How to calibrate the pH meter (acidity meter)?
Tools and Materials for Calibrating a pH Meter
Standard buffer solution (the standard buffer solution with a pH value close to that of the test solution should be selected), currently there are 7 kinds of standard solutions, and the following 3 solutions are generally used in my country: (when the pH value is at 25°C)
1. Potassium hydrogen phthalate (KHC8H4O4) pH 4.003; 0.05M
2. Mixed phosphate (Na2HPO4): potassium dihydrogen phosphate and disodium hydrogen phosphate mixed salt solution pH6.864; 0.025M
3. Borax (Na2B4O7 l0H2O) pH 9.182; 0.01M
The configuration methods of these three standard buffers are as follows:
1. pH4, Potassium Hydrogen Phthalate Standard Buffer:
Accurately weigh 10.12g of potassium hydrogen phthalate [KHC8H4O4] dried at 115±5°C for 2 to 3 hours, add water to dissolve and dilute to 1000ml.
2. pH7, phosphate standard buffer solution (pH7.4):
Accurately weigh 4.303 g of anhydrous disodium hydrogen phosphate and 1.179 g of potassium dihydrogen phosphate, which were dried at 115±5°C for 2 to 3 hours, add water to dissolve and dilute to 1000 ml.
Additional supplement: Phosphate standard buffer solution (pH6.8) Accurately weigh 3.533g of anhydrous disodium hydrogen phosphate and 3.387g of potassium dihydrogen phosphate, dried at 115±5°C for 2 to 3 hours, add water to dissolve and dilute to 1000ml .
3. pH9, borax standard buffer:
Precisely weigh 3.80g of borax [Na2B4O7·10H2O] (note: avoid weathering), add water to dissolve and dilute to 1000ml, put it in a polyethylene plastic bottle, seal it tightly, and avoid contact with carbon dioxide in the air.
Steps and Methods of Calibrating a pH Meter
1. Calibration of pH meter | acidity meter used in the laboratory:
When calibrating the commonly used laboratory pH meter instrument, the slope of the instrument should be adjusted to the maximum, and the rubber plug on the upper part of the electrode should be removed to expose the small hole. Otherwise, negative pressure will be generated during calibration, and the solution will not work normally. Ion exchange will make the measurement data inaccurate.
Take the electrode out of the beaker containing distilled water, and use filter paper to blot up the remaining distilled water on the electrode. Then put the electrode into the beaker with the mixed phosphoric acid basin, wait for more than 15 minutes, and then adjust the positioning knob on the instrument to make the instrument display 6.86pH, which is to set the reference point for the instrument first. After setting the reference point, take the electrode out of the beaker containing the mixed phosphoric acid solution, wash the electrode with distilled water, and put it in the beaker filled with distilled water, wait for about 3 minutes to dissolve the remaining part of the mixed phosphoric acid solution.
2. Take the electrode out of the beaker filled with distilled water later, and use filter paper to dry up the remaining distilled water on the electrode. Then put the electrode into the solution containing potassium hydrogen phthalate or borax, wait for more than 15 minutes, and observe whether the instrument shows 4.00 or 9.18 pH. If not, adjust the slope knob on the instrument to make the instrument display 4.00 or 9.18 pH, which is the commonly used two-point calibration. If a three-point calibration is required, just use another solution and follow the above steps to do it one more time. This is the calibration method of the pH meter.
3. After calibration, put back the rubber plug. If you don’t use it temporarily, remember to put a saturated solution in the protective sleeve of the electrode to keep the electrode wet, which can prolong the life of the electrode and reduce the asymmetrical potential of the electrode. Electrodes have a service life and are fragile. Therefore, each laboratory needs to replace the electrodes frequently. Don’t think that the electrodes will not be replaced if they are not damaged during use.
4. Before using the composite electrode, first check whether the glass bulb is cracked or broken. If not, when using pH buffer solution for two-point calibration, when the positioning and slope knobs can be adjusted to the corresponding pH value, it is generally considered acceptable, otherwise The electrode activation treatment can be carried out according to the instruction manual. The activation method is to soak in 4% hydrogen fluoride solution for about 3-5 s, take it out and rinse it with distilled water; then soak it in 0.1 mol/L basin acid solution for several hours, rinse it with distilled water, and then calibrate it. For non-closed composite electrodes, if the inner solution is less than 1/3, it is necessary to add an external reference solution, that is, 3mol/L potassium fluoride solution. If the potassium fluoride solution exceeds the position of the small hole, shake off the excess potassium fluoride solution and check whether there are bubbles in the solution. If there are bubbles, flick the electrode to drive the bubbles out completely, so as not to cause inaccurate measurement data.
5. Calibration method of pen pH meter:
Immerse the electrode of the test pen in the mixed phosphate standard buffer solution with a pH value of 6.86 (at 25°C), and shake it gently; adjust the calibration potentiometer with a small screwdriver until the displayed value matches the pH value of the standard buffer solution at ambient temperature ; Insert the electrode into the pH4.01 potassium hydrogen phthalate or pH9.18 borax standard buffer solution; the displayed value should be within the allowable error range compared with the pH value of the buffer solution.
