The standard “ISO 6060 Water quality - Determination of chemical oxygen demand” developed by the International Organization for Standardization (ISO) describes a method for determining the chemical oxygen demand (COD) of water. This method is valid for water with a COD value between 0 mg/l and 700 mg/l. The chloride content should not exceed 1000 mg/l. A water sample that meets these conditions is used directly for analysis. If the COD value exceeds 700 mg/l, the water sample is diluted. For the highest accuracy, it is preferred that the COD value of the sample be between 300 mg/l and 600 mg/l.
Under the given reaction conditions, organic compounds are intensively oxidized, with the exception of compounds with certain structural elements (e.g. pyridine nucleus, quaternary nitrogen compounds).
Volatile hydrophobic substances can evaporate and thus escape oxidation. Examples of inorganic compounds that are oxidized under the reaction conditions are:
On the other hand, certain compounds may react as oxidizing agents under the reaction conditions. These conditions should be kept in mind when determining the use of the test results. This standard should be followed, especially for interferences from chlorides.
Chemical oxygen demand is a test that measures the amount of oxygen required to chemically oxidize organic matter and inorganic nutrients such as ammonia or nitrate in water. Chemical oxygen demand is measured by a laboratory test in which a sample is incubated with a strong chemical oxidant for a specified time and at a constant temperature (usually 150 hours at 2 degrees). The most commonly used oxidant is potassium dichromate combined with boiling sulfuric acid. The chemical oxidant is not specific to organic or inorganic compounds, and so both sources of oxygen demand are measured in a chemical oxygen demand test. The measurements are not directly comparable to biochemical oxygen demand, but can be used as a complement.
For the purposes of this international standard, chemical oxygen demand (COD) expresses the mass concentration of oxygen equivalent to the amount of dichromate consumed by dissolved and suspended matter when a water sample is treated with that oxidant under defined conditions.
This method is based on the principle of refluxing a test portion with a known amount of potassium dichromate and a silver catalyst in strong sulphuric acid for a specified time in the presence of mercury(II) sulphate, during which time a portion of the dichromate is reduced by the oxidisable material present. It is also based on the principle of titrating the remainder of the dichromate with ammonium iron sulphate and calculating the chemical oxygen demand value from the amount of dichromate reduced. 1 mole of dichromate is equivalent to 1,5 moles of oxygen. If the test sample contains more than 1000 mg/l of chloride, a modified procedure should be applied.
This method involves the handling and boiling of strong solutions of sulphuric acid and dichromate. Protective clothing, gloves and full face protection are required when performing the tests. In the event of a spill, immediate washing with plenty of clean water is the simplest and most effective solution. Concentrated sulphuric acid should always be added to water with care, gently shaking the contents of the bottle. Care should be taken when preparing and using solutions containing silver sulphate and mercuric sulphate, as these substances are toxic. The reagents used contain mercury, silver and chromium salts. The reagents used should be disposed of in accordance with national or local regulations.
Only reagents of recognized analytical quality and only distilled water or water of equivalent purity are used during the analysis. The quality of the water is of great importance for the accuracy of the results. The quality of the water is checked by running blanks and similar parallel tests without any boiling, but otherwise exactly as specified. In both cases, attention should be paid to the consumption of ammonium ferrous sulphate solution. A difference of more than 0,5 ml indicates poor water quality. For the determination of chemical oxygen demand values below 100 mg/l, the difference should not exceed 0,2 ml. The quality of the distilled water can usually be improved by redistillation from an acidified solution of potassium dichromate or potassium permanganate using all-glass distillation equipment. Two blank tests are carried out in parallel with each set of determinations according to the procedure given in this standard, but the test portion is replaced with 10,0 ml of water.
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