Solvent properties of PVC membranes

173

J. Electronnol. Chem., 266 (1989) 173-174 Elsevier Sequoia S.A., Lausanne - Printed

in The Netherlands

Short communication

Solvent properties of PVC membranes R.D. Armstrong, Hong Wang and M. Todd Depurtment of Chemistry NE1 7R U (Great Britam) (Received

9 February

Be&m

Buildrng, Unroersrty of Ntwcrrsrle upon rune, Newccrstle upon Tsne

1989)

PVC based membranes which are used for the determination of Ca“ are frequently [l] plasticized with o-nitrophenyl octyl ether (NPOE). We report here a study of the dielectric constant of such a membrane as the proportion of PVC to plasticizer is varied. We have also studied the variation of the conductivity of membranes with composition when NaBPh, is dissolved in them. The object of research was the comparison of the properties of these membranes with those which we have studied recently [2] and which were plasticized by di-octyl sebacate (DOS). In the latter case we found that for membranes containing 33% PVC + DOS, Er = 4.8, a value somewhat higher than that for the pure liquid plasticizer (c r = 4.2). However, the most surprising feature of this work was that on going from the pure liquid plasticizer to a 33% PVC + DOS membrane, the degree of dissociation of 1 mM NaBPh, increased from < 2 X 10 ’ to 0.74, probably as a result of the solvation of Na+ by Cl-. This caused a maximum to be found when the conductivity of 1 mM NaBPh, was plotted against PVC content. Measurements of dielectric constant and conductivity were made at 25°C for a series of PVC + NPOE membranes as described earlier [2]. The dielectric constant is shown as a function of composition in Fig. 1, from which it can be seen that c, falls monotonically from its value for pure liquid NPOE of 21 as PVC is added. The fall is particularly sharp between 25% PVC and SO%PVC. It seems probable that this feature is due to the restriction of the free rotation of NPOE molecules as PVC chains are added. The conductivities also fall monotonically with the increase in the proportion of PVC (Fig. 2) so that in this case as distinct from PVC + DOS, the dominant factor is the decrease of ionic mobilities with increasing PVC content. There is. however, a feature which is similar to the PVC + DOS case. For pure NPOE liquid the degree of distortion of 1 mM NaBPh, is - 0.70 whereas for PVC levels of > 21% it is 1 .O. Thus in conclusion it appears that PVC + NPOE membranes behave somewhat similarly to PVC + DOS membranes. However, the degree of dissociation of a 0022-0728/89/$03.50

0 1989 Elsevier Sequoia

S.A.

Ob. WEIGHT%PVC

Fig. 1. Variation

of dielectric

constant

Fig. 2. Variation of conductivity and 2 mM (A) NaBPh,.

with composition

with composition

for PVC + NPOE

for PVC+NPOE

membranes.

membranes

containing

1 mM

(B)

typical salt such as NaBPh, at the 1 mM level is always very considerable as is to be expected from the dielectric constants involved (compared with those for PVC + DOS membranes). For this reason a maximum in the conductivity with increasing PVC content is not observed in this case. REFERENCES 1 W.E. Morf, The Principles of Ion-selective Electrodes and of Membrane Transport, Elsevier, Amsterdam, 1981. 2 R.D. Armstrong, A.K. Covington and W.G. Proud, J. Electroanal. Chem., 257 (1988) 155.