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This is due to the fact that most liquid membranes used as ion-selective electrode contain between 60 and 70% by weight of a plasticizer. The latter can be explained by considering that the p Ka of the plasticized PVC is above its glass transition temperature ( Tg) at room temperature.

For all plasticizers used the selectivity for nitrate over chloride (log KN03 a = 2.5) is the lowest, because chloride is the least hydrophilic anion in this series. A more detailed examination of the data in Table 2 reveals that qualitative deviations from the Hofmeister series occur.

In addition to their use as an additive to improve physical properties of solvent polymeric membranes, plasticizers have a considerable impact on the specificity and sensitivity of membrane-modified… Expand PDF View 1 excerpt, cites background Chemical sensors based on field effect transistors : selective recognition of cations and anions

This membrane has been attached physically on top of a pH-buffered hydrogel, which itself has been attached covalently to the gate oxide of the field-effect transistor. The influence of the plasticizer in the PVC membrane on the selectivity pattern of the CHEMFET for the detection of nitrate has been investigated.

Influence of plasticizer on the selectivity of nitrate-sensitive CHEMFETs