Dichromate chemical oxygen demand

Oxidizability with potassium dichromate (chemical oxygen demand, COD) as mg... 

COD (Chemical Oxygen Demand) Legislation and controls Oxidation by dichromate... 

Discussion. One very important application of potassium dichromate is in a back-titration for the environmental determination16 of the amount of oxygen required to oxidise all the organic material in a sample of impure water, such as sewage effluent. This is known as the chemical oxygen demand (C.O.D.) and is expressed in terms of milligrams of oxygen required per litre of water, mg L l. The analysis of the impure water sample is carried out in parallel with a blank determination on pure, double-distilled water. 

The chemical method for the determination of the chemical oxygen demand of non-saline waters involves oxidation of the organic matter with an excess of standard acidic potassium dichromate in the presence of silver sulfate catalyst followed by estimation of unused dichromate by titration with ferrous ammonium sulfate. Unfortunately, in this method, the high concentrations of sodium chloride present in sea water react with potassium dichromate producing chlorine ... 

Consequently the consumption of dichromate is many times higher than that due to organic material in the sample. To complicate matters, any amines in the sample consume and release chlorine in a cyclic process, leading to high chemical oxygen demands. 

Chemical oxygen demand (COD) is another wet chemical oxidation method. The chemical basis is the reaction of dichromate in concentrated sulfuric acid matrix according to Eq. (10.3). COD values are given mg liter1 02. 

Chemical Oxygen Demand. COD refers to the amount of oxygen consumed during chemical oxidation with hot dichromate. 

Chemical oxygen demand (COD)— A measure of the oxygen equivalent content of a given waste obtained by using a chemical such as potassium dichromate to oxidize the organic content of the waste. 

Chemical Oxygen Demand. The chemical oxygen demand (COD) test measures the oxygen equivalent of the organic matter in a sample that is susceptible to oxidation by a strong oxidant (20). The sample is refluxed with a known amount of potassium dichromate in sulfuric acid for 2 hs. Silver sulfate is added to catalyze the oxidation of straight-chain compounds, and mercuric sulfate is added to the sample prior to refluxing to react with chloride ion and prevent its oxidation. The amount of unreacted dichromate is then titrated with standard ferrous ammonium sulfate. 

The chemical oxygen demand (COD) test may be performed by Standard Method 5220, which has three variants (Standard Methods, 1998). The most common method is 5220 D, the closed reflux colorimetric method. Like the BOD test described above, COD is a measure of the amount of oxygen required to stabilize wastewater. COD is the quantity of oxygen required to stabilize wastewater, reported as mg 1 when exposed to a strong oxidizer, that is, dichromate. 

Chemical oxygen demand, COD Chemical oxygen demand refers to the amount of oxygen consumed during chemical oxidation with hot acid dichromate. It gives an indication of the oxidisable material present, but only as a second best to ThOD. Not all chemical compounds are oxidised by dichromate, particularly oil products and lubricants. 

T. Korenaga, H.Y. Ikatsu, The determination of chemical oxygen demand in waste-waters with dichromate by flow injection analysis, Anal. Chim. Acta 141 (1982) 301. 

T. Korenaga, The Continuous Determination of Filtered Chemical Oxygen Demand with Potassium Dichromate by Means of Flow Injection Analysis. Bull. Chem. Soc. Jpn., 55 (1982) 1033. 

T. Korenaga and H. Ikatsu, The Determination of Chemical Oxygen Demand in Waste Waters with Dichromate by Flow Injection Analysis. Anal Chim. Acta, 141 (1982) 301. 

The amount of oxygen used up in microbial oxidation is - the biochemical oxygen demand (BOD) - another important water-quality indicator. The BOD is taken as a realistic measure of water quality - a clean river would have a BOD value of less than 5 ppm, whereas a very polluted river could have a BOD value of 17 ppm or more. A BOD determination takes a few days, so another parameter called the chemical oxygen demand (COD) is sometimes measured. In a COD determination, acid dichromate is used to oxidize the organic matter in a sample of water. The measurement takes only a couple of hours. 

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