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compensators that electromcally adjust for temperature differences Refer to manufacturer's mstructlOns ' 84 After nnsmg and gently wlpmg the electrodes, If necessary, Immerse them mto the sample beaker or sample stream and stir at a constant rate to provide homogeneity and suspenslOn ofsohds Rate ofstIffing should mmimize the air transfer rate at the aIr water mterface of the sample Note and record sample pH and temperature Repeat measurement on succeSSIve volumes of sample untll values differ by less than 0 1 pH umts Twoorthreevolumechangesareusuallysufficient 85 For aCId rain samples It IS most Important that the magnetic stirrer IS not used Instead, sWIfI the sample gently for a few seconds after the mtroductIon of the e1ectrode(s) Allow the electrode(s) to eqUIlIbrate The air-water mterface should not be disturbed whtle measurement IS bemg made If the sample IS not m equdlbnum with the atmosphere, pH values wdl change as the dIssolved gases are either absorbed or desorbed Record sample pH and temperature 9 CalculatIon 9 1 pH meters read dIrectly m pH umts Report pH to the nearest 0 1 umt and temperature to the nearest °C 10 PrecIsIon and Accuracy 10 1 Forty-four analysts m twenty laboratorIes analyzed SIX synthetic water samples contaImng exact mcrements ofhydrogen-hydroxyl Ions, WIth the followmg results pH UnIts 35 35 71 72 80 80 Standard DeVIatiOn BIas, pH UnIts % 010 -029 011 -000 020 +101 018 -003 013 -012 012 +016 BIas, pH UnIts -001 +007 -0002 -001 +001 (FWPCA Method Study 1, Mmeral and PhysIcal Analyses) 10 2 In a smgle laboratory (EMSL), usmg ~urfacewater samples at an average pH of7 7, the standarddeVIatIonwas ±O1 BIblIography 1 Standard Methods for the Exammation of Water and Wastewater, 14th EditlOn, p 460, (1975) 2 Annual Book ofASTM Standards, Part 31, "Water", Standard DI293-65, p 178 (1976) 3 Peden, M E and Skowron, L M, Iomc StabIlIty of PreCIpItatIOn Samples, Atmosphenc EnvIfonment, Vol 12, pp 2343-2349, 1978 1501-3 Accuracy asPDF Image | Methods for Chemical Analysis of Water and Wastes
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