Oxidation‐Reduction Potentials Measured with the Dropping Mercury Electrode: Part I . Studies with Quinhydrone and Estimation of Limits of Measurement

A slowly dropping mercury electrode is shown to function as a reliable indicator electrode of oxidation‐reduction potentials which are sufficiently negative. The potentials are instantaneously established on the constantly renewed, fresh surface of mercury and are not influenced by any previous polarization of the electrode. If ions with which mercury forms complexes or precipitates are absent or sufficiently dilute, even potentials as positive as quinhydrone may be measured accurately. Experiments with quinhydrone and the dropping mercury electrode were carried out in buffered solutions of a pH ranging between 2 and 8. Studies were also made on the effect of dilution of quinhydrone and on weakly buffered solutions. The most positive potential which can be measured reliably may be predicted from a consideration of the solubility product and concentration of the precipitant on the basis of experiments on the effect of chloride ion on the quinhydrone potential. A test verifying such a prediction has been made in the case of methylene blue, the potential of which was altered by the addition of sulfide ions.