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magnetic stirring ba . If field measurements are being made, the electrodes may be immersed <br /> directly into the sample stream to an adequate depth and moved in a manner to ensure <br /> sufficient sample movement across the electrode-sensing element as indicated by drift-free <br /> readings (< 0.1 pH). <br /> 7.3 If the sample temperature differs by more than 2 •C from the buffer solution, the <br /> measured pH values must be corrected. Instruments are equipped with automatic or manual <br /> compensators that electronically adjust for temperature differences. Refer to manufacturer's <br /> instructions. <br /> 7.4 Thorouc hly rinse and gently wipe the electrodes prior to measuring pH of samples. <br /> Immerse the electro es into the sample beaker or sample stream and gently stir at a constant <br /> rate to provide homogeneity and suspension of solids. Note and record sample pH and <br /> temperature. Repeat measurement on successive aliquots of sample until values differ by < 0.1 <br /> pH units. Two or th ee volume changes are usually sufficient. <br /> 8.0 QUALITY CO TROL <br /> 8.1 Refer to Chapter One for the appropriate QC protocols. <br /> 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 water samples <br /> containing exact inc ements of hydrogen-hydroxyl ions, with the following results: <br /> Accuracy as <br /> Standard Deviation Bias Bias <br /> H Units H Units % H Units <br /> 3.5 0.10 -0.29 -0.01 <br /> 3.5 0.11 -0.00 <br /> 7.1 0.20 +1.01 +0.07 <br /> 7.2 0.18 -0.03 -0.002 <br /> 8.0 0.13 -0.12 -0.01 <br /> 8.0 0.12 +0.16 +0.01 <br /> 9040C - 3 Revision 3 <br /> November 2004 <br />