Box 16-1 Environmental Carbon Analysis and Oxygen Demand

Industrial waste streams are partly characterized and regulated on the basis of their carbon content or oxygen demand. Total carbon (TC) is defined by the amount of CO2 evolved when a sample is completely oxidized by combustion  

Color Plate 11 shows an alternative photochemical method to oxidize all carbon to CO2 without the high temperature required for combustion with O2. 

Total carbon includes dissolved organic material (called total organic carbon, TOC) and dissolved COf and HCOJ (called inorganic carbon, IC). By definition, TC = TOC + IC. To distinguish TOC from IC, the pH of a fresh sample is lowered below 2 to convert COl and HCO J into CO2, which is purged from (bubbled out of) the solution with N2. After IC has been removed, combustion analysis of the remaining material measures TOC. IC is the difference between the two experiments. 

TOC is widely used to determine compliance with discharge laws. Municipal wastewater may contain 1 g of TOC per liter. At the other extreme, high-purity water required for microelectronic processes may contain 1 xg of TOC per liter. 

Biological oxygen demand (BOD) is defined as the O2 required for biological d radation of organic materials by microoiganisms. The procedure calls for incubating a 

Instruments using different oxidation techniques produce different values for TOC, because not all organic matter is oxidized by each technique. The current state of the art is such that TOC is really defined by the result obtained with a particular instrument. 

Ultraviolet radiation absorbed by Ti02 creates an electron-hole pair. The hole oxidizes H20 to the powerful oxidant HO The electron reduces 02 to H20 by a chain of reactions. Ti02 is a catalyst and 02 is consumed in the net reaction organic C + 02 — C02. 

Total oxygen demand (TOD) tells us how much 02 is required for complete combustion of pollutants in a waste stream. A volume of N2 containing a known quantity of 02 is mixed with the sample and complete combustion is carried out. The remaining 02 is measured by a potentiometric sensor (Box 17-1). Different species in the waste stream consume different amounts of O,. For example, urea consumes five times as much 02 as formic acid does. Species such as NH3 and H2S also contribute to TOD. 

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