It is difficult to obtain a satisfactory result in measuring the pH value of pure water, especially when using conventional composite electrodes for open measurement in the laboratory. The performance is often slow response, drift of indication, poor repeatability, and low accuracy.

1、 Reason for occurrence:

1. Because the ion concentration in pure water is very low, and the reference electrode salt bridge solution selects high concentration of 3mol/L kcl, the concentration difference between each other is large, which is very different from its situation in ordinary solution. In pure water, it increases the penetration rate of the salt bridge solution, promoting the loss of the salt bridge, thereby accelerating the decrease in the concentration of K+and CL -. It causes the change and instability of liquid junction potential, while the potential of Ag/AgCl reference electrode itself depends on the concentration of CL -. When CL - concentration changes, its reference electrode self potential will also change. So it causes the reading to drift. This is especially true for composite electrodes that cannot supplement internal reference liquids.

2. Due to the high internal resistance of glass resistance, the higher the internal resistance, the thicker the glass film, and its asymmetric potential will increase. The inertia of the electrode will also increase, and the generation of electromotive force will be slower. Meanwhile, due to the fact that pure water is a non buffering liquid with completely different properties from standard buffer solutions, electrodes that perform well in standard buffer solutions are immediately moved to non buffering pure water, and the establishment time of electrode potential undoubtedly becomes delayed.

3. Pure water is easily contaminated, and when measured open in a beaker, it is easily affected by CO2 absorption. The pH value will continuously decrease, and relevant international standards stipulate that the measurement must be carried out in a special device and sealed. But it is difficult to implement in general laboratories.

4. The pH value reflects the activity of H+, (H+) rather than the concentration of H+[H+], and its relationship is (H+)=f × [H+]。 F is the activity coefficient of H+. It is determined by the total concentration of all ions in the solution, not just by the concentration of the measured ions. In theoretical pure water, the activity coefficient f is equal to 1, but as long as there are other ions, the activity coefficient will change, and the PH value will also change. The pH value is affected by the total ion concentration in the solution, and changes in the total ion concentration will cause the pH value to change. Due to the close proximity of the composite electrode liquid interface to the pH sensitive glass bulb, the salt bridge solution leaked from the liquid interface first gathers around the sensitive bulb, changing the total ion concentration near it. From the above reasons, it can be inferred that the measured value only changes the pH value near the sensitive bulb, and cannot reflect its true pH value. Although stirring or shaking the beaker can change this situation, practice has shown that different stirring speeds can result in different test values, and stirring or shaking can also accelerate the dissolution of CO2, so it is not advisable.

5. In order to ensure the pH zero potential of the composite electrode, the salt bridge must use high concentration of Kcl. At the same time, to prevent the Ag/AgCl coating from being dissolved by high concentration of Kcl, powdered AgCl must be added to the salt bridge to saturate the salt bridge solution with AgCl. However, as mentioned in Article 1 above, due to the decrease in Kcl concentration in the salt bridge solution, the AgCl originally dissolved in it becomes supersaturated and precipitates, thereby blocking the liquid interface. 

2、 Overcoming and improving methods:

In summary, it is difficult to test the pH value of pure water, mainly due to the structural performance of the electrode and the instability of the electrode liquid interface potential caused by this, as well as the non buffering performance of pure water and CO2 in the air. 

To overcome and improve this, the following methods can be used.

1. Using a pH electrode suitable for pure water should be the first consideration.

2. When using the separation plate, that is, separate PH glass electrode and reference electrode for measurement, keep the reference electrode as far away from the glass electrode as possible, so as to improve the phenomenon analyzed in Clause 4 above. 

3. Increase the sampling amount of the tested pure water and possibly reduce the contact surface with air, while avoiding stirring and shaking to reduce CO2 absorption.

4. Add neutral salt (such as Kcl) into the measured water sample as a ionic strength regulator to change the total ionic strength in the solution, increase conductivity, and make the measurement fast and stable. This method is stipulated in the national standard GB/T6P04.3-93: "In order to reduce the impact of liquid junction potential and quickly reach stability when measuring water samples, a drop of neutral 0.1mol/LKCL solution is added to every 50ml of water samples." Although this method changes the ionic strength in the water samples, causing the PH value to change to a certain extent, the experiment shows that this change only changes about 0.01PH in value, which is completely acceptable. But when using this method, it is important to note that the added Kcl solution should not contain any alkaline or acidic impurities. Therefore, Kcl reagents should be of high purity, and the water quality of the prepared solution should also be of high purity neutral water quality.