Why Is the Ph Value of Pure Water Difficult to Measure?

Why is it that the pH value of pure water measured with a pH meter (glass electrode head) is particularly difficult to stabilize compared to that of a solution?

Ignoring environmental factors (such as carbon dioxide absorption, agitation of the solution, temperature, voltage, etc.), the following reasons apply:

1. Because the ion concentration in pure water is very low, the concentration difference between it and the high concentration of Kcl in the reference electrode salt bridge solution is significant, unlike its behavior in ordinary solutions. Pure water increases the permeation rate of the salt bridge solution, promoting its wear and tear, thus accelerating the decrease in the concentrations of K+ and Cl-. This causes changes and instability in the liquid junction potential. Since the potential of the Ag/AgCl reference electrode itself depends on the Cl- concentration, changes in the Cl- concentration will also change the potential of the reference electrode, resulting in a drift in the measured value.

2. To ensure the composite electrode has a zero pH potential, the salt bridge must use a high concentration of KCl. Simultaneously, to prevent the Ag/AgCl coating from being dissolved by the high concentration of KCl, powdered AgCl must be added to the salt bridge solution to saturate it. However, the decrease in KCl concentration in the salt bridge solution leads to AgCl supersaturation, causing precipitation and clogging the liquid junction.

3. Due to the high internal resistance of the glass resistor, a higher internal resistance results in a thicker glass film, increasing the asymmetry potential and electrode inertia, thus slowing the generation of electromotive force. Pure water has no buffering effect and its properties are completely different from standard buffer solutions, resulting in a very slow electrode potential build-up time.

4. Because pure water has very low conductivity, the friction between the flowing water sample and the electrode surface is similar to friction between insulators, generating static charge. This static charge produces a ΔEr in the measuring cell, independent of the measured pH value. This ΔEr is superimposed on the measurement signal, causing pH measurement error.

5. Theoretically, a pH meter cannot accurately measure the pH value of ultrapure water because there are not enough ions in the water for the pH meter to function correctly.

In reality, carbon dioxide from the air will quickly dissolve into ultrapure water.

Do not attempt to measure the pH value of ultrapure water, because the resistivity of "ultrapure water" at 18.2 MΩ*cm (at 25°C) already ensures that it is neutral.