State of oxidation-reduction indicators in solution

Summary

• 1. It was shown that the method of determining protolytic dissociation constants of reversible oxidation-reduction systems with the aid of cells operating without transfer formed by oxidation-reduction and hydrogen electrodes is theoretically and experimentally sound. This method differs from Clark's method in having greater thermodynamic rigor, permitting more convenient interpretation of results, and being more accurate.

• 2. The method was applied to the determination of protolytic dissociation constants of leuco methylene blue, toluidine blue, and leuco toluidine blue.

• 3. The proposed method made it possible to correct values of protolytic dissociation constants obtained previously for leuco thionine by Clark's method.

• 4. It was found that change in the ionic strength of the solution has no appreciable effect on the emf of the above-mentioned cell.

• 5. Spectrophotometric measurements confirm the results of potentiometric measurements.

• 6. Diagrams were constructed showing the dependence on pH of the distribution of leuco dyes, and also of their additive properties (optical density), among the various products formed in equilibrium by their dissociation.