300 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS H• O (i) R and R'=H,OH,NH2,NHR, alkyl, alkoxy. of indo-aniline formed to that of the PPD consumed, depends on the concentration of hydrogen peroxide used. Kinetic modeling is employed to explore the mechanistic steps responsible for the indo-aniline formation. EXPERIMENTS All experiments were performed in a pH = 9.2 borax buffer solution at room temper- ature. To maintain quantitative measurement of the rates of the indo-aniline formation and to minimize H202 decomposition, all reaction solutions were prepared in either Teflon or glass containers. The solution was then transferred by a plastic pipette into cuvettes for spectroscopic measurements. p-Phenylenediamine (PPD, 99%) and 2,6-dimethylphenol (2,6-DMP, 99.8%) were chosen as the primary reactants. They were obtained from Aldrich and were used directly as received. The hydrogen peroxide used as oxidizing agent was obtained from Clairol (6% Clairoxide). Reactant concentrations varied from 5 X 10 -5 to 1 X 10-3 M for both PPD and 2,6-DMP and from 5 X 10-3 to 2 X 10- • M for hydrogen peroxide. The lowest reactant concentration that can be used was limited by the experimental techniques used. The highest limits of reactant concentrations of PPD and 2,6-DMP were selected to assure that only primary reactions responsible for indo-aniline formation were studied. The hydrogen peroxide concentration is controlled to be less than 0.2 M under the experimental conditions used, in order to maintain the pH of the buffer solution. The rate of indo-aniline dye formation was obtained from the initial slope of indo-aniline concentration profiles, which were obtained by monitoring the optical density of indo- aniline formed at maximum absorption wavelength using a UV/VIS spectrophotometer (Lambda 6, Perkin Elmer). The PPD disappearance rate was determined by monitoring the PPD consumption as a function of time. At a given time, the PPD concentration was determined by adding sufficient ferricyanide salt to a reaction mixture to convert all the unreacted PPD rapidly to the corresponding indo-aniline of known extinction coefficient (1 X 10 -4 M/O.D. for 2,6-dimethyl-indo-aniline) (2). Effects of reactant concentrations on the rates of indo-aniline formation were studied by varying one reactant concentration at a time while others were kept constant at fixed pH. All kinetic data were collected at the time when the amount of indo-aniline (product) formed accounted for •5% of the initial PPD concentration. All kinetic modeling described below was performed using "scientist" software from Micromath, Inc.

INDO-ANILINE DYE FORMATION 301 RESULTS AND DISCUSSION Although hydrogen peroxide is known as a strong oxidizing agent, without a catalyst its ability to oxidize PPD in aqueous solution at alkaline pH is very limited. As it is shown in Figure 1, at t • 30 minutes after mixing ofPPD, 2,6-DMP, and H202, the amount of indo-aniline formed accounts for less than 5% of the PPD concentration. Unlike the ferricyanide system, where the rate of indo-aniline formation is a function of the con- centrations of both PPD and phenol (2), the rate of indo-aniline formation in the aqueous hydrogen peroxide solution is essentially independent of the phenol concentra- tion. In addition, enhancement of the initial rates of indo-aniline formation by H202 can only be observed within a very narrow range of [H202] (5 X 10-3 M -- 1.8 X 10 -2 M). When [H202] is greater than 1.8 X 10 -2 M under the experimental conditions used, the apparent rate of indo-aniline formation (obtained from the initial slope of indo-aniline concentration profiles) is inversely proportional to [H•O•]. EFFECTS OF REACTANT CONCENTRATIONS ON RATE OF INDO-ANILINE FORMATION Three sets of indo-aniline concentration profiles obtained using different concentrations of p-phenylenediamine (PPD) and 2,6-dimethylphenol (2,6-DMP) are shown in Figure 1. As we can see, ten-time increase in concentration of 2,6-DMP results in little increase in the rate of indo-aniline formation (from circles to squares). However, the rate of indo-aniline formation is proportional to PPD concentration. An increase in PPD 1.2 1.0 0.8 • 0.6 .•_ -- '•= o 'o 0.4 0.2 03,:9,:9000 i i I 50 100 150 Time / Min. 0.0 • 0 200 250 Figure 1. Plots of concentrations of indo-aniline obtained using three different experimental conditions: O, [PPD] = 9 X 10 -5 M, [2,6-DMP] = 9 X 10 -5 M, [H202] = 1.8 X 10 -2M, pH = 9.2 [•, [PPD] = 9 X 10 -5 M, [2,6-DMP] = 9 X 10 -4 M, [H202] = 1.8 X 10 -2 M, pH = 9.2 and A, [PPD] = 9 X 10 -4 M, [2,6-DMP] = 9 X 10 -4 M, [H202] = 1.8 X 10 -2 M, pH = 9.2.