I need to calculate theoretical COD for one organic dye molecule, I know the nitrogen and sulphur oxidized as ammonia and sulfur dioxide. Can it be converted to any other forms like nitrate, nitrite etc.?
The theoretical oxygen demand includes the oxygen required to convert the ammonia to nitrate, which is sometimes called the "nitrogenous oxygen demand".
So, the theoretical oxygen demand (ThOD) is calculated as:
ThOD = (Oxygen required to convert your compound into CO2, NH3 and SO2) + (Oxygen required to convert the produced NH3 to NO3-)
The ammonia oxidation reaction is:
NH3 + 2O2 --> NO3- + H2O + H+
So, for each mole of NH3 your compound produces, two additional moles of O2 will be required to satisfy the nitrogenous oxygen demand.
you will never get any N oxidized to NO2- or No3-. It will remain as NH3 in COD test. Thus, if your dye contains NH2 group, dont bother to consider any demand of O for this. Sulfur can be oxidized to SO4 but in most of the dyes it is in SO3H or SO3Na form which is the most oxidized form and does not need additional oxygen. If sulfur is in sulfide form (S-2) it will consume O2. So you need to mainly concentrate on C and H for COD calculation of a dye.
I accept the answer of Christine Dykstra about N-group and am agree with the answer of Upendra Patel about S-group.
S-group except dye can have fate such as N-grop in wastewater
Theoretical Oxygen Demand is defined in OECD Guideline 301 as
The total amount of Oxygen required to completely oxidize the chemical and is calculated from chemical formula
ThOD = 1/4*(4x+y-2z)/(12x+y+16z) gm COD/gm of substance
OECD 301 states that for a compound CcHhClclNnNanaOoPpSs (where the small letters are suffixes) gives the formula
ThOD without Nitrification
ThOD (NH3) = 16( 2c+ 1/2(h-cl-3n)+3s+5/2(p)+1/2na-o)/MW mg COD/mg
substance
ThOD with Nitrification
ThOD (NO3) = 16( 2c+ 1/2(h-cl)+5/2n+ 3s+5/2(p)+1/2na-o)/MW mg COD/mg substance
BOTH above formulas give comparable results
If u desire the THOD to approximate the COD value - calculate without Nitrification, since as UPENDRA Patel points out above, COD measurement doesnot consider Nitrogenous Oxygen demand which manifests after 6th day (manifests slowly).
If u actually want to find complete oxygen demand , then u determine with Nitrification
I need to calculate ThOD of food waste CaHbOcNdSe
- After oxidation N will be converted to NH3 but what about S??
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