Simply put TOC is Total Organic Carbon. It matters when making drinking water because carbon can react with chlorine forming by products that could possibly make the water more hazardous than it was prior to disinfection. TOC matters in wastewater because it is the source of food that allows bacteria to consume all the oxygen in receiving streams and cause fish kills. TOC matters in stream sediments because heavy metals and toxic organic pollutants are trapped in it and it occupies the upper sediment layer where bottom feeders feed ingesting and accumulating poisons. TOC matters in soil because it provides essential nutrients and conditions soil helping to grow strong healthy plants, crops, and forests. TOC matters because it comprises up to 60 % or more of human and animal waste products that either need to be treated or end up buried and taking up space in land fills. TOC matters because if buried in land fills it either decomposes to form hazardous gases, or can dissolve in water resulting in organic acids that dissolve and potentially carry toxic organics and metals into the groundwater. TOC simply matters.

We measure TOC with TOC analyzers. Instrument companies argue which one is best, but they all pretty much measure TOC. The only real interference in measuring TOC comes from inorganic carbon which, if not accounted or corrected for, is measured as TOC also. Inorganic carbon is the carbon that comes from carbonate minerals and rocks, such as calcite, limestone, and caliche. In water inorganic carbon is present as carbonate, bicarbonate, or dissolved carbon dioxide. In most cases acidifying the solution and purging it with air removes inorganic carbon. Once inorganic carbon is removed, the organic carbon compounds are oxidized, converted to carbon dioxide, and measured. It is the oxidation techniques that instrument manufacturers and method users usually debate.

Initially organic carbon was oxidized chemically with a heated acid persulfate solution and the evolved carbon dioxide measured. Later UV light was used to help speed and promote decomposition. Then along came the heated catalyst. In catalytic combustion water-containing TOC is added directly onto the surface of a heated catalyst immediately oxidizing organics and converting all carbon to carbon dioxide. These three oxidation techniques all work, all have merits and deficiencies, and the differences in recoveries of molecules are not worth the debates they have caused. Find what you like best and know that it works. If you are looking for an instrument to measure TOC study all aspects of each technique and get the one that best suits your purposes. Know that chemical methods take longer, but allow you to “see” less carbon. Combustion methods are fast, but not as sensitive. Those are the main differences.

William Lipps

(979) 690-1375 ext. 230