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

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Abstract
The electrode equation for the oxazine dye, prune (Rowe, 1924, No. 893), has been detd. for the pH range 0-10. As an oxidation-reduction indicator, it usefully occupies the region intermediate between Lauth''s violet and the indophenols (E[omicron]= +0.528 v.). It is most useful in acid ranges, since neutral solns. show colloidal tendencies and become aggregated on keeping. The striking color change from red to blue at pH 2.7-4 is useful in adjusting pH. As a reagent for the titration of ascorbic acid (warmed at pH3 under CO2), it represents the limit to which oxidizing agents may be weakened without hindering successful titration. It is, however, not weak enough to eliminate other biological reducing agents from the titration. Oxidative titration methods must continue to be regarded as having only tentative significance.