Electrodes, null types

The servomotor itself receives its signal from a conventional servo-amplifier, which in turn receives a chopped error signal from a null type pH electrometer. The electrodes which supply the signal to the electrometer are located in the reaction cell. Since the electrometer is of the null-balance type, a preset end point is required and this presupposes a knowledge of the titration curve. This electrometer system has been described previously [3]. 

The inhinsic disadvantage of this method is its two-terminal nature the facts that a dc potential cannot be applied to the electrode of interest with respect to a suitable reference electrode and that the potential e, across the specimen varies during the balance procedure. Since the in-phase and quadrature null signals usually are derived from a PSD, instruments of this type are limited at low frequencies to approximately 1 Hz due to the instability of analog filters with longer time constants. 

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