Demanda Química de Oxígeno (DQO)

Chemical oxygen demand

Chemical oxygen demand
In environmental chemistry, the chemical oxygen demand (COD) 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 (e.g. lakes and rivers) or wastewater, making COD a useful measure of water quality. It is expressed
inmilligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution. Older references
may express the units as parts per million (ppm).

Overview
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 tocarbon
dioxide, ammonia, and water is given by:

This expression does not include the oxygen demand caused by the oxidation of ammonia into nitrate. The process of
ammonia being converted into nitrate is referred to as nitrification. The following is the correct equation for the
oxidation of ammonia into nitrate.

It is applied after the oxidation due to nitrification if the oxygen demand fromnitrification must be known.
Dichromate does not oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard chemical
oxygen demand test.
The International Organization for Standardization describes a standard method for measuring chemical oxygen
demand in ISO 6060 [1].

History
For many years, the strong oxidizing agent potassium permanganate (KMnO4) was usedfor 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 potassiumpermanganate 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 topurify, 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 stillremaining in the sample.

1

Chemical oxygen demand

Using potassium dichromate
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:

where d = 2n/3 + a/6 - b/3 - c/2. Most commonly, a 0.25 N solution of potassium dichromate is used forCOD
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, andis used as an indirect measure of the organic contents of the water
sample.

Blanks
Because COD measures the oxygen demand of organic compounds in a sample of water, it is important that no
outside organic material be accidentally added to the sample to be measured. To control for this, a so-called blank
sample is required in the determination of COD (and BOD -biochemical oxygen demand -...