A study of the electrical impedance of a glass electrode in adverse environments

Abstract

The frequency response of pH-sensitive glass electrodes has been measured over the range 0.01 to 100 Hz using an SCE counter electrode. Observations were made firstly in standard buffers (pH2,4,7 and 9) with electrodes as supplied and secondly with electrodes having membranes which had suffered abrasion or surface contamination. These later observations were intended to simulate effects which might well be encountered when electrodes of this type were used in harsher commercial environments. Surface abrasion was simulated by scratching the surface with emery paper, and surface contamination by smearing it with petroleum jelly or causing a precipitate to form over it. The precipitate employed was that of aluminium hydroxide brought down by the addition of aqueous sodium hydroxide to a solution of hydrochloric acid and aluminium, chloride. The glass electrodes employed were Polymetron type 8404S and 8400B and in a standard buffer the frequency response of the former comprised two semi-circles when plotted in the complex plane and that of the latter a single semi-circle. Impedances on -the real axis ranged from virtually zero at the highest frequency to values of about 300 to 500 Mn 'at the lower frequency, and on the imaginary axis they peaked at values between 150 and 300 M. Precise values varied both between electrode types and between electrodes of the same type. Ageing and PH values were also found to affect impedances, by UP to some 20%, but did not change the shape of the curves. Abrasion had little effect on the impedance characteristics, and petroleum jelly appeared to have a "stopping off" effect, increasing impedance values but not impairing electrode integrity. The precipitate, however, caused severe distortion of both the impedance characteristic and the pH response. It was concluded that frequency response could be developed into a method of electrode integrity testing.