previousChemical Oxygen Demand (COD) Interrelationship between BOD, COD and TOCnext
Total Organic Carbon (TOC)

Another means for measuring the organic matter present in water is the TOC test, which is especially applicable to small concentrations of organic matter. Wastewater content of carbon bound in organic molecules is the TOC (total organic carbon). Organic carbon comprises nearly all carbon compounds except a few carbon species which are looked at as inorganic (carbon dioxide, hydrogen carbonate, carbonate, cyanide and some further examples which are not commonly found in wastewaters). For detection of organic carbon in aqueous samples, the whole sample is subdued to oxidation (commonly by incineration of a particular volume of the wastewater sample in the presence of a catalyst at 900°C using CO2-free air). The CO2 formed by incineration of the organic wastewater constituents is quantified in the off-gas of the furnace using an infrared cell (CO2 absorbs infrared light at a wavenumber of 2349 cm-1, corresponding to a wavelength of 4257 nm) by comparison with measurements using aqueous calibration solutions of a pure organic compound with known concentration. The theoretical TOC of such a calibration solution can be calculated as follows (for the calculation, the formula, the molar mass M and the mass concentration m/V of the standard compound have to be known using the atomic weight of carbon [12 g/mol] and the number of carbons z in the standard compound) using phenol as an example for a standard substance:

Standard compound: phenol

Formula: C6H6O

Molar mass of standard compound: (6 x 12 + 6 x 1 + 16) g/mol = 94 g/mol

Carbon mass per mol of standard compound: z x 12 g/mol = 6 x 12 g/mol = 72 g/mol

Carbon mass portion of standard compound mass: (72 g/mol)/(94 g/mol) = 0.766

Phenol mass concentration of standard solution: 200 mg/l

Theoretical TOC of standard solution: 0.766 x 200 mg/l = 153.2 mg/l

However, although the analytical method described above will deliver appropriate results if performed with standard solutions in deionized water, it will not work with real wastewater samples or solutions of organics in tap water, because these matrices also contain inorganic carbon compounds (mainly hydrogen carbonate). The carbon bound in inorganic molecules or ions is designated as total inorganic carbon (TIC). The sum of TOC and TIC gives total carbon (TC). As wastewater usually contains hydrogen carbonate (or dissolved carbon dioxide or carbonate, depending on pH), the TIC has to be removed prior to TOC analysis. This is easily obtained by acidification of the sample and stripping the formed CO2 with CO2-free air. However, this method also removes volatile organic compounds and can only be applied if no volatile organics are present in the wastewater sample. In the presence of volatile organics TOC has to be calculated after analysis of TC and TIC by subtraction of both concentrations. TOC analyzers exhibit high technical standards and are thus rather expensive. A German company also offers cuvette tests for TOC analysis which is performed using a photometric method.

previousChemical Oxygen Demand (COD) Interrelationship between BOD, COD and TOCnext