Chloride Interferences\nin Wet Chemical Oxidation Measurements:\nPlausible Mechanisms and Implications

Wet chemical oxidation\n(WCO) methods measure total organic\ncarbon\n(TOC) in aqueous solutions through the formation and detection of\ncarbon dioxide (CO<sub>2</sub>). Prior research documents chloride\n(Cl<sup>–</sup>) interference during WCO. However, the mechanism\nthat determines WCO interference is not established. We investigate\nWCO and find that formic acid exhibits TOC recovery (89–108%)\nwithin measurement uncertainty in the presence of Cl<sup>–</sup>, while acetic acid recovery is substantially reduced (3–67%).\nWe postulate that chlorine radical (<sup>•</sup>Cl) formation\nduring WCO alters oxidation pathways for organic compounds with methyl\ngroups to form stable halogenated organic species that are thus not\ndetected as CO<sub>2</sub>, reducing observed TOC recovery. We develop\na kinetic model of elementary step reactions that reproduces observed\nTOC recoveries at multiple organic (1 and 5 ppm of C) and Cl<sup>–</sup> (>0.01 M) concentrations for both acetic and formic acids. Independent\nexperiments with pyruvic acid and different halogen salts are consistent\nwith the proposed mechanism. Our findings provide a plausible mechanistic\nexplanation for Cl<sup>–</sup> interference in WCO-derived\nTOC measurements of environmental samples for which halogenated salts\nare present. A plausible mechanism provides a more complete understanding\nof how and why the TOC is biased low in environmental aquatic samples\nfrom saline environments when WCO is employed.