AUTOXIDATION OF p-PHENYLENEDIAMINE 687 75 5O 98 96 50 I00 150 Time (min) Figure 3. Rate of oxygen consumption by p-phenylenediamine (3.4 x 10- M) at various pH and 30øC from pure oxygen at atmospheric pressure (manometric measurement). NH2 (I) slow • + 0 2 •'- + ß O•- NH2 (IV) (IV) NH -- fast 0 -f ß 02 "- -{- H2 02 or or 02 H02- -F H + NH (III) (I)+(III)+2H* • 2(IV) Figare 4. Autocatalytic autoxidation of p-phenylenediamine.

688 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 75 25 112 •08 5O t-lrne Figure 5. Consumption of oxygen from air saturated solutions of p-phenylenediamine (10.0x 10-aM) in closed cells at 30øC and various pH. consumption of only 2• of the p-diamine, most of which will be converted, via the di-imine, to Bandrowski's base. It can be seen that, except at pH 9.6, there is a marked induction period which increases in duration with increasing pH, up to pH 10.4, and then decreases in duration with further increase in pH. Subsequent to the induction period the rate of oxygen consumption increases slightly with increasing pH. Fig. 6 shows first order rate plots for the consumption of oxygen from air and from oxygen saturated solutions at pH 10.4. It can be seen that the reactions are auto catalytic and that the induction period extends to 32 min (42•o reaction) for the oxygen saturated solution and to 80 min (88•o reaction) for the air saturated solution. Subsequently the two curves have the same slope. This result can be explained by invoking the autocatalytic mechanism of Fig. 4. Thus, the reaction is initiated by the slow oxidation of the p- diamine and is dependent on oxygen concentration. The oxidation of semi- quinone to di-imine is rapid and independent of oxygen concentration. The resulting di-imine can then give more semiquinone by interaction with p-diamine to produce the catalysed route.