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December 23, 1991 Page C-!Z <br />• carbon copy will be retained by the samplers. Chain-of-custody will be maintained until final <br />disposition of the samples by the laboratory. <br />C.6 INSTRUMENT CALIBRATION <br />C.6.1 pH Meter <br />The pH meter must be calibrated before taking any readings of samples and if erroneous <br />readings are suspected. Calibration and operation of the pH meter should follow the manufacturer's <br />specific instructions. In general, calibration is done by adjusting the meter with standard buffers that <br />bracket the expected pH of field water. Calibration will consist of the following general procedures: <br />Adjust the reading of the pH meter with the electrode placed in the pH 7 buffer by <br />using the calibration knob. Rinse the electrode with distilled water between buffer <br />adjustments. <br />Adjust the reading ofthe meter with the electrode placed in the pH 10 buffer with the <br />slope (or temperature) knob. <br />• ~ Repeat steps i and 2 untii the meter gives acceptable readings (t0.1 pH unit} for all <br />the buffers used for calibration. <br />Note: Always use the same electrode for measurements that was used in the calibration. <br />Recalibrate the meter if the electrode is replaced. Also, the temperature setting on the pH meter <br />often does not match the sample temperature after calibration. The pH readings will still be accurate <br />in these cases, provided that the response to the buffers is correct. <br />Record the time and temperature in the field notebook whenever the pH meter is calibrated. <br />When recording pH values from a sample, check pH meter and recalibrate (as necessary) if sample <br />pH values are significantly different from previous values. <br />C.6.2 Conductivity Meter <br />The temperature-compensated conductivity meter must be calibrated before taking field <br />measurements. Record time, temperature, and instrument response in the field notebook. <br />Calibration is done by noting the response of [he meter to several standard conductivity solutions <br />which bracket the values expected to be measured in the Held. Standards of 100, 1,000, and ] 0,000 <br />• should be adequate for the samples expected. If the instrument has a calibration adjustment, set the <br />response to match the standards. Otherwise, simply record the instrument response to each stmrdard <br />in the field notebook. <br />Agapito Associates, Inc. <br />