Analysis of the Reasons for Unstable PH Measurement Values
1. Check if the electrodes are damaged;
2. It should be because the electrode has been used for too long. First, calibrate it to see if it is effective;
3. You can try soaking the probe in a 2.5MMOL/L KCL solution;
4. Clean the glass ball, is it possible that some organic matter has adhered to it over time, causing the reaction to be insensitive;
5. There exists a chemical equilibrium in water~CO2+H2O → H++HCO3-. Due to the weak alkalinity of pure water or surface water, this equilibrium shifts towards the positive reaction direction, resulting in a continuous increase in pH. Personally, I think this is the case;
6. Adding neutral salts (such as Kcl) as ion strength regulators to the tested water sample can change the total ion strength in the solution, increase conductivity, and make the measurement fast and stable. This method is specified in the national standard GB/T6P04.3-93, which states: "In order to reduce the influence of liquid contact potential and quickly achieve stability when measuring water samples, a neutral 0.1mol/LKCL solution is added to every 50ml of water sample." Although this method changes the ion strength in the water sample to some extent, it has been experimentally proven that this change only changes the pH value by about 0.01PH, which is completely acceptable. However, when using this method, it is important to note that the added Kcl solution should not contain any alkaline or acidic impurities. Therefore, the Kcl reagent should be of high purity, and the water quality of the prepared solution should also be of high purity neutral water quality;
7. During the measurement, CO2 is absorbed and the pH continues to rise;
8. The principle of measuring pH value with a pH meter is to insert an electrode consisting of an indicator electrode and a reference electrode into a solution to form a primary battery. At room temperature (25 ℃), each unit of pH value is equivalent to a change in electromotive force of 59.1mv, which is directly expressed as the pH reading on the instrument. The temperature difference is compensated by a device on the instrument. Because pure water has very few ions and cannot form a stable primary battery, neutral salts (such as KCl) are added to the measured water sample as ion strength regulators to change the total ion strength in the solution, increase conductivity, and make the measurement fast and stable;
9. The pH meter reading is unstable: if the measured solution is acidic, use a pH 4 buffer to correct the slope, and if the measured solution is alkaline, use a pH 9 buffer to correct the slope. The closer the pH of the solution used to adjust the slope is to the pH of the measured solution, the better;
10. It may be due to poor contact;
11. If the pH meter shakes slightly, the reading will also change;
12. It shouldn't be due to the electrode, I think it's because your electrode has dried up. You can take a look yourself. Generally, there is a small hole on the electrode, and it should be filled with 3mol/LKCL to fix it;
13. The unstable pH value of electrodes during use is basically not related to calibration. It is related to fluctuations in power supply voltage, electrode performance, electrode guide wire, electrode socket contact, temperature of the measured solution, etc. When calibrating, if the measured solution is close to acidic, use "6" positioning, and if it is alkaline, use "9" positioning. The two cannot be chosen arbitrarily;
14. It should be an issue with the electrode, activate it before use. And regardless of whether the electrode is used or not, it will be eliminated within a year. Measure the biased acidic solution and calibrate it with buffer solutions close to 4 and 7; For alkaline solutions, buffer solutions of 7 and 10 should be used for calibration;
15. It's strange that the pH of pure water can be measured directly with a pH meter. First, understand the principle of pH meter measurement: it displays the reading by the directional movement of ions in the water under the action of an electrode to form an electric current. If there are too few ions in pure water and the electrolysis rate is low, how can the measurement be stable;
16. It may be due to poor contact, insufficient electrode immersion solution (3MOL/L KCL solution), or incomplete immersion of the electrode in the electrode immersion solution. Additionally, the electrode may have aged or needs to be replaced;
17. I think it's related to room temperature. When the temperature is too low or the air flows quickly, it can affect the pH meter. It's important to maintain a stable room temperature and close the doors and windows when measuring;
18. If measuring pure water, instability is normal. Originally, it contains fewer ions, weak buffering capacity, and the environment has a significant impact on it. Generally, a relatively stable value is sufficient.
