Transient Response of the Glass Electrode to pH Step Variations

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Abstract
A dc amplifier with electrometer input stage is described making it possible to display pH changes on the oscilloscope screen for physiological applications. Sensitivity is 0.003 pH unit per cm beam deflection; noise 50 μv; drift 300 μv/hr(≃0.005 pH unit). A simple hydraulic mechanism is used to produce very fast pH steps under the electrode. The speed of the electrode response is shown to tend towards a finite limit when the speed of the pH change increases. The form of the transient in the fastest records is close to an exponential curve e−αt, with 1/α≃30 msec. The response is independent of the membrane thickness (5 to 10 μ). Some evidence exists to show that the response of the electrode to pH steps is analogous to the response of a low‐pass RC‐filter to a corresponding voltage step, where R and C are, respectively, the resistance and the capacity of the glass membrane. R and C values determined from the analysis of the electrode response to step voltages in series give RC≃30 msec in agreement with the experimental results.

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