The Two-Step Oxidation-Reduction of Phthiocol

Abstract

Having found that semiquinones can be formed from the anthraquinone sulfonates 1 in alkaline solution, it was suspected that phthiocol, a pigment contained in the human tubercle bacillus, isolated by Anderson and Newman 2 and proved by them to be 2-methyl-3-hydroxy-1, 4 naphthoquinone, 3 , 4 might behave similarly. Ball 5 studied the electrochemical behavior of phthiocol at pH's ranging from 1.12 to 12.56 and reported no indication of a 2-step oxidation.

Our suspicions were partially confirmed colorimetrically when solutions of phthiocol at pH > 10 were reduced slowly and an intermediate violet color was observed. Solutions of phthiocol were titrated reductively over a pH range from 7.53 to 14.3 (3M-NaOH), and it was found from an analysis of the titration curves that the separation of the 2 steps begins at about pH 9.0 and is more distinct at greater pH's. The separation never becomes so great as to produce a jump of the potential at 50% of the whole titration. The maximum amount of the ratio semiquinone: total dye is about 50% at pH 13.4-14.3. The E_o curve (midpoint potentials of the system: fully oxidized-fully reduced dye) changes from a slope of .06 to .03 at pH 11.5 (pK_r2) and from .03 to 0.0 in the neighborhood of pH 14.3 (pK_r3). The curves of the midpoint potentials of the systems : semiquinone form-fully reduced form, and totally oxidized form-semiquinone form (E₁ and E₂, respectively as per Elema 6 ) cross the E_o curve at pH 12.9, at a potential of - -.540 volt.