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<br />~- <br />compensators that electronically adjust For temperature differences. Refer to <br />manufacturer's instructions. <br />8.4 After rinsing and gently wiping the electrodes, if necessary, immerse them into the <br />sample beaker or sample stream and stir at a constant rate to provide homogeneity and <br />suspension of solids. Rate of stirring should minimize the air transfer rate at the air water <br />interface of the sample. Note and record sample pH and temperature. Repeat <br />measurement on successive volumes of sample until values differ by less than 0.1 pH <br />units. Two or three volume changes are usually sufficient. <br />9. Calculation <br />9.1 pH meters read directly in pH units. Report pH Co the nearest 0.1 unit and temperature <br />to the nearest °C. <br />I0. Precision and Accuracy <br />f0.1 Forty-four analysts in twenty laboratories analyzed six synthetic water samples <br />containing exact increments ofhydrogen-hydroxyl ions, with the following results: <br /> Accuracy as <br />pH Units Standard Deviation Bias, Bias, <br /> pH Units % pH Units <br /> <br />3.5 0. IO -0.29 -0.Ol <br />3.5 0.1 I -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. I2 -0.01 <br />8.0 0.12 +0.16 +0.01 <br />(FWPCA Method Study 1, Mineral and Physical Ana]yses) <br />10.2 In a single laboratory (EMSL), using surface water samples at an average pH of 7.7, the <br />standard deviation was t0.1. <br />Bibliography <br />1. Standard Methods for the Examination of Water and Wastewater, 14th Edition, p 460, (1975). <br />2. Annual Book ofASTM Standards, Part 31, "Water",Standard D1293-65, p 178 (1976). <br />L <br />150.1-3 <br />