Prune as an oxidation-reduction indicator. Its suitability for titration of ascorbic acid

stimulates interest in other positive oxida- tion-reduction indicators. In particular, the character of the somewhat elusive potentials of ascorbic acid The oxazine "prune" was found to have such a range and seemed to be a substance of general utility for three reasons: (a) prune is readily available as a commercial dyestuff [No. 893, Rowe, 1924], or is easily synthesised from methyl gallate and p-nitrosodimethylaniline, (b) its potential range is such as to minimise fortuitous oxidation of phenols, etc., by the reagent and to provide the greatest specificity consistent with titration of the somewhat weakly reducing ascorbic acid, and (c) its acid-solubility, stability and considerable tinctorial power, as compared with the acid-instability and "rather pale reddish colour" of indophenols [Gibbs et al., 1925] offer considerable advantages where the desired degree of specificity requires the use of acid solutions [Birch et al., 1933]. As other likely indicators (indamines such as toluylene blue [Phillips et al., 1927], amino- indophenols [Cohen and Phillips, 1929], or oxazines such as cresyl blue [Cohen and Preisler, 1931] were chemically unstable in solution, it became desirable to submit the system, prune-reduced prune, to potentiometric investigation.