Chloride Interferences in Wet Chemical Oxidation Measurements: Plausible Mechanisms and Implications

Wet chemical oxidation (WCO) methods measure total organic carbon (TOC) in aqueous solutions through the formation and detection of carbon dioxide (CO₂). Prior research documents chloride (Cl^-) interference during WCO. However, the mechanism that determines WCO interference is not established. We investigate WCO and find that formic acid exhibits TOC recovery (89-108%) within measurement uncertainty in the presence of Cl^-, while acetic acid recovery is substantially reduced (3-67%). We postulate that chlorine radical (^•Cl) formation during WCO alters oxidation pathways for organic compounds with methyl groups to form stable halogenated organic species that are thus not detected as CO₂, reducing observed TOC recovery. We develop a kinetic model of elementary step reactions that reproduces observed TOC recoveries at multiple organic (1 and 5 ppm of C) and Cl^- (>0.01 M) concentrations for both acetic and formic acids. Independent experiments with pyruvic acid and different halogen salts are consistent with the proposed mechanism. Our findings provide a plausible mechanistic explanation for Cl^- interference in WCO-derived TOC measurements of environmental samples for which halogenated salts are present. A plausible mechanism provides a more complete understanding of how and why the TOC is biased low in environmental aquatic samples from saline environments when WCO is employed.