690 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS recording the visible spectrum of samples of the solution at appropriate intervals. The duration of such studies are limited by the solubility of Bandrowski's base, to the conversion of only 1-2}/0 of the p-diamine. In Fig. 7 the rates of colour development with air and oxygen bubbling are compared at three pH values. Under these conditions we observe an induc- tion period due to the time necessary to build up to a steady state di-imine concentration for the maximum rate of Bandrowski's base formation. Thus, the duration of the induction period increases with increasing pH and decreasing oxygen concentration. The subsequent rate of Bandrowski's base formation increases with increasing pH and increasing oxygen concen- tration. This latter effect can be ascribed to the increasing rate ot• oxidation of p-diamine by molecular oxygen as pH increases. Similar effects are observed when the colour development in 0.01 M p-diamine solutions saturated with air (0.2 x 10 -a M oxygen) is followed in closed cells (Fig. 8). 0.75 II 4 0.50 0,25 9 85 O7 25 50 75 Time (mn) Figure 7. Rate of formation of Bandrowski's base during the oxidation of solutions of •o- phenylenediamine (10.0x 10-aM) at various pH and 30øC with air (broken lines) or oxygen (full lines) bubbling through.

AUTOXIDATION OF tO-PHENYLENEDIAMINE 691 75- 25 25 50 75 l•me (m•n) Figure $. Rate of formation of Bandrowski's base in the oxidation of p-phcnylcncdiamin½ (10.0x 10-a•) by oxygen (oxygen= 0.2x 10•a•) at various pH and 30øC in closed •lls. DISCUSSION The results presented above serve to show the complexity of the autoxi- dation of p-phenylenediamine, and the importance of both the reaction conditions and the method of following the reaction on the subsequent interpretation of the experimental results. The only way to isolate the initial oxidation reaction is to work with very dilute solutions of the p-diamine so as to inhibit subsequent reactions of the resulting di-imine. From such studies we find that the oxidation is auto- catalytic and probably involves a semiquinone intermediate. Measurement of oxygen consumption is generally a more reliable method of determining the rate of oxidation than is colour development. This can be seen by comparing the data for the reaction in closed cells shown in Fig. 6 with that shown in Fig. 8. Thus, oxygen consumption shows the induction period at pH 11.2 to be shorter than that at pH 10.8 while spectrophotometric measurements show a longer induction period for Bandrowski's base formation at the higher pH. This is understandable when it is remembered that the reaction of di-imine with p-diamine is slower at the higher pH and that a larger proportion of the di-imine undergoes