882 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS studies, was also very significant in the sense of a general method for melanin solubilization and its usefulness toward better characteriza- tion of this highly resistant polymer. The reaction appeared amenable to spectroscopic techniques, and both the visible and the UV spectra of the melanin solutions were ex- amined. The visible region proved uninformative a monotonic rise in optical density with increasing time of reaction was observed. The rise was very gradual and did not appear to reflect the qualitative changes taking place in the system under investigation. Initial attempts to utilize the UV region were also of little avail because of the high absorption intensity of H•O2, which overshadowed any absorption changes caused by the solubilization of the melanin. However, by resorting to the technique of differential spectroscopy, the peroxide absorbance was suppressed and the spectroscopic changes due to the reaction could be readily followed. A typical recording is reproduced in Fig. 2. Within a few moments of the contact of the reagents, a well-defined absorbance peak was developed. The peak intensity in- creased with the time of the reaction and reached a maximum at the time of complete dissolution of the melanin. Then a slow decrease in absorbance was observed as the bleaching of melanin by H202 continued. mm.•..• -- - 0.4 õ mirl•_ 15mi•----•"'• \ •4 mirt 0.5 8 mi o7 • •N 0m.i,• 0.s •z 8mln 0,9 • 1.0 1,1 1.2 3 0 350 3 WAVELENGTH (millimicrons) 1,3 1.4 1,5 Figure 2. Solubilization of intact melanin in 1% H202 at pH 11.5

HAIR BLEACHING 883 The time required for dissolution of melanin in aqueous H:O2 can be readily determined from the absorbance peak and then used as a convenient parameter in further investigations of the reaction. The general experimental procedure employed in the solubilization studies was as follows: 1 mg of melanin was introduced into a volumetric flask containing 10 ml of aqueous H20.,. The reaction mixture was stirred magnetically at 25øC at a given time, aliquots were withdrawn, transferred into a 5-ram quartz cell, and their spectra recorded. The reference cell contained a solution of H:O2 at a concentration identical to that in the sample. Concomitant with the recording' of spectra a visual observation was made of the state of the melanin dispersion, and the time of its complete dissolution was noted. Usually the reaction was followed for at least 60 min after the dissolution time. Effect o[ pH on the Rate of Solubilization--The pigment was treated with 1% aqueous solutions of HeO2 adjusted to different pH values by means of sodium hydroxide. Both the dissolution times (tD) and the absorbances at tD were recorded, and are given in Table II. The reac- tion appears to have a maximum rate in the region of pH just below the pK value of H20,(11.75), indicating that although the peroxide anion is definitely involved in the solubilization process, it may not be the sole attacking species. Table II Effect of pH on the Dissolution of Melanin in 1% H202 at 25 øC Time for Complete Dissolution (t•) pH (Min) Absorbance 10.45 30 1.29 10.80 14 1.27 11.25 9 1.15 11.55 10 1.21 12.20 14 1.21 12.70 13 1.02 Duke and Haas, who studied the homogeneous base-catalyzed de- composition of H== (25), noted a similar pH dependence and ac- counted for it by postulating a reactive, cyclic intermediate formed from the neutral H=• molecule and its anion: H,,O,, + HO,- • [ .4' decomposition _ _ _ H•o/OxH' ß H20 + OH- + 0,2