Methanol as a Carrier Solvent Can Influence Chlorination Rates of Phenolic Compounds in Chlorinated Waters

Methanol is commonly used as a carrier solvent in environmental chemistry experiments; however, the possible influence of methanol on the kinetics of chemical transformations is often overlooked. The effects of methanol and other frequently used carrier solvents on the chlorination rates of aromatic precursors of disinfection byproducts during water chlorination were investigated. At concentrations as low as 0.50 vol %, methanol increased chlorination rates of ethylparaben, phenol, 4-hydroxybenzoic acid, ethyl 3-hydroxybenzoate, and ethyl 2-hydroxybenzoate. Methanol did not increase the chlorination rates of salicylic acid, dimethenamid, or 1,2-dimethoxybenzene. Ethylparaben and phenol chlorination were especially sensitive to methanol, with pseudo-first-order rate constants (k_obs) increasing by a factor of >2 in water containing 2.0 vol % methanol compared to those in methanol-free controls. Rate enhancements persisted across differing reaction conditions (pH 6–10 and in buffers containing borate or phosphate). The rate enhancements of unsubstituted and para-substituted phenols were larger than those of meta- and ortho-substituted phenols. The carrier solvents acetone, acetonitrile, and tert-butanol had no appreciable impact on the chlorination rates of ethylparaben. Overall, our findings suggest that methanol as a carrier solvent can cause systematic errors in lab-scale chlorination experiments. To avoid experimental artifacts, researchers should prepare stock solutions in water (when feasible) or minimize carrier solvent concentrations present in reaction solutions.