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.

Experimental and modeling results suggest chlorine radical formation and subsequent adduct formation interferes with wet-chemical oxidation measurements of organic carbon.