Decomposition of hydrogen peroxide by haemin. Dependence of the reaction velocity on pH in the alkaline range

The rate of the haemin-catalyzed decomposition of H₂O ₂ decreases with increasing pH in weakly alkaline solutions. The decrease is due to the acid dissociation of haemin, the anion form being catalytically inactive. The dissociation constant is 2.4 × 10^-7 M. The rate constants of the association of haemin with peroxide and of its reverse process were calculated as 1.6 × 10⁶M^-1 min^-1 and 3 × 10⁸ min^-1 respectively. The ineffective coupling of rate constants in haemin, as contrasted with catalase, is discussed.