RESULTS AND DISCUSSION SOLUTIO N EXPERIMENTS The role of copper in accelera tion of color formation was tested fi rst in solution to simplify the sy stem to just dye precursors and hydrogen peroxide in the presence of varying levels of copper. Low levels of sodium sulfi te and ascorbic acid were added as reducing agents with the purpose of stabilizing the solution after making so that there was a reproducible starting solution color. Other precautions were taken to minimize air exposure before the reaction started, and tests were carried out to ensure vials were free of redox-metal con- taminants. Color formation as a function of time was measured with different copper levels for three dye couples—PPD–AHT, PPD– Res, and HDAP–AHT—after addition of hydrogen peroxide to the dye solutions with added copper levels. Color was measured from images taken of solution vials under controlled lighting. The three dyes showed dif- ferent ove rall rates of color formation, with HDAP–AHT being the fastest and PPD–Res being the slowest, but in all dye systems, copper addition increased the rate of color for- mation signifi cantly. Initial images were only taken after ~30 s after addition of hydrogen peroxide, and dye formation during this time was not measured. All dye solutions started with an L value of 99. Color formation values for L (lightness) for HDAP–AHT color formation show a dose response for increasing copper levels (Figure 1). Similar dose- response curves were obtained for PPD–Res and PPD–AHT. Change in L versus initial was calculated at 7 min for all dyes at each copper level and confi rmed a dose response of color formation acceleration versus copper level added for all three dye couples (Figure 2 ). Figure 1. L value change versus time for the HDAP–AHT dye couple. JOURNAL OF COSMETIC SCIENCE 272

The mechanism by which copper accelerates color formation is proposed to be accelera- tion of the initial primary intermediate to diamine reaction, as shown in Figure 3. This oxidation with hydrogen peroxide in a “clean” solution with no added redox metals is kinetically a slow step that proceeds via two one-electron oxidations. Ad ded redox metals accelerate the initial formation of semiquinone radical cation, which then rapidly forms the diimine. HAIR EXPERIMENTS Hair experime nts were carried out to determine how copper acceleration of dye formation impacts color uptake on hair. Previously colored hair was fi rst rinsed in tap water contain- ing 0.06–0.08 μg/g of copper to create sets of 12 tresses with four different copper levels (Ta ble I). Water hardness was medium (~9 US gpg), and no signifi cant differe nces in calcium and magnesium were measured between tresses. These copper levels are within the range of levels seen in consumers’ hair from previously reported hair harvesting (4). Each set of tresses was then co lored with three different shades [intense medium blonde/ red (77/44), medium brown/violet (4/6), and intensive red (66/46)] to asses the impact of copper levels in hair on color formation. The shades were chosen as ones that have the same dye primary intermediates and coupler combinations used in the solution testing. Figure 4A–C shows L*, a*, and b* values for initial color results. In all shades, a signifi - cant difference is measured in color readings as a function of copper levels. A higher L* value is seen with higher copper levels, indicating the shade is lighter due to lower dye Figure 2. Change in L at 7 min versus initial for PPD–AHT, HDAP–AHT, and PPD–Res dye couples. REDOX METALS IN COLOR FORMATION IN A HAIR COLORANT 273