Pivalic acid as combined buffer and scavenger for studies of cloud water chemistry with pulse radiolysis

Abstract

The reactions of cloud water with transition-metal ions may be studies using pivalic acid as buffer and ˙OH scavenger. Commonly used phosphate buffers form strong complexes that may have an important effect on reaction kinetics. Low solubility products limit transition-metal ion concentrations to very low levels (micromolar). With pivalic acid as buffer, we can increase the Cu^II concentration typically by a factor of 340. At the same time better buffer characteristics and less complex formation is achieved. The scavenger efficiency equals that of tert-butyl alcohol.

The rate data and the spectra of pivalic acid radicals have been measured in N₂O-saturated solutions of pivalic acid. The acid constant was found to be 4.9 ± 0.1. The rate constants for the reaction of ˙OH with pivalic acid have been re-evaluated: k[(CH₃)₃CCO₂^–+˙OH]=(7 ± 2)× 10⁸ dm³ mol^–1 s^–1 and k[(CH₃)₃CCO₂H +˙OH]= 6.5 × 10⁸ dm³ mol^–1 s^–1. The importance of including reactions like H˙+˙CH₂(CH₃)₂CCO₂H in the reaction model is shown. The rate constants for the recombination of pivalic acid radicals are: k[2˙CH₂(CH₃)₂CCO₂H]=(7 ± 1)× 10⁸ dm³ mol^–1 s^–1, k[˙CH₂(CH₃)₂CCO₂H +˙CH₂(CH₃)₂CCO₂^–]=(7 ± 3)× 10⁸ dm³ mol^–1 s^–1 and k[2˙CH₂(CH₃)₂CCO₂^–]=(3.7 ± 0.4)× 10⁸ dm³ mol^–1 s^–1.