302 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS concentration by the same amplitude leads to a significant increase in indo-aniline concentration (from circles to triangles). The effect of [H202] on the rate of indo-aniline formation was studied in deaerated solution to minimize the influence of atmospheric oxygen. Buffer solution used for this purpose was degassed using freezing pumping-thaw-pumping cycles in an acetone/dry ice bath. Two different types of concentration curves for indo-aniline formation were observed as [H202] increases shown in Figure 2: (a) at low [H202], concentration profiles of indo-aniline formation are linear and their initial slope increases with [H202] (down triangles and up triangles), and (b) at high [H202], concentration profiles of indo-aniline are nonlinear. After a brief linear accumulation of indo-aniline concentra- tion, the curvature of the concentration profile starts to increase as a function of time and of [H202] (squares and circles in Figure 2). Clearly, hydrogen peroxide plays two opposite roles in indo-aniline formation. Under fixed concentration of PPD and 2,6- DMP, low [H202] will result in a slow formation of indo-aniline, and consequently, the small amount of indo-aniline will be formed within the observation time. Therefore, indo-aniline consumption by secondary reaction will be negligible. At this stage, only indo-aniline formation is the dominant process in the reaction system, and the concen- tration profiles observed will linearly accumulate with time. As [H202] is increased, both the rates of indo-aniline formation and the rates of the secondary consumption reactions of indo-aniline are increased. Therefore, the experimental concentration pro- files obtained are the result of the indo-aniline formation process and its secondary reactions. Decomposition of the indo-aniline by hydrogen peroxide under our experimental con- ditions is insufficient to cause the observed peroxide concentration effect on indo- 0.30 0.25 0.20 o =• o.15 ._ _c 0.10 0.05 0.00 • 0 80 100 Time/Min. Figure 2. Plots of indo-aniline concentrations vs time. Four plots shown in this figure represents indo- aniline concentration profiles obtained at four [H202] under the fixed concentration of [PPD] = 1.9 X 10 -4 IV[, [2,6-DMP] = 2.9 X 10 -4 M, and constant pH = 9.2. W, [H202] = 5.3 X 10 -3 M F], [H202] ---- 3.5 X 10 --2 M /•, [H202] ---- 1.8 X 10 -2 M O, [H202] ---- 7.0 X 10 -2 M.

INDO-ANILINE DYE FORMATION 303 aniline formation (Figure 2). Kinetic data of indo-aniline decomposition were separately obtained by using indo-aniline as reactant, which was synthesized from the oxidation of the corresponding diphenylamine. The rate of the indo-aniline decomposition was mon- itored at its maximum absorbance (Xma x = 525 rim) as a function of reactant concen- trations. The reaction has first order kinetics on both indo-aniline and hydrogen per- oxide in pH = 9.2 buffer solution. At t = 30 minutes, estimated loss of indo-aniline by peroxide decomposition can only account for 1% of total indo-aniline concentra- tion. A group of experiments was conducted to simultaneously monitor both initial rates of indo-aniline formation and rates of PPD depletion as a function of [PPD]. Concentration of hydrogen peroxide used for this purpose was chosen such that indo-aniline concen- tration is linearly accumulated as a function of time. Over a tenfold change in [PPD], the rate of indo-aniline formation was found to be comparable to the PPD depletion rate shown in Figure 3. Evidently, PPD depletion is the rate-limiting step in the overall indo-aniline formation process. Additional experiments were conducted to study the influence of [H202] on the rate of PPD consumption. A set of such data is plotted in Figure 4. Although data are scattered due to the low accuracy caused by a small depletion of PPD under experimental con- ditions used, it is evident that within the range of [H202] used in this study, the rate of PPD depletion increases as [H202] increases. A fractional order of •0.5 on [H202] was obtained, which indicates that the hydrogen peroxide molecule is not directly responsible for the oxidation of PPD. If a molecule of hydrogen peroxide reacted with PPD in the rate-determining step, the effect of the peroxide concentration on the rate of PPD disappearance should be first order. lx10-1 1 xl 0-2 1 xl 0-3 1 xl 0-4 lx10-5 ........ ' ........ ' ' ' ' 0.1 1.0 10.0 [PPD]x104/M Figure 3. Comparison of the indo-aniline formation rates to the PPD consumption rates as a function of [PPD] under the constant concentration of hydrogen peroxide, [H202] = 1.8 X 10 -2 M, and pH. O, Rates of indo-aniline formation &, rates of PPD consumption.