pH meter use and precautions
The pH meter is a precision instrument for measuring pH and can also be used to measure electromotive force.
1. How to use (1) Installation 1 The voltage and frequency of the power supply must be in accordance with the data specified on the instrument's nameplate and must be grounded. Otherwise, the pointer may be unstable during measurement.
2 The instrument is equipped with a glass electrode and a calomel electrode. The bakelite cap of the glass electrode was clamped on the small clip of the electrode holder. The calomel electrode's metal cap is clamped on the large clip of the electrode holder. The height of the two electrodes can be adjusted using the head screws on the electrode holder.
3 Glass electrodes must be soaked in distilled water for more than 24 hours before initial use. Should not be soaked in distilled water.
4 The calomel electrode should be soaked in saturated potassium chloride solution before its initial use. Do not soak it in distilled water with the glass electrode. When not in use, soak in saturated potassium chloride solution or cap the lower capillary of the calomel electrode with a rubber cap.
(2) Calibration 1 Turn the pH-mv switch to the pH position.
2 Turn on the power indicator light at the beginning and warm it up for 30 minutes.
3 Remove the small beaker of distilled water, and use the filter paper to gently remove the excess water on the glass electrode. In a small beaker, select a standard buffer solution of known pH. Immerse the electrode. Note that the capillary of the glass electrode tip and the calomel electrode are immersed in the solution. Gently shake the small beaker so that the electrode is in contact with the solution.
4 Screw the range switch to 0-7 or 7-14 depending on the pH of the standard buffer.
5 Adjust the temperature control knob so that the temperature indicated by the knob is the same as the room temperature.
6 Adjust the zero point so that the pointer is at pH 7.
7 Press lightly or slightly turn the reading switch to make the switch jam. Adjust the positioning knob so that the pointer is exactly at the pH value of the standard buffer. Release the reading switch and repeat until the value is stable.
8 After alignment, do not turn the positioning knob again, otherwise it will need to be re-aligned. Remove the standard small beaker and rinse the electrode with distilled water.
(3) Measurement 1 The excess water on the electrode should be sucked dry or washed twice with the tested solution, then the electrode should be immersed in the tested solution, and the beaker should be gently rotated or shaken to make the solution evenly contact the electrode.
2 The temperature of the test solution should be the same as that of the standard buffer solution.
3 Zero calibration, press the reading switch, the value indicated by the pointer is the pH of the test solution. If the pointer reading exceeds the scale when measuring in the range of pH 0~7, the range switch should be placed at pH 7~14 and then measured.
4 After the measurement is completed, after releasing the reading switch, the pointer must be at pH 7, otherwise readjust.
5 Turn off the power, rinse the electrodes, and soak as described above.
2. Precautions (1) Prevent the instrument from coming into contact with moisture. Immersion of moisture will reduce the insulation of the instrument, making it less sensitive, accurate, and stable.
(2) The glass film of the glass electrode ball is extremely thin and easily damaged. Avoid contact with hard objects.
(3) The glass film of the glass electrode should not be stained with oil. If it is accidentally stained with oil, it can be washed with carbon tetrachloride or ether, rinsed with alcohol, and finally washed with distilled water.
(4) The presence of air bubbles in the potassium chloride solution of the calomel electrode is not allowed, and there is little crystallization to maintain saturation. If there is too much crystallization and the pores are clogged, it is best to add new saturated potassium chloride solution.
(5) If the pointer of the pH meter has serious jitter, replace the glass electrode.
3. Preparation of Standard Buffers The reagents for standard buffers used in pH meters are also easily purified and stable. The commonly used preparation methods are as follows:
(1) Standard buffer with pH=4.00 Weigh 5.07 g of potassium hydrogen phthalate which was dried at 105° C. for 1 hour, dissolved with distilled water, and set the volume to 500 ml.
(2) Standard buffer solution of pH=6.88 Weigh 3.4001 g of potassium dihydrogen phosphate (KH2PO4) dried at 130°C for 2 hours, 8.95 g of disodium hydrogen phosphate (Na2HPO·12H2O) or disodium hydrogen phosphate anhydrous (Na2HPO4) 3.549g, add distilled water to dissolve and set the volume to 500ml.
(3) Standard buffer with pH=9.18 Weigh 3.8144 g of sodium borate (Na2B4O7·10H2O) or anhydrous sodium borate (Na2B4O7) 2.02 into distilled water and dissolve to 100 ml.
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