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52 53 54 5 5 6 Reagents 6 1 6 2 Glass electrode Reference electrode-a calomel, sl1ver-sl1ver chlonde or other reference electrode of constant potential may be used NOTE 1: CombmatIOn electrodes mcorporatIng both measuring and reference functIons are convenient to use and are aval1able With sohd, gel type filhng matenals that requIre minimal maintenance MagnetiC stirrer and Teflon-coated stlrnng bar Thermometer or temperature sensor for automatic compensation Pnmary standard buffer salts are aval1able from the NatIOnal Bureau of Standards and should be used 10 SituatIOns where extreme accuracy IS necessary 6.1 1 PreparatIOn ofreference solutions from these salts reqmre some speCial precautions and handlIng(l) such as low conductiVity dl1utIOn water, drymg ovens, and carbon diOXide free purge gas These solutIons should be replaced at least once each month Secondary standard buffers may be prepared from NBS salts or purchased as a solutIOn from commerCial vendors Use of these commercially aval1able solutIOns, that have been vahdated by companson to NBS standards, are recommended for routme use. 7. CahbratIOn 7.1 Because of the Wide vanety of pH meters and accessones, detal1ed operatmg procedures cannot be Incorporated Into thIs method Each analyst must be acquamted With the operatIOn ofeach system and famlhar With all mstrument functions SpeCial attentIOn to care of the electrodes IS recommended 72 Each mstrument/electrode system must be cahbrated at a mmlmum of two pomts that bracket the expected pH of the samples and are approximately three pH Units or more apart 7 2 1 V anous mstrument designs may mvolve use of a "balance" or "standardize" dial 8 Procedure 81 8.2 8 3 Standardize the meter and electrode system as outhned In SectIOn 7 Place the sample or buffer solutIOn In a clean glass beaker usmg a suffiCient volume to cover the sensmg elements of the electrodes and to give adequate clearance for the magnetic stlmng bar 8 2 1 If field measurements are bemg made the electrodes may be Immersed directly m the sample stream to an adequate depth and moved In a manner to msure suffiCient sample movement across the electrode sensing element as mdlcated by dnft free (<0 1pH) readings Ifthe sample temperature dIffers by more than 2°e from the buffer solutIOn the measured pH values must be corrected Instruments are eqmpped With automatic or manual (I)NatIOnal Bureau of Standards SpeCial Pubhcatlon 260 1501-2 and/or a slope adjustment as outhned In the manufacturer's instructIOns Repeat adjustments on successive portions of the two buffer solutIOns as outhned m procedure 8 2 untl1 readings are wlthm 0 05 pH Units of the buffer solutIOn value ,PDF Image | Methods for Chemical Analysis of Water and Wastes
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