Common points for attention in pH calibration
The laboratory needs to be calibrated a little more times, because it is necessary to do experiments frequently and the measured solutions are different. At present, calibration methods are generally divided into one or two-point calibration and three-point calibration. The more calibration times, the higher the accuracy, and the more accurate the acidity meter measurement.
When calibrating the instrument before determination, a standard buffer close to the pH value of the test solution should be selected. After the instrument is calibrated with standard buffer, it should be checked with another standard buffer with a pH difference of about 3, and the error should not exceed 0.1. Before replacing the standard buffer or test solution every time, the electrode of the acidity meter should be fully washed with water, and then the water should be completely absorbed, and the replaced standard buffer or test solution can also be used for washing.
To calibrate the pH probe, two kinds of buffer solutions are enough. If the sample to be tested is acidic, please adjust the probe with buffer solution with pH of 4 and 7. If the sample to be tested is alkaline, please adjust it with buffer solution with pH of 7 and 10. Buffer solution has powder granules and solution products for sale. Granules have the advantages of long storage time and economy, but they are inconvenient. The finished solution is easy to use, but it is easy to deteriorate after opening the lid. You can use the finished solution separately in small beakers.
If you use powder granules, please use 50ml deionized water to prepare buffer before calibration. After the buffer solution is used for calibration, it should be dumped. Therefore, the finished buffer should be used separately. Never pour the used buffer back into the solution bottle.
When determining the sample with high pH value, we should pay attention to the problem of alkali error, and choose the appropriate glass electrode for determination when necessary.
When measuring the pH value of the weak buffer solution by an acid meter, first calibrate the instrument with potassium hydrogen phthalate standard buffer solution, then measure the test solution, and then re-measure the test solution until the reading of the pH value does not change more than 0.05 within 1 minute;
Then use borax standard buffer to calibrate the instrument, and then determine it as above;
The difference between the readings of the secondary pH value shall not exceed 0.1, and the average value of the secondary readings shall be taken as its pH value.
The water for preparing the standard buffer and dissolving the test sample should be newly boiled cold distilled water, and its pH value should be 5.5 ~ 7.0.
The standard buffer can generally be stored for 2 ~ 3 months, but it can't be used any more when it is found to be turbid, moldy or precipitated.
