Theoretical oxygen demand (ThOD) is the calculated amount of

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

required to

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

a compound to its final

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

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

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

released from

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s is generated as

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

, whereas others allow for ammonia

nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...

. 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. Glycine is one of the proteinogenic amino acids. It is encoded by all the codons starting with GG (G ...

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

organic carbon Total organic carbon (TOC) is an analytical parameter representing the concentration of organic carbon in a sample. TOC determinations are made in a variety of application areas. For example, TOC may be used as a non-specific indicator of wa ...

and

are converted to

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(CO₂) and

(NH₃), respectively. #In the second and third steps, the ammonia is oxidized sequentially to

nitrite The nitrite polyatomic ion, ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name ...

and

. #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 depends on the biodegradability of the compound and the specific organism metabolizing the compound. The actual

demand can be measured experimentally and is called the

biochemical oxygen demand Biochemical oxygen demand (also known as BOD or biological oxygen demand) is an analytical parameter representing the amount of dissolved oxygen (DO) consumed by aerobic bacteria growing on the organic material present in a water sample at a s ...

(BOD).

References

{{reflist Environmental chemistry Water quality indicators

See also

References