NITRATE ION-SELECTIVE ELECTRODES BASED ON QUATERNARY PHOSPHONIUM SALTS IN PLASTICIZED POLY(VINYL CHLORIDE) AND INFLUENCE OF MEMBRANE HOMOGENEITY ON THEIR PERFORMANCE

Nitrate ion-selective electrodes based on quaternary phosphonium salts in plasticized poly(vinyl chloride) (PVC) were found to be superior in stability and sensitivity to those based on quaternary ammonium salts in the same matrix and to Orion 93-07 electrodes. The difference in the characteristics of the electrodes tested was attributed to the different extent of homogeneity existing in the sensing membrane. Scanning electron microscopy revealed that quaternary phosphonium salts were homogeneously dispersed in the sensing membrane because of some specific interaction with the plasticized PVC. On the other hand, the other sensors tested were segregated to various extents. This specific interaction of quaternary phosphonium salts with the plasticized PVC, resulting in a homogeneous sensing membrane, was considered to be the cause for the improved characteristics of the nitrate electrodes. In line with this, additional rules for constructing ion-selective electrodes in non-porous polymer membranes are proposed.