TOC Test
The Total Organic Carbon (TOC) test uses heat, ultra- violet light, and a strong chemical oxidant (or a combina- tion of these three) to oxidize organic compounds to CO2 and H2O. Oxygen demand is measured indirectly by determining the amount of CO2 produced using infrared spectroscopy, conductivity, or coulometry (an electro- chemical technique). The test can take several minutes to several hours to complete, and information obtained from TOC analysis is less useful than information obtained from BOD and COD analysis. Also, the TOC test does not differentiate between compounds with the same number of carbon atoms in different stages of oxidation and will thus produce different oxygen demand results. Because BOD and COD tests directly measure the amount of oxy- gen required to stabilize a waste sample, results reflect the original oxidation state of the chemical pollutants. This is demonstrated using the following example, where two compounds with the same number of carbon atoms in different oxidation states are oxidized to CO2 and H2O.
Oxalic Acid: C2H2O4 + 1/2 O2 ——> 2 CO2 + H2O
Ethanol: C2 H6O + 3 O2 ——> 2 CO2 + 3 H2O While TOC results are identical for both compounds,
the oxygen demand of ethanol is six times greater than oxalic acid, and will thus have a much greater effect on the dissolved oxygen content of a receiving water.
COD Test
The Chemical Oxygen Demand (COD) test uses a strong chemical oxidant in an acid solution and heat to oxidize organic carbon to CO2 and H2O. By definition, chemical oxygen demand is “a measure of the oxygen equivalent of the organic matter content of a sample that is suscep- tible to oxidation by a strong chemical oxidant.”* Oxy- gen demand is determined by measuring the amount of oxidant consumed using titrimetric or photometric methods. The test is not adversely affected by toxic substances, and test data is available in 1-1/2 to 3 hours, providing faster water quality assessment and process control.
COD test results can also be used to estimate the BOD results on a given sample. An empirical relationship exists between BOD, COD and TOC. However, the specific relationship must be established for each sample. Once correlation has been established, the test is useful for monitoring and control. Table 1 compares the attri- butes of BOD, COD and TOC tests.