CORRESPONDENCE Surface Films on Glass Membrane Electrodes zc --
SIR: The presence of a film of hydrolyzed glass at the surface of a glass membrane electrode may exert a considerable effect on the mechanism of response of such an electrode. In particular, mass transport of ions across the membrane may be rate-limited by the surface film. It is therefore important that the properties of a surface film on glass should be well understood so that allowances can be made in quantitative treatments of electrode behavior. We have proposed ( I ) that the surface film on glass can be represented as an equivalent circuit comprising a semiinfinite Warburg diffusional impedance (2) in parallel with a large fixed resistance. Buck (3) has suggested that the surface film corresponds to a finite transmission line showing a phase angle of 45" at high frequencies and a decrease in phase angle as the frequency decreases. The presence of a fixed resistance in series with the transmission line was suggested to account for measured phase angles of less than 45 O at high frequencies. There is a third possibility which must be considered because film impedances are observed experimentally only at very low frequency ac signals (typically
