0 × 10 -5 M (n=6) respectively. The influence of potential interfering ions on the determination of La(III) and Cd(II) were studied and the main interferents were removed by extraction methods. The sensor was applied for determination of La(III) and Cd(II) in real water samples.
