Potential Unintended Artifacts of Ascorbic and Sulfuric Acids for DBP Sample Preservation

Disinfection byproducts (DBPs) form when chemical disinfectants, such as chlorine or chloramine, react with natural organic matter and halides in drinking water. Accurate DBP analysis requires quenching of residual disinfectants and pH adjustment. Ascorbic acid (AA) and sulfuric acid (H2SO4) are commonly used for these purposes, particularly for organic DBPs, yet no standardized guidance exists on their use. This study evaluated the impact of AA and H2SO4 premixing on the quenching of DBP samples under both controlled laboratory and field sampling conditions. Results showed premixing of these reagents prior to sampling substantially increased the formation of haloacetaldehydes (HALs), particularly dichloroacetaldehyde (DCAL) and trichloroacetaldehyde (TCAL) under chlorination, due to acid-catalyzed degradation of AA and subsequent formation of reactive aldehydic intermediates. This effect is magnified in the presence of elevated bromide, leading to higher levels of brominated HALs that are among the most cytotoxic DBPs. Field data from four treatment plants confirmed the laboratory findings and demonstrated that premixing introduced substantial bias into HAL measurements. To prevent overestimation of DBP levels, premixing of AA and H2SO4 should be avoided; AA should be added at the time of sampling, followed immediately by pH adjustment with H2SO4.