Chapter 5 169 reducing agent (R-SH) throughout the hair.46 The pKa of the thiol group on thioglycolic acid is ~10.2–10.4 and at pH 9.0 less than 10% of the thiol will be in the active anionic form.47 At higher pH with thioglycolate or with dithiol reducing agents such as lipoic acid or Figure 9 Effect of temperature and pH on SFTK reaction kinetics.