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adjustments o ccessive portions of the two buffer solutions until <br /> readings are within 0.05 pH units of the buffer solution value. <br /> 7.2 Place the sample or buffer solution in a clean glass beaker usinga <br /> P .... sufficient volume to cover the sensing elements of the electrodes and to <br /> adequate clearance for the magnetic stirringbar, give <br /> being made, the electrodes may be immersed irectly intothesampleestream to <br /> are <br /> an adequate depth and moved in a manner to ensure sufficient sample movement <br /> across the electrode-sensing element as indicated by drift-free readings <br /> (<0.1 pH) . <br /> 7.3 If the sample temperature differs by more than 2'C from the buffer <br /> solution, the measured pH values must be corrected. <br /> Instruments with automatic or manual compensators that electronically are <br /> ad ustipfod <br /> temperature differences. Refer to manufacturer's instructions. r <br /> 7.4 Thoroughly rinse and gently wipe the electrodes prior to measurin <br /> pH of samples. Immerse the electrodes into the sample beaker or s rea <br /> le st g <br /> and gently stir at a constant rate to provide homogeneity and suensi m <br /> solids. Note and record sample pH and temperature. Re eat measp on of <br /> successive volumes of sample until values differ b p s. Two on <br /> y <0.1 pH units. Two <br /> three volume changes are usually sufficient. or <br /> 8.0 QUALITY CONTROL <br /> 8.1 Duplicate samples and check standards should be analyzed routinely- <br /> 8.2 <br /> ouin lY•8.2 Electrodes must be thoroughly rinsed between samples. <br /> 9.0 METHOD PERFORMANCE <br /> 9.1 Forty-four analysts in twenty laboratories analyzed six synthetic <br /> water samples containing exact increments of hydrogen-hydroxyl ions, with the <br /> following results: <br /> Standard Deviation Bias Accuracy as <br /> H Units pH Units Bias <br /> % H Units <br /> 3.5 0.10 <br /> 3.5 0.11 -0.29 -0.01 <br /> 7.1 0.20 -0.00 <br /> 7.2 0.18 +1.01 +0.07 <br /> . _ , 8.8.® ^_ 13 -0.12 -0.002 <br /> 0.12 +0.16 : <br /> 10.0 REFERENCES <br /> 1. National "Bureau of Standards, M <br /> Standard Reference Material 87, Special Publication 260, al Catalog 1986- <br /> i . <br /> 9040 — 3 <br /> Revision 0 <br /> Date Septem er 1986 <br />