pH- and Temperature-Dependent Kinetics of the Oxidation Reactions of OH with Succinic and Pimelic Acid in Aqueous Solution

Abstract

Rate constants for the aqueous-phase reactions of the hydroxyl radical with the dicarboxylic acids, succinic acid and pimelic acid were determined using the relative rate technique over the temperature range 287 K ≤ T ≤ 318 K and at pH = 2.0, 4.6 or 4.9 and 8.0. OH radicals were generated by H2O2 laser flash photolysis while thiocyanate was used as a competitor. The pH values were adjusted to obtain the different speciation of the dicarboxylic acids. The following Arrhenius expressions were determined (in units of L mol−1 s−1):  succinic acid, k(T, AH2) (2.1 ± 0.1) × 1010 exp[(−1530 ± 250 K)/T], k(T, AH−) (1.8 ± 0.1) × 1010 exp[(−1070 ± 370 K)/T], k(T, A2−) (2.9 ± 0.2) × 1011 exp[(−1830 ± 350 K)/T] and pimelic acid, k(T, AH2) (7.3 ± 0.2) × 1010 exp[(−1040 ± 140 K)/T], k(T, AH−) (1.8 ± 0.1) × 1011 exp[(−1200 ± 240 K)/T], k(T, A2−) (1.4 ± 0.1) × 1012 exp[(−1830 ± 110 K)/T]. A general OH radical reactivity trend for dicarboxylic acids was found as k(AH2) < k(AH−) < k(A2−). By using the pH and temperature dependent rate constants, source and sinking processes in the tropospheric aqueous phase can be described precisely.