In environmental chemistry, chemical oxygen demand (COD) tests the measurement of oxygen that is consumed during the test by particular organic compound. In other words the COD test is used to measure the amount organic matter present in the particular organic compound. Its application is commonly in the measurement of organic pollutants present in the surface water or river water and to clean the water. A common method for chemical oxygen demand analysis is to react the strong oxidizing chemical such as potassium dichromate or dichromate ion with the organic matter. Potassium dichromate oxidizes the whole organic matter to carbon dioxide, ammonia and water under acidic condition.
Chemical oxygen demand is an important parameter for the quality check of the water as it helps to know the effect that will discharged wastewater effect will have on the environment. High chemical oxygen demand level means the greater amount of oxidixable organic matter which in result less amount of dissolved oxygen in the water. COD is the most valuable test to know the level of pollution, water contains. As it takes long time to oxidizing chemical to oxidize the organic compound we can also use UV-VIS technology. By using this technology, we can know the COD in real time. This system works with the help of UV lights. At certain wavelength the organic compound present in the water, such as tannin, lignin and other aromatic compounds absorbs the UV rays. By measuring wavelengths and making exact calibration we can obtain COD data.
For many years, strong oxidizing substance potassium permanganate is being used for measuring COD. Later oxidizing substance such as potassium iodate and ceric sulphate. But they were too costly to afford and were not giving the perfect results. When potassium dichromate was used it gives the perfect COD value. As it as relatively cheaper it was most commonly used..
COD WITH POTASSIUM DICHROMATE: mostly every type of organic compounds is oxidizied by means of boiling mixture containing organic compound with chromic and sulphuric acid. The sample of organic matter is refluxed with potassium dichromate in strong acidic solution. The unused potassium dichromate used in the process is being titrated with the ferrous ammonium sulphate to know the exact amount of potassium dichromate which had been consumed and thus oxidizable matter is calculated in terms of the potassium dichromate terms. Some samples with very low chemical oxygen demand need to be analyzed in replicate to get the accurate results.
LIMITATIONS: it is mostly observed that the oxidation of the given sample organic compound is about 90% to 95% that of theoretical value. Organic compounds such as pyridine and related compound resist the oxidation. Volatile organic compounds also react in proportion to which they are in contact with the oxidant. Straight chain aliphatic organic compound can easily be oxidized in the presence of silver sulphate catalyst. Chloride ions or other halogen ions of bromide, iodide easily reacts with silver ion to form the precipitate of silver chloride. This type of interference is negative, which tend to restrict the oxidizing action of potassium dichromate. This results in an error on the high side.