Dissolved organic halogen (DOX) is a measurement used to estimate the total quantity of dissolved halogenated organic material in a water sample, also referred to as total organic halogen (TOX), adsorbable organic halogen (AOX), and carbon-adsorbable organic halogen (CAOX). The presence of halogenated organic molecules is indicative of disinfection byproducts and other synthetic chemical contamination. Halogenated compounds that contribute to a DOX result include, but are not limited to:
• the trihalomethanes (THMs);
• organic solvents such as trichloroethene, tetrachloroethene, and other halogenated alkanes and alkenes;
• chlorinated and brominated pesticides and herbicides;
• polychlorinated biphenyls (PCBs);
• chlorinated aromatics such as hexachlorobenzene and 2,4-dichlorophenol; and
• high-molecular-weight, partially chlorinated aquatic humic substances.
Compound-specific methods, such as gas chromatography, typically are more sensitive than DOX measurements.
The adsorption-pyrolysis-titrimetric method for DOX measures only the total molar amount of dissolved organically bound halogen retained on the activated carbon adsorbent. It yields no information about the structure or nature of the organic compounds to which the halogens are bound or about the individual halogens present. The method is sensitive to organic chloride, bromide, and iodide, but does not detect fluorinated organics.
DOX measurement is an inexpensive and useful method for screening large numbers of samples before specific (and often more complex) analyses. For example, DOX measurement can be used for:
• extensive field surveying for pollution by certain classes of synthetic organic compounds in natural waters;
• mapping the extent of organohalide contamination in groundwater;
• monitoring the breakthrough of some synthetic organic compounds in water treatment processes; and
• estimating the level of formation of chlorinated organic byproducts after disinfection.
When used as a screening tool, a large positive (i.e., above background measurements) DOX test result indicates the need for identifying and quantifying specific substances. In saline or brackish waters, the high inorganic halogen concentrations interfere. The possibility of overestimating DOX concentration because of inorganic halide interference always must be considered when interpreting results.