THERMAL-DEGRADATION OF AQUEOUS OXALATE SPECIES

The aqueous thermal degradation of oxalic acid (a naturally occurring dicarboxylic acid) and its anions has been examined experimentally under variable conditions of temperature (160-230-degrees-C), time (24-96 h), buffered pH (4-7), and ionic strength (0.2-0.5 M). Decomposition of oxalate species follows a first-order rate law. Degradation rate increases with decreasing pH; the effects of ionic strength are not significant. Changes in reaction rate over the temperature range studied give activation energies that range from 29-50 kcal/mol over a starting solution pH range of 5-7. Extrapolation of these data to sedimentary basin temperatures suggests that oxalate species may be long-lived in formation waters. At 80-degrees-C, half-lives range from 2500 to 28,000 years at pH values of 5 and 7, respectively. Formate species are relatively stable degradation products over the range of conditions studied. Comparison of these data with results of other thermal degradation studies of carboxylic acids and their anions indicate that acetate stability >> formate stability > oxalate stability > gallate and malonate stability. These results are consistent with the high concentrations of acetate relative to oxalate and malonate observed in formation waters.