Theoretical oxygen demand
Encyclopedia
Theoretical Oxygen Demand (ThOD) is the calculated amount of oxygen
required to oxidize a compound to its final oxidation products. However, there are some differences between standard methods that can influence the results obtained: for example, some calculations assume that nitrogen
released from organic compound
s is generated as ammonia
, whereas others allow for ammonia oxidation to nitrate
. Therefore in expressing results, the calculation assumptions should always be stated.
(CH2(NH2)COOH) using the following assumptions:
1. In the first step, the organic carbon and nitrogen are converted
to carbon dioxide
(CO2) and ammonia (NH3), respectively.
2. In the second and third steps, the ammonia is oxidized
sequentially to nitrite
and nitrate
.
3. The ThOD is the sum of the oxygen required for all three steps.
We can calculate by following steps:
1. Write balanced reaction for the carbonaceous oxygen demand.
CH2(NH2)COOH + 1.5 02 ->NH3 + 2CO2 + H20
2. Write balanced reactions for the nitrogenous oxygen demand.
NH3 + 1.5 02 -> HNO2 + H20
HNO2 + 0.5 O2 -> HNO3
NH3 + 2 O2 -> HNO3 + H2O
3. Determine the ThOD.
ThOD= (1.5 + 2) mol O2/mol glycine
= 3.5 mol O2/mol glycine x 32 g/mol O2 / 75 g/mol glycine
= 1.49 g O2/g glycine
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
required to oxidize a compound to its final oxidation products. However, there are some differences between standard methods that can influence the results obtained: for example, some calculations assume that nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
released from organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s is generated as ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, whereas others allow for ammonia oxidation to nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
. Therefore in expressing results, the calculation assumptions should always be stated.
Example
In order to determine the ThOD for glycineGlycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
(CH2(NH2)COOH) using the following assumptions:
1. In the first step, the organic carbon and nitrogen are converted
to carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2) and ammonia (NH3), respectively.
2. In the second and third steps, the ammonia is oxidized
sequentially to nitrite
Nitrite
The nitrite ion has the chemical formula NO2−. The anion is symmetric with equal N-O bond lengths and a O-N-O bond angle of ca. 120°. On protonation the unstable weak acid nitrous acid is produced. Nitrite can be oxidised or reduced, with product somewhat dependent on the oxidizing/reducing agent...
and nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
.
3. The ThOD is the sum of the oxygen required for all three steps.
We can calculate by following steps:
1. Write balanced reaction for the carbonaceous oxygen demand.
CH2(NH2)COOH + 1.5 02 ->NH3 + 2CO2 + H20
2. Write balanced reactions for the nitrogenous oxygen demand.
NH3 + 1.5 02 -> HNO2 + H20
HNO2 + 0.5 O2 -> HNO3
NH3 + 2 O2 -> HNO3 + H2O
3. Determine the ThOD.
ThOD= (1.5 + 2) mol O2/mol glycine
= 3.5 mol O2/mol glycine x 32 g/mol O2 / 75 g/mol glycine
= 1.49 g O2/g glycine
See also
- Biological oxygen demand
- Chemical oxygen demandChemical oxygen demandIn environmental chemistry, the chemical oxygen demand test is commonly used to indirectly measure the amount of organic compounds in water. Most applications of COD determine the amount of organic pollutants found in surface water or wastewater, making COD a useful measure of water quality...
- Carbonaceous biochemical oxygen demandCarbonaceous biochemical oxygen demandCarbonaceous biochemical oxygen demand or CBOD is a method defined test measured by the depletion of dissolved oxygen by biological organisms in a body of water in which the contribution from nitrogenous bacteria has been suppressed. CBOD is a method defined parameter is widely used as an...