Chapter 5 171 K is a rate constant proportional to both the initial diffusion constant of the reducing agent prior to reduction and reaction rate constant. T is a constant proportional to the major and diameters of the elliptical hair shaft [T= (a2 +b2)/2)1/2]. Eq. 7 was found to give excellent fit to SFTK data with dithiol reducing agents for at the first half-life of the reaction and to effectively account for differences in reaction rate due to differences in reactant concentration or hair diameter.29,30 It is well known that some individuals have hair that is harder to perm than the hair of others. Figure 12 shows SFTK curves for four hairs each from one individual with easy to perm hair (solid line) and one with hard to perm hair (dashed line). Treatment was with 0.6 M ammonium thioglycolate pH 9.5. Figure 11 Transmission electron micrographs of hair stained with silver methenemine, displayed at 16,200X resolution. Reduction for 5 min at pH 9.3. A. Untreated B. 0.5M ammonium thioglycolate C. 0.15M dihydrolipoic acid. Modified from Wickett and Barman30 with permission of the Society of Cosmetic Chemists Figure 12 Individual differences in reactivity to ATG at pH 9.5. Modified from Wickett29 with permission of Society of Cosmetic Chemists.