ASTM D1287 Standard Test Method for pH of Engine Coolants and Antirusts
8. Preparation of Electrode System
8.1 Maintenance of Electrodes Clean the glass electrode or combination electrode at frequent intervals (not less than once every week during continual use) in accordance with the manufacturer's instructions. Drain the calomel electrode at least once each week and till with KCI electrolyte. Keep the electrolyte level in the calomel electrode above that of the solution being tested at all times. When not in use, immerse the lower halves of the electrodes in distilled water, or the appropriate buffer solution recommended by the manufacturer. Do not immerse the electrodes in the solution being tested for any appreciable period of time between determinations. Although the electrodes are not extremely fragile, they should be handled carefully at all times.
8.1.1 A combination electrode system can also be used for this test method.
8.2 Preparation of Electrodes - Before and after using, wipe the electrode or combination electrode thoroughly with a clean cloth, or a soft absorbent tissue, and rinse with distilled water. Follow the manufacturer's recommendations if further preparation is required. Prior to each pH determination, soak the prepared electrode in distilled water for at least 2 min. Immediately before use. touch the tips of the electrodes with a dry cloth or tissue to remove excess water.
9. Standardization and Testing of Electrode System in pH Units
9.1 The assembly shall always be standardized with more than one of the standard buffer solutions. A single standardization point serves to establish the electrodes actual zero potential. but the electrode's slope is assumed to be a default or already known value. A pH measurement with one-point standardization should be restricted to a narrow pH range, very near the standardization point. A second standard buffer solution shall be used to check the linearity of the response of the electrodes at different pH values and to detect a faulty glass electrode or incorrect temperature compensation. The two buffer solutions selected should bracket the anticipated pH of the solutions being tested.
9.1.1 Turn on the instrument, allow it to warm up. and adjust according to the manufacturer's instructions. Immerse the tips of the electrodes in the standard buffer solution chosen and allow sufficient time for the temperature of the buffer solution and the electrodes to equalize. Set the temperature knob at the temperature of the buffer solution. Adjust the standardization or asymmetry potential control until the meter registers a scale reading, in pH units, equal to the known pH of the standardizing buffer solution.
9.1.2 Rinse the electrodes with distilled water and touch the lips with a dry cloth or tissue to remove excess water. Immerse the electrodes in a second standard buffer solution. The reading of the meter shall agree with the known pH of the second standard buffer solution within +/- 0.05 unit without changing the setting of the standardization of asymmetry potential control. If it docs not agree or if the meter gives a slow response time and drifts, the electrode should be cleaned in accordance with the manufacturer's instructions.
9.1.3 The presence of a faulty electrode is indicated by failure to obtain a correct value for the pH of the second standard buffer solution after the meter has been standardized with the first. A cracked electrode will often yield pH values that arc essentially the same for both standard buffer solutions.
9.2 Multi-point standardization allows measurements to be performed over a very broad pH range, but required the additional effort of carrying out further standardizations. With multiple buffer points, the meter uses the electrode slope applicable to the pH range of the current sample. This linear segments method is always in pH operation. When standardization by auto recognition of U.S. standard buffers or custom buffers is selected, three buffer points are usable. With auto- recognition of NIST buffers or a combination of U.S. standard, custom, and NIST buffers, or manual entry of buffers, up to five standardization buffers are permitted.
