Couples in the Titration of Acids and Bases

Continuing the investigations previously reported upon, a large number of metal pairs were tried out for the potentiometric titration of acids and bases, and for the estimation of ammonia and other nitrogen bases. The potential developed by a metal in acid solution is not the same as that developed in basic solution. This difference in the two potentials is large and abrupt in the case of W, Pt, Sb, and Mo which serve well as "indicator" electrodes. In the case of other metals this difference is comparatively slight. However, a metal of this second class when coupled with a good indicator electrode makes a serviceable pair for titration purposes. W as an indicator electrode is better in the titration of an acid with a base than in the reverse titration. The final choice of a suitable galvanic pair depends not only upon the potentials developed but also on the solubility of the metals in the titrating solutions. Accordingly, the most serviceable pairs for acid‐alkali titrations were W‐graphite, Sb‐graphite and W‐Ag. A small amount of quin‐hydrone makes the W‐Ag pair even more sensitive. In general the potentials were quite constant and equilibrium was attained quite rapidly. No satisfactory explanation is found why one couple serves well in potentiometric titration and another does not.