Why Is It Difficult to Measure the PH of Ultrapure Water?

In the application of ultrapure water, many people find that its pH value is exceptionally difficult to measure, and multiple measurements can yield significantly different results. This stems from a "contradiction" between the unique properties of ultrapure water and the principle of pH measurement.

I. Extremely Low Ion Concentration in Ultrapure Water

This is the core reason for the measurement difficulty. pH measurement relies on the conduction current of ions in the solution. Ordinary water contains a small amount of anions and cations, allowing the electrodes to function normally. However, ultrapure water has a high resistivity (typically exceeding 18 MΩ·cm) and a negligible ion content, making it difficult for the electrodes to form a stable current loop. This results in a weak and unstable measurement signal, and the reading is prone to drift.

II. Ultrapure Water is Highly Susceptible to Environmental Influences

Ultrapure water lacks buffering capacity. Carbon dioxide in the air quickly dissolves into it, forming carbonic acid, making the water weakly acidic. Even trace amounts of impurities dissolved in containers during experiments, or other solutions remaining on the electrodes during measurement, can alter the pH of the ultrapure water. For example, freshly prepared ultrapure water may have a pH close to 7, but after exposure to air for a few minutes, the pH can drop to around 5.5, making it difficult to capture the true initial value during measurement. III. Conventional pH Electrode Design is Inadequate for Ultrapure Water

The reference solution of ordinary electrodes leaks slowly, which has minimal impact in ion-rich solutions. However, in ultrapure water, this leakage directly contaminates the water sample. Furthermore, it is difficult to form a stable hydration layer on the electrode surface, further reducing measurement accuracy. Even when using a dedicated ultrapure water pH electrode, measurement conditions must be strictly controlled.

III. Improving Ultrapure Water pH Measurement Results

Measure rapidly in a closed environment to minimize air contact; use low-leakage electrodes and ensure they are fully activated beforehand; rinse the electrode multiple times with ultrapure water before measurement to avoid cross-contamination. Only by paying attention to these details can the pH measurement results of ultrapure water be more reliable.