PhD ThesisIn 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.University of Razi Bakhtaran, Ira