Four types of lipophilic calixarene ionophores have been used in the design of sodium-selective optode membranes. Plasticized poly(viny1 chloride) (PVC) membranes incorporating the tetraester of calix[4]arene and a neutral H+-selective chromo-ionophore (ETH 5294) have proved to be excellent reversible sensing devices for the optical determination of the sodium cation in pH-buffered solutions. At pH 9.0, the absorbance response of the optode shows a good correlation with the theoretically derived formula in the range from 1 X 10-6 to 10-2 moll-' sodium ion. Dynamic range, reproducibility, response time and the selectivity of the new optode membranes are discussed.
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