Methods for Chemical Analysis of Water and Wastes

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Methods for Chemical Analysis of Water and Wastes ( methods-chemical-analysis-water-and-wastes )

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72 7 3 74 8 Calculation 8 1 7 1.1 If volattle orgamcs are present m the sample, use an AllIhn condenser and add the sulfunc aCid-stlver sulfate solutIOn through the condenser, whtle coolIng the flask, to reduce loss by volatilIzatIOn Apply heat to the flask and reflux for 2 ho~rs For some waste waters, the 2-hour reflux penod is not necessary The time reqmred to give the maXimum oXidatIOn for a wastewater of constant or known compOSitIOn may be determmed and a shorter penod of refluxmg may be permissible Allow the flask to cool and wash down the condenser with about 25 ml of disttlled water If a round bottom flask has been used, transfer the mixture to a 500 ml Erlenmeyer flask, washmg out the reflux flask 3 or 4 times With disttlled water Dtlute the aCid solutIOn to about 300 ml with disttlled water and allow the solutIOn to cool to about room temperature Add 8 to 10 drops of ferrom mdicator (6 6) to the solutIOn and titrate the excess dichromate with 0025 N ferrous ammomum sulfate (64) solutIOn to the end pomt The color change Will be sharp, changmg from a blue-green to a reddish hue Blank-Simultaneously run a blank determmatIOn followmg the detatls given m (7 1) and (7 2), but usmg low COD water m place ofsample Calculate the COD m the sample m mg/l as follows COD, mg/l = (A - B)~ x 8,000 where A = mtllIlIters ofFe(NH4)iS04)2 solutIOn reqmred for titration ofthe blank, B = mtllIlIters of Fe(NH4)iS04)2 solutiOn reqmred for titratIOn of the sample, N = normalIty of the Fe(NH4)lS04)2 solutIOn, and S = mtllIlIters ofsample used for the test 9 PreCiSIOn and Accuracy 9 1 Eighty-six analysts m fifty-eIght laboratones analyzed a distilled water solutIOn contammg OXidizable orgamc matenal eqmvalent to 123 mg/l COD The standard deViatIOn was ±4 15 mg/l COD With an accuracy as percent relative error (bias) of o3% (EP A Method Research Study 3 ) 4102-3

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