Knowledge on Using pH Meters.doc

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Knowledge on using pH meters – Precautions for using pH meters: 1. Under normal circumstances, when the instrument is used continuously, it should be calibrated once a day； Generally, the instrument does not need to be recalibrated within 24 hours. 2. Before use, pull down the rubber sleeve on the upper end of the electrode to expose the small hole at its top. 3. For the calibration buffer solutions, a pH of 6.86 is generally used for the first time, while for the second use a buffer with a pH value close to that of the solution being tested is employed; if the solution being tested is acidic, a buffer with a pH of 4.00 should be chosen ； If the solution under test is alkaline, a buffer with pH=9.18 should be used. 4. During measurement, the lead wires connecting the electrodes should remain stationary; otherwise, it will cause instability in the measurements. 5. The electrode must never be immersed in distilled water. 6. Keep the electrode bulb moist; if it becomes dry, it should be soaked in a 3 mol/L potassium chloride solution or a slightly acidic solution for several hours before use, in order to reduce the electrode’s asymmetric potential. 7. Table showing the relationship between the pH value of buffer solutions and temperature: Temperature (°C): 0.05 mol/kg, 0.025 mol/kg, 0.01 mol/kg; Bases used: Potassium hydrogen phthalate, mixed phosphates, sodium tetraborate. Values for pH: 5 – 4.00, 6.95, 9.39; 10 – 4.00, 6.92, 9.33; 15 – 4.00, 6.90, 9.28; 20 – 4.00, 6.88, 9.23; 25 – 4.00, 6.86, 9.18; 30 – 4.01, 6.85, 9.14; 35 – 4.02, 6.84, 9.11; 40 – 4.03, 6.84, 9.07; 45 – 4.04, 6.84, 9.04; 50 – 4.06, 6.83, 9.03; 55 – 4.07, 6.83, 8.99; 60 – 4.09, 6.84, 8.97. 8. The electrode should be used with a pH meter that has a high input impedance (≥1012 Ω) in order to maintain its optimal performance. 9. The water used to prepare buffers at pH=6.86 and pH=9.18 should be boiled in advance for 15–30 minutes to remove dissolved carbon dioxide. During the cooling process, it should be kept away from air to prevent contamination by carbon dioxide. 10. The external reference replenishment solution for the composite electrode is a 3 mol/L potassium chloride solution; this solution can be added through the small hole at the upper end of the electrode. When the composite electrode is not in use, a rubber sleeve should be placed over it to prevent the replenishment solution from drying up. The external reference replenishment solution for the composite electrode is a 3 mol/L potassium chloride solution (an attached small bottle contains potassium chloride powder; the user only needs to add deionized water up to the 20 ml mark on the bottle and shake well – this solution then constitutes the 3 mol/L external reference replenishment solution). The replenishment solution can be added through the small hole at the top. 11. After long-term use, if a slight decrease in slope is observed in the electrode, its lower end can be immersed in 4% HF (hydrofluoric acid) for (3–5) seconds, washed with distilled water, and then immersed in a 0.1 mol/L hydrochloric acid solution to restore its original properties. 12. Reference table for pollutants and cleaning agents: Pollutants Cleaning Agents Inorganic metal oxides Dilute acid with a concentration of less than 1 mol/L Organic oils and fats Dilute detergent (weakly alkaline) Resin polymers Alcohol, acetone, ether Protein and blood cell precipitates 5% pepsin + 0.1 mol/L hydrochloric acid solution Pigments Dilute bleach, hydrogen peroxide Usage and precautions for the pH meter: 1. Open the back cover and insert one battery. 2. Install the composite glass electrode. Note: 2.1 The lower end of the composite electrode consists of a fragile glass bulb; care must be taken when using and storing it to prevent it from coming into contact with other objects. 2.2 A saturated KCl solution is used as the conductive medium inside the composite electrode; if it dries up, accurate measurements cannot be obtained, so it is necessary to constantly check for any remaining liquid, and when only a small amount is left, it must be replenished in the laboratory. 2.3 The interface of the composite electrode instrument must not be contaminated, including by water droplets. 2.4 The wiring of composite electrodes must not be pulled forcefully to prevent breakage at the connections. 3. After turning on the power switch, set it to the PH measurement mode. 4. Use a thermometer to measure the temperature of the PH6.86 standard solution, and then adjust the pH meter’s temperature compensation knob to match that measured temperature. 5. Rinse the composite electrode thoroughly with deionized water and dry it with filter paper. 6. Pour 2–5 ml of the PH6.86 standard solution into a plastic beaker that has been washed and dried with water; wash the beaker as well as the composite electrode and then discard the liquid. Add another 20 ml of the PH6.86 standard solution to the plastic beaker, insert the composite electrode into the solution, and use the instrument’s adjustment knob to set the reading to 6.86 until it stabilizes. Note: 6.1 must be adjusted to its position using the PH6.86 standard. 6.2 After adjustment, the positioning knob must not be moved again. 7. Wash the composite electrode with deionized water, dry it with filter paper, measure the temperature of the pH 4.00 solution using a thermometer, and adjust the instrument’s temperature compensation knob to the measured temperature value. 8. Pour 2–5 ml of the PH4.00 standard solution into another plastic beaker; wash the beaker and the composite electrode and discard the wash water. Then add 20 ml of the PH4.00 standard solution, insert the composite electrode into the solution, and once the reading stabilizes, adjust it to PH4.00 using the slope knob. Note: Once the 8.1 slope knob is adjusted, it must not be moved again. 9. Use a thermometer to measure the temperature of the liquid to be tested, and adjust the instrument’s temperature compensation to match that temperature. 10. Insert the composite electrode into the solution to be tested and read the pH value; this value represents the pH of the solution in question. Note: The temperature should not be too high during the 10.1 measurement; if it exceeds 40°C, the measurement results will be inaccurate, and the sample needs to be taken out of the beaker and allowed to cool slightly. 10.2 Composite electrodes should be kept away from contact with organic substances; if contact or contamination occurs, they must be cleaned thoroughly with anhydrous ethanol. 11. Precautions: 11.1 The instrument must be calibrated before use, that is, through the operations listed in points 4 to 8 above. If the instrument is not turned off, measurements can be taken continuously; once it is turned off, calibration is required. But it must be calibrated once every 12 hours, even if it is not turned off. 11.2 The use and storage of pH meters must be the responsibility of individual employees; during shift handovers, the status of the pH meters must be clearly explained. The handover process must be thorough, and the person taking over must be diligent. In the event of any damage, the responsibility lies with the employee on duty at that time.