Methanol as\na Carrier Solvent Can Influence Chlorination\nRates of Phenolic Compounds in Chlorinated Waters

Methanol is commonly used as a carrier solvent in environmental\nchemistry experiments; however, the possible influence of methanol\non\nthe kinetics of chemical transformations is often overlooked. The\neffects of methanol and other frequently used carrier solvents on\nthe chlorination rates of aromatic precursors of disinfection byproducts\nduring water chlorination were investigated. At concentrations as\nlow as 0.50 vol %, methanol increased chlorination rates of ethylparaben,\nphenol, 4-hydroxybenzoic acid, ethyl 3-hydroxybenzoate, and ethyl\n2-hydroxybenzoate. Methanol did not increase the chlorination rates\nof salicylic acid, dimethenamid, or 1,2-dimethoxybenzene. Ethylparaben\nand phenol chlorination were especially sensitive to methanol, with\npseudo-first-order rate constants (<i>k</i><sub>obs</sub>) increasing by a factor of >2 in water containing 2.0 vol % methanol\ncompared to those in methanol-free controls. Rate enhancements persisted\nacross differing reaction conditions (pH 6–10 and in buffers\ncontaining borate or phosphate). The rate enhancements of unsubstituted\nand <i>para-</i>substituted phenols were larger than those\nof <i>meta-</i> and <i>ortho</i>-substituted phenols.\nThe carrier solvents acetone, acetonitrile, and <i>tert</i>-butanol had no appreciable impact on the chlorination rates of ethylparaben.\nOverall, our findings suggest that methanol as a carrier solvent can\ncause systematic errors in lab-scale chlorination experiments. To\navoid experimental artifacts, researchers should prepare stock solutions\nin water (when feasible) or minimize carrier solvent concentrations\npresent in reaction solutions.