Glass pH electrodes with improved temperature characteristics: use of a low-impedance pH sensor as the inner reference electrode

Abstract

Glass electrodes are generally preferred to the various types of low-impedance pH electrodes, despite their greater susceptibility to electrical noise and need for a better grade of electronics. This preference arises from the reversibility of the glass electrode and its freedom from interference by redox and complexing agents, with the result that it has an ideal (Nernstian) response over a wide pH range. pH meters apply a linear correction for changes in temperature, but for most electrodes this is a good approximation only over a range of ±10 °C, because of the properties of the solution inside the glass electrode. By use of a low-impedance pH electrode in controlled (interference-free) conditions inside the glass electrode, it was hoped to eliminate the variation of the internal pH as a factor in temperature compensation. A preliminary assessment of various candidate electrodes indicated that the mercury-mercury(II) oxide electrode was the best choice. Glass electrodes with Hg-HgO inner electrodes had good general characteristics, i.e., their slope factors and standard potentials agreed with theory, and their temperature characteristics were better than those of some commercial pH electrodes, being linear from 10 to 50 °C. Deviation from linearity was, however, observed at lower temperatures. Because of the limitations of many pH meters, it is desirable that electrodes have the characteristic that both the isopotential pH (pH_iso) and the pH at which the e.m.f. of the cell is zero (pH°)= 7, i.e., the e.m.f. at pH 7 is 0 mV at all temperatures. With the Hg-HgO inner electrode, however, pH_iso= 3.8 and pH°= 11.3.