pH measurement of ultrapure water

Ultrapure water has its unique characteristics, making it difficult to measure its pH. In practice, it is difficult to obtain reproducible and stable measurements in low ionic strength samples. Low ion samples, ie low alkaline, low ionic strength, or low electrical conductivity (conductivity generally 0.056-10 μs/cm) high impedance, such as deionized water. Due to the varying junction potentials in the reference junction, different pH values ​​are produced when measuring deionized water, even for new, sealed electrodes, and are well corrected in buffered standard solutions.

Why do you need to measure the pH of pure water? During the production of pure water, measuring pure water can be used as an indicator of process contamination. When air or carbon dioxide invades the pure water production or distribution system, it will cause only a drift in pH.

Selecting the pH electrode The more expensive double junction electrode has many advantages over the conventional electrode. However, it does not show any particular advantage in measuring ultrapure water compared to the single junction electrode. Zui's good choice is a refillable, liquid reference liquid electrode, and is made of low-impedance glass. A liquidity reference junction has a relatively high flow rate, reducing the junction potential. The sealed electrode, usually gel-filled, has a long life advantage because of its very slow outflow rate of the reference solution. However, the faster effluent rate is very suitable for ultrapure water because the pH potential can be quickly stabilized.

Static interference Because the ultrapure water machine has very poor conductivity, it is also a static power source. These static electricity can be expressed when measuring the pH. In order to compensate for these disturbances, the electrodes are specially shielded. Such electrodes are relatively expensive, and are particularly designed for the pH measurement of ultrapure water.

Impact of Carbon Dioxide (CO2) Intrusion on Measurements Pure water (typical pH 7 range) exposure to the air atmosphere will absorb CO2 until equilibrium. CO2 will acidify the sample and lower the pH. Depending on temperature and pressure, the pH of pure water will drop to 6.2. Sampling the sample into the laboratory, the CO2 in the atmosphere will quickly invade the sample, so it should be avoided. So it is a good idea to bring a laboratory acidity meter to the site for measurement.

Relationship between pH and temperature of pure water Water molecules can be considered to be acid and alkalized and can be expressed by the following equation: 2H2O « H3O+ + OH-
Usually we say that pure water has a ph of 7, but this is a misleading statement. In fact, pure water only exhibits a pH at a specific temperature, which is a sample having a Kw value of 1.00 x 10-14 mol2dm-6. To calculate the pH, you first calculate the hydrogen ion concentration (or hydroxide ion concentration, the meaning is the same), and then convert to pH. The Kw value in pure water at room temperature is: [H+]2 = 1.00 x 10-14

In pure water, the hydrogen ion concentration must be equal to the hydroxide ion concentration. That is, each hydrogen ion must have a hydroxide ion. That is, you can replace [OH-] in the Kw formula with [H+]: [H+] = 1.00 x 10-7, that is: pH = - lg[H+]; pH = 7.

Since the Kw values ​​of pure water are different at different temperatures, their pH values ​​are different.

T (°C) Kw (mol2 dm-6) pH
0 0.114 x 10-14 7.47
10 0.293 x 10-14 7.27
20 0.681 x 10-14 7.08
25 1.008 x 10-14 7.00
30 1.471 x 10-14 6.92
40 2.916 x 10-14 6.77
50 5.476 x 10-14 6.63
100 51.3 x 10-14 6.14

Some people say that the temperature increases and the pH decreases, so the sample becomes acidic. This is wrong. Because the acidity is expressed as the concentration of hydrogen ions is higher than the concentration of hydroxide ions, in fact, the temperature increases, the concentration of hydrogen ions and hydroxide ions are the same, so the temperature of pure water increases, the pH decreases, but it is still neutral.

Measurement points:
• Use as small a sample as possible.
• Keep the sample temperature as stable as possible, for example between 25-30 °C.
• Adding a small amount of KCl to the sample increases ionic strength and improves response time. However, high purity KCl must be used, and low purity KCl may affect the pH.
• Use clean glassware to avoid cross-talk and clean the electrode with deionized water after calibration.
• Temperature compensation should be used and corrected daily with high quality calibration standards.
• Reduce the contact between the sample and the air. The carbon dioxide gas will be absorbed by the sample and will affect the pH value.


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