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8 72 7 3 74 CalculatlOn 9 1 2 PreCIsIon and Accuracy 9 1 EIghty-SIX analysts m fifty-eIght laboratones analyzed a dist1I1ed water solutiOn contammg OXIdIzable orgamc matenal eqUlvalent to 270 mg/l COD The standard deVIatiOn was ±1776 mg/l COD With an accuracy as percent relative error (bIas) of -47% (EPAMethodResearchStudy3) BIblIography Standard Methods for the ExammatiOn of Water and Wastewater, 14th EditlOn, p 550, Method 508 (1975) Annual Book of ASTM Standards, Part 31, "Water", Standard D1252-67, p 473 (1976) 8 1 sulfate solutlOn (6 3) to the cooled reflux flask, agam usmg slow addItiOn WIth sWIrlmg motiOn CautiOn Care must be taken to assure that the contents ofthe flask are well mIxed Ifnot, superheatmg may result, and the mIxture may be blown out of the open end of the condenser 7 1 1 If voiatlie orgamcs are present m the sample, use an alhhn condenser and add the sulfunc aCId-sliver sulfate solutiOn through the condenser, whIle coohng the flask, to reduce loss by volatihzatiOn Apply heat to the flask and reflux for 2 hours For some waste waters, the 2-hour reflux penod IS not necessary The tIme reqUlred to gIve the maXImum oXIdatlOn for a wastewater ofconstant 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 distliled 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 dIStilled water Dliute the aCId solutiOn to about 300 ml WIth dIStilled water and allow the solutlOn 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 025 N ferrous ammomum sulfate (64) solutlOn 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 detalis 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/hter = (A - B)N x 8,000 S where A = mlIhhters of Fe(NH4)z(S04)2 solutlOn reqUlred for titratiOn of the blank, B = mlIhhters of Fe(NH4)2 (S04)2 solutiOn reqUlred for titratiOn ofthe sample, N = normahty of the Fe(NH4MS04)2 solutiOn, and S = mIlhhters ofsample used for the test 410 1-3PDF Image | Methods for Chemical Analysis of Water and Wastes
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