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temperature compensation and wIth a recorder or alarm function 6 2 Glass electrode - with shielded cable between electrode and monrtor unless preampllflcatlon IS used 6.3 Reference electrode - a reference electrode wIth a constant potential and with eIther a vIsible electrolyte or VISCOUS gel fill NOTE 1 Combmation electrodes corporatmg both measuring and refer- ence functions are convenrent to use and are available wIth solid, gel-type fil- ling matenals that require minimal maintenance 6 4 Temperature sensor - for automatic compensator covering general ambient temperature range 6.6 Electrode mounting - to hold electrodes, may be flow through (for small flows), pipe mounted or Immersion 6. Reagents 6.1 Primary standard buffer salts are available from the National Bureau of Standards and should be used In situations where extreme accuracy IS required 6 1 1 Preparation of reference solutions from these salts require some s~clal precautions and handhng' such as low conductivity dilution water, drying ovens, and carbon dIoxide free purge gas These solutions should be replaced at least once each month 6.2 Secondary buffers may be prepared from NBS salts or purchased as a solution from commercIal vendors. Use of these commercially available solutIOns, whIch have been validated by comparison to NBS standards, IS recommended for routine operatIOn These buffers may be retamed for at least SIX months If kept stoppered 7. Calibration 7.1 Immersion type electrodes - easily removed from mountmg 7.1 1 The electrode should be cahbrated at a mmlmum of two pomts that bracket the expected pH of the water/waste and are approxImately three pH units or more apart 'Nallonll Bureau of Standards SpeclSl PublicatIon 260 7 1 2 Repeat calibration adjustments on successive portions of the two buffer solutIons until readings are wlthm ±O 05 pH Units of the buffer value If calibration problems occur, see43 7 1 4 Calibration against two buffers should be carrred out at least dally If the pH of the flUid being measured fluctuates conSiderably, the calibration should be carrred out more often Calibration frequencIes may be relaxed If hIstorical data supports a longer penod between calibration 7 2 Immersion type electrodes - not easily removed from mountmg 7 2 1 Collect a grab sample of the flOWing matenal from a POint as close to the electrode as pOSSible Measure the pH of thiS grab sample as qUickly as pOSSible With a laboratory - type pH meter Adjust the calibration control of the continuous monrtor to the reading obtained 7 2 2 The temperature and condition of the grab sample must remain constant untIl ItS pH has been measured by the laboratory pH meter The temperature of the sample should be measured and the temperature compensator of the laboratory pH meter adjusted 7 2 3 The laboratory - type pH meter should be calibrated pnor to use against two buffers as outlined In 7 1 7 2 4 The continuous pH monrtonng system should be initially calibrated against two buffers as outlined In 7 1 before being placed Into service Recallbratlon (every 30 days) at two POints IS recommended If at all pOSSIble to ensure the measunng electrode IS In working order If thiS IS not pOSSible, the use of electrode testing features for a broken or malfunctioning electrode should be conSidered when purchasing the eqUipment 7 2 5 The indirect calibration should be carrred out at least once a day If the pH of the flUid being measured fluctuates conSiderably, the calibration should be carrred out more often Calibration frequencies may be relaxed If hlstoncal data support a longer penod between calibration 7 2 6 If the electrode can be removed from the system, but With difficulty, It should be directly calibrated as In 7 1 at least once a month 7 3 Flow-through type electrode - easily removed from ItS mounting 73 1 Calibrate uSing buffers as In 7 1 The buffers to be used may be the plocess stream Itself as one buffer and as a second buffer after adjustment of pH by additIon of an aCid or base ThiS Will prOVide the larger volumes necessary to calibrate thiS type electrode 73 2 Since the velOCIty of sample flow-through a flow through electrode can produce an offset error In pH reading, the user must have data on hand to show that the offset IS known and compensation has been accomplished 7 4 Flow-through type electrode· not easily removed from ItS mounting 74 1 CalibrateasIn 72 74 2 Quality control data must be on hand to show the user IS aware of pOSSible sample flow velOCity effects 8 Procedure 8 1 Calibrate the monitor and electrode system as outlined In SectIon 7 8 2 Follow the manufacturer's recommendation for operation and installation of the system 8 3 In wastewaters, the electrode may require penodlc cleaning After manual cleaning, the electrode should be calibrated as In 7 1 or 7 2 before returning to servICe Calculations I n - 7 1 3 mstruments available, no detailed operatmg mstructlons are prOVided Instead, the analyst should refer to the particular manufacturer s mstructlons Because of the Wide vanety of Dec 1982 1502-2 9 The electrode must be placed so 8 4 that the water or waste flOWing past the electrode IS representative of the system 9 1 Units Reports pH to the nearest 0 1 Unit and temperature to the nearest °C pH meters read directly In pH 10 10 1 of eqUipment and conditions and the changeable character of the pH of many process waters and wastes, the preCISion of thiS method IS probably less than that of Method 150 1, however, a preCISion of 0 1 pH unrt PrecIsion and Accuracy Because of the Wide vanabilltyPDF Image | Methods for Chemical Analysis of Water and Wastes
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