Chemical Oxygen Demand Calculator

Chemical oxygen demand expresses the oxidizable content of a water sample as the mass of oxygen that would be needed to burn it, reported in mg/L even though no oxygen is used in the test. The sample is refluxed two hours at 150 degrees C with a measured excess of potassium dichromate in concentrated sulfuric acid, with silver sulfate to catalyse oxidation of straight-chain aliphatics and mercuric sulfate to complex chloride, which would otherwise oxidize to chlorine and inflate the result. What survives is back-titrated with ferrous ammonium sulfate against ferroin indicator, the endpoint turning blue-green to reddish brown. Since dichromate consumed equals oxidizable matter, the answer is the difference between blank and sample titres. The factor 8000 folds together the equivalent weight of oxygen, 8 g per equivalent since the 32 g mole undergoes a four-electron change, and the thousandfold conversion from mL to L. Typical values run 250 to 800 mg/L for raw domestic sewage, below about 125 mg/L for well-treated effluent, which is the European urban wastewater directive discharge limit, and from several thousand to well over 10,000 mg/L for food, pulp, or chemical process streams, which is why the dilution factor exists. Comparison with biochemical oxygen demand indicates treatability: COD always equals or exceeds BOD, and a ratio much above 4 signals a large refractory fraction biological treatment will not remove. The test has real blind spots. Benzene, toluene, and pyridine resist dichromate and are under-reported, while ammonia is not oxidized unless chloride is present. Reduced inorganics including sulfide, nitrite, and ferrous iron register as demand despite not being organic. The reagents are hazardous, generating hexavalent chromium and mercury waste.

The idea of rating water by the oxidant it consumes is Victorian. British sanitary chemists from the 1860s onward reported an oxygen absorbed figure based on potassium permanganate, a test that was quick and became a routine index of sewage contamination but suffered from irreproducibility, since the result depended heavily on contact time and temperature and permanganate oxidized only a variable portion of the organic load. Dichromate in strong sulfuric acid was recognized as the far more aggressive and more complete oxidant, and the modern procedure grew out of efforts in the mid-twentieth century to make that reaction reproducible enough for regulatory use. The key refinements came in the late 1940s, when work in the United States established the two-hour reflux with silver sulfate catalysis and mercuric sulfate masking of chloride that gives the method its consistency, and the procedure was subsequently codified in Standard Methods for the Examination of Water and Wastewater, where it is now section 5220. A major practical change followed in 1975, when Jirka and Carter described in Analytical Chemistry a sealed-tube micro-scale version read colorimetrically rather than by titration; that closed-reflux approach became the basis of the pre-dosed vial kits and small block digesters used in most laboratories today, cutting reagent volumes and hazardous waste by more than an order of magnitude. Concern over chromium and mercury disposal continues to push some laboratories toward total organic carbon analysis as a substitute.