Theoretical oxygen demand (ThOD) is the calculated amount of

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

required to

oxidize Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

a compound to its final

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

products. However, there are some differences between standard methods that can influence the results obtained: for example, some calculations assume that

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

released from

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...

s is generated as

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

, whereas others allow for ammonia

to nitrate. Therefore, in expressing results, the calculation assumptions should always be stated. In order to determine the ThOD for

glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...

(CH₂(NH₂)COOH) using the following assumptions: #In the first step, the

organic carbon Total organic carbon (TOC) is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic c ...

and

are converted to

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

(CO₂) and

(NH₃), respectively. #In the second and third steps, the ammonia is oxidized sequentially to nitrite and nitrate. #The ThOD is the sum of the

required for all three steps. We can calculate by following steps: #Write balanced reaction for the carbonaceous oxygen demand.
CH₂(NH₂)COOH + 1.5O₂ → NH₃ + 2CO₂ + H₂O #Write balanced reactions for the nitrogenous oxygen demand.
NH₃ + 1.5O₂ → HNO₂ + H₂O
HNO₂ + 0.5O₂ → HNO₃
NH₃ + 2O₂ → HNO₃ + H₂O #Determine the ThOD.
ThOD = (1.5 + 2) mol O₂/mol glycine
= 3.5 mol O₂/mol glycine × 32 g/mol O₂ / 75 g/mol glycine
= 1.49 g O₂/g glycine The theoretical oxygen demand represents the worst-case scenario. The actual oxygen demand of any

compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive struc ...

depends on the

biodegradability Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...

of the compound and the specific organism

metabolizing Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

the compound. The actual

demand can be measured experimentally and is called the

biochemical oxygen demand Biochemical oxygen demand (BOD) is the amount of dissolved oxygen (DO) needed (i.e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. ...

(BOD).

References

{{reflist Environmental chemistry Water quality indicators

See also

References