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.

AUTOXlDATION OF •O-PHENYLENEDIAMINE 689 2O i I I 25 50 75 [trne (min) Figure 6. First order rate plots for oxygen consumption in solutions of p-phenylenediamine (10.0X 10-aM) in closed cells at pH 10.4 and 30øC. If di-imine were not consumed by other reactions, a simple kinetic pattern might emerge. However, di-imine is consumed by hydrolysis and in the formation of Bandrowski's base. SPECTROSCOPIC TECHNIQUES We have shown previously (5) that by using very dilute (0.1 x 10-a•d) solutions of the p-diamine, its oxidation to di-imine can be followed spectrophotometrically. Such studies showed that the rate of formation of di-imine, subsequent to an induction period, increased with increasing pH over the range 8.50-11.20. Further studies have now shown that the dura- tion of the induction period decreases as oxygen concentration increases while the rate of the subsequent reaction is independent of oxygen con- centration. Similarly, provided the reaction conditions are such that Bandrowski's base is not formed, the duration of the induction period is decreased as the initial p-diamine concentration is increased. These results again point to an autocatalytic mechanism involving the semiquinone. We have also studied the early stages of the oxidation of p-diamine (0.01 M) by bubbling air or pure oxygen through the solution and