Studies of membrane composition and factors affecting the analytical use of potassium responsive ion-selective electrodes
In this work, optimization of the performance of potassium / PVC electrodes based on valinomycin, with respect to the composition of the membrane, i. e. the ratio of valinomycin to plasticizer, and the type of PVC matrix has been investigated, in an attempt to reduce the cost of the commercially available electrode. The behaviour of a number of PVC electrodes based on a series of novel bis-crown compounds towards potassium and sodium ions has been evaluated. The influence of the internal reference system, particularly the internal reference solution, on the electrode charateristics was examined by varying the concentration and type of the reference solution. The presence of negative sites in the PVC membrane as an origin of the selectivity, maintaining the electroneutrality in the bulk of the membrane, has been investigated by making pH measurements on solutions in contact with the membrane constituents. The anion response of the potassium / valinomycin electrode with the variation of the concentration of valinomycin, type of plasticizer and PVC have been clarified. Automatic calibration and selectivity determination has been carried out on the membrane using a continuous dilution method, and examination of the effect of the liquid junction potential, type of junction and concentration of the bridge solution on the response of the electrode were ascertained. A high concentration of the valinomycin (>1% w/w) and plasticizer (<70% w/w), independent of the type of PVC and plasticizer, was found to show the best performance. The bis-crowns, except bis-naphthol crowns, did not display competitive results in the determination of potassium and sodium ions. The potentials of the internal and external surfaces of the membrane are established separately of one another, and the magnitude of the membrane potential did not simply depend on the ratio of sensed ion activity in the inner and outer solutions. There is however, strong supporting evidence that the presence of impurities in the membrane components are responsible for the selectivity and electroneutrality of the membrane. The anion response of the membrane is shown to be influenced by the nature of membrane and concentration of valinomycin. The source of the bias in the Corning 902 Analyser in relation to flame photometry was explained and the continuous dilution method was applied to the evaluation of the sensor instruments.