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

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, A^2−) (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, A^2−) (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(A^2−). By using the pH and temperature dependent rate constants, source and sinking processes in the tropospheric aqueous phase can be described precisely.

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