Chemical oxygen demand

The basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions. The amount of oxygen required to oxidize an organic compound to carbon dioxide, ammonia, and water is given by:

${\displaystyle {\mbox{C}}_{n}{\mbox{H}}_{a}{\mbox{O}}_{b}{\mbox{N}}_{c}+\left(n+{\frac {a}{4}}-{\frac {b}{2}}-{\frac {3}{4}}c\right){\mbox{O}}_{2}\rightarrow n{\mbox{CO}}_{2}+\left({\frac {a}{2}}-{\frac {3}{2}}c\right){\mbox{H}}_{2}{\mbox{O}}+c{\mbox{NH}}_{3}}$

This expression does not include the oxygen demand caused by nitrification, the oxidation of ammonia into nitrate:

${\displaystyle {\mbox{N}}{\mbox{H}}_{3}+2{\mbox{O}}_{2}\rightarrow {\mbox{N}}{\mbox{O}}_{3}^{-}+{\mbox{H}}_{3}{\mbox{O}}^{+}}$

Dichromate, the oxidizing agent for COD determination, does not oxidize ammonia into nitrate, so nitrification is not included in the standard COD test.

The International Organization for Standardization describes a standard method for measuring chemical oxygen demand in ISO 6060 .

Potassium dichromate is a strong oxidizing agent under acidic conditions. Acidity is usually achieved by the addition of sulfuric acid. The reaction of potassium dichromate with organic compounds is given by:

${\displaystyle {\ce {C}}_{n}{\ce {H}}_{a}{\ce {O}}_{b}{\ce {N}}_{c}\ +\ d{\ce {Cr2O7^2-}}\ +\ (8d\ +\ c){\ce {H+ ->}}n{\ce {CO2}}\ +\ {\frac {a+8d-3c}{2}}{\ce {H2O}}\ +\ c{\ce {NH4+}}\ +2d{\ce {Cr^3+}}}$

where ${\displaystyle d=2n/3+a/6-b/3-c/2}$. Most commonly, a 0.25 N solution of potassium dichromate is used for COD determination, although for samples with COD below 50 mg/L, a lower concentration of potassium dichromate is preferred.

In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (since in all redox reactions, one reagent is oxidized and the other is reduced), forming Cr³⁺. The amount of Cr³⁺ is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample.

A solution of 1.485 g 1,10-phenanthroline monohydrate is added to a solution of 695 mg FeSO₄·7H₂O in distilled water, and the resulting red solution is diluted to 100 mL.

Some samples of water contain high levels of oxidizable inorganic materials which may interfere with the determination of COD. Because of its high concentration in most wastewater, chloride is often the most serious source of interference. Its reaction with potassium dichromate follows the equation:

${\displaystyle \mathrm {6Cl^{-}+Cr_{2}O_{7}^{2-}+14H^{+}\rightarrow 3Cl_{2}+2Cr^{3+}+7H_{2}O} }$

Prior to the addition of other reagents, mercuric sulfate can be added to the sample to eliminate chloride interference.

The following table lists a number of other inorganic substances that may cause interference. The table also lists chemicals that may be used to eliminate such interference, and the compounds formed when the inorganic molecule is eliminated.

Many governments impose strict regulations regarding the maximum chemical oxygen demand allowed in waste water before they can be returned to the environment. For example, in Switzerland, a maximum oxygen demand between 200 and 1000 mg/L must be reached before waste water or industrial water can be returned to the environment .

For many years, the strong oxidizing agent potassium permanganate (KMnO₄) was used for measuring chemical oxygen demand. Measurements were called oxygen consumed from permanganate, rather than the oxygen demand of organic substances. Potassium permanganate's effectiveness at oxidizing organic compounds varied widely, and in many cases biochemical oxygen demand (BOD) measurements were often much greater than results from COD measurements. This indicated that potassium permanganate was not able to effectively oxidize all organic compounds in water, rendering it a relatively poor oxidizing agent for determining COD.

Since then, other oxidizing agents such as ceric sulphate, potassium iodate, and potassium dichromate have been used to determine COD. Of these, potassium dichromate (K₂Cr₂O₇) has been shown to be the most effective: it is relatively cheap, easy to purify, and is able to nearly completely oxidize almost all organic compounds.

In these methods, a fixed volume with a known excess amount of the oxidant is added to a sample of the solution being analyzed. After a refluxing digestion step, the initial concentration of organic substances in the sample is calculated from a titrimetric or spectrophotometric determination of the oxidant still remaining in the sample. As with all colorimetric methods blanks are used to control for contamination by outside material.

• Biochemical oxygen demand – Amount of dissolved oxygen needed by aerobic biological organisms to break down organic material
• Carbonaceous biochemical oxygen demand
• Theoretical oxygen demand
• Wastewater quality indicators discusses both BOD and COD as measures of water quality.

• ISO 6060: Water quality - Determination of the chemical oxygen demand