D6423_9

Turn on the pH meter and allow it to warm up thoroughly in accordance with the manufacturer's instructions. Note the temperature of the sample to be tested. If the temperature compensation is to be manual, adjust the temperature dial of the meter to correspond to the temperature of the sample to be tested. And allow time for all buffers, solutions, and the electrode to equilibrate thermally. Clean or rehydrate the electrode after every 10 samples and new electrodes before first use by alternately soaking four times in 1 molar sodium hydroxide solution and 1 molar sulfuric acid, or 1 molar hydrogen chloride for about 30 seconds each. Remove the electrode and rinse it with distilled water into a waste container. Calibrate the pH meter to pH 7.00, with a water-based pH 7.00 buffer solution. Remove the electrode and rinse it with distilled water into a waste container. Calibrate the pH meter to pH 4.00, with a water-based pH 4.00 buffer solution using the slope adjustment. The slope shall be in the 95% to 100% range, or the electrode will have to be cleaned or replaced. Remove the electrode and rinse it with distilled water into a waste container. Return the electrode to the pH 7.00 buffer. And if the measured pH is not between 6.90 and 7.10, then return to subsection 9.2 of the written standard.