involved, the movement of Ag+ ions within the membrane being the potential-governing factor.
Selectivity ratios for some of these electrodes are given in Table 6.2.
(c)—
Liquid Membrane Electrodes
Two types have been developed. One utilizes liquid ion-exchangers with a selective response to certain
polyvalent cations or anions. The other involves the selective complexing ability of univalent cations by
neutral macro-cyclic antibiotics and cyclic polyethers. In earlier versions of both types, a water-
immiscible liquid supported on an inert porous plastic disc formed the membrane but gradual leakage
and dissolution of the membrane liquid necessitated the frequent replenishment of a reservoir inside the
electrode body. Later versions employ PVC-gelled membranes incorporating the appropriate liquid and
these behave more like a solid-state membrane, no replenishment of the liquid being required during
use.
Electrodes with liquid ion-exchange membranes are typified by a calcium-sensitive electrode (Figure
6.4). The membrane-liquid consists of the calcium form of a di-alkyl phosphoric acid, [(RO) 2 POO–]
2 Ca2+, which is prepared by repeated treatment of the acid with a calcium salt. The internal solution is
calcium chloride and the membrane potential, which is determined by the extent of ion-exchange
reactions at the interfaces between the membrane and the internal and sample solutions, is given by
Figure 6.4
Calcium-sensitive ion-exchange membrane electrode.