Common Precautions to Take When Calibrating the pH
The laboratory requires more calibrations because experiments need to be done frequently and the measured solutions are different. Calibration methods are currently generally divided into one, two-point calibration, and three-point calibration. The more calibration times, the higher the accuracy, and the more accurate the acidimeter measurement will be.
When calibrating the instrument before measurement, a standard buffer solution with a pH value close to that of the test solution should be selected. After calibrating the instrument with a standard buffer during measurement, it should be checked again with another standard buffer with a pH value that differs by about 3, and the error should not exceed ±0.1. Before each replacement of the standard buffer or test solution, the electrode of the acidimeter should be fully washed with water, and then the water should be sucked out. It can also be washed with the replaced standard buffer or test solution.
To calibrate the pH probe, two buffer solutions are enough. If the sample to be tested is acidic, please use buffer solutions with pH 4 and 7 to calibrate the probe. If the sample to be tested is alkaline, please use buffer solutions with pH 7 and 10 to calibrate. Buffer solutions are sold as powdered granules and finished solutions. Granules have the advantages of long shelf life and economy, but they are just inconvenient. The finished solution is easy to use, but it is easy to deteriorate after opening the lid. You can divide the finished solution into small beakers for use.
If you use powdered granules, please use 50ml of deionized water to make a buffer before calibration. Buffer solutions should be discarded after use for calibration. Therefore, the finished buffer solution should be packaged separately for use. Never pour used buffer solution back into the solution bottle.
When measuring test samples with high pH values, attention should be paid to the problem of alkali error, and appropriate glass electrodes should be used for measurement if necessary.
To measure the pH value of a weak buffer solution with an acidometer, first calibrate the instrument with potassium hydrogen phthalate standard buffer solution and then measure the test solution, and then retake the test solution and test again until the pH value reading does not change by more than 1 minute. to ±0.05; then use borax standard buffer to calibrate the instrument, and then measure as above; the difference between the two pH readings should not exceed 0.1, and take the average of the two readings as the pH value. The water used to prepare the standard buffer solution and dissolve the test sample should be freshly boiled cold distilled water, and its pH value should be 5.5 to 7.0. The standard buffer solution can generally be stored for 2 to 3 months. However, if it is found to be turbid, moldy or precipitated, it cannot be used further.
