Please provide a detailed explanation: Why can’t conventional pH meters measure pure water?

Because before the development of pure water pH meters, measuring the pH of pure water with ordinary pH meters was inaccurate and unstable. Later, pH meters specifically designed for pure water were developed, so conventional pH meters can no longer be used to measure the pH of pure water. As for the principle, check out my previous posts.

It’s impossible to measure it. Even when we use domestic pure water pH electrodes, the results are not good; a certain resistivity value is required as well

When any pure water pH electrode is used to measure the pH of pure water, a certain level of conductivity is required. However, by properly configuring the measurement cell, electrodes, and measurement conditions, pH can be measured stably. Otherwise, how could those thermal power plants and nuclear power stations operate properly?

It has been found that pure water has a low conductivity and an extremely low amount of dissolved gases. Since freshly prepared pure water is free from any contamination, its pH value is determined by the trace amounts of acidic or alkaline substances dissolved in it. When water comes into contact with the atmospheric environment, a small amount of carbon dioxide dissolves in it, causing the pH value to decrease; in other words, the pure water becomes contaminated. Additionally, to measure the pH of pure water, a pH measurement system designed for pure water must be used; ordinary pH electrodes and containers cannot provide accurate and reliable results. Additionally, the pH of pure water should preferably be measured online. If sampling offline measurement is used, it requires high levels of expertise from both the equipment and the personnel involved, and it is difficult to achieve accurate measurements.

For pure water, it is best to measure the pH value online; this ensures the reliability of the data and eliminates the hassle of sampling in the laboratory and transferring samples. In fact, a pure water pH measurement system doesn’t require much money: an electrode, a flow-through cell, a converter or transmitter, and possibly a simple pre-treatment unit are all that’s needed. However, for pure water pH measurement systems, it is essential that the selection be carried out responsibly by the manufacturer’s technical staff; otherwise, some distributors may supply you with ordinary pH meters at low prices.

It’s quite challenging in the laboratory to achieve a high purity for water in terms of pH, and it’s also difficult to reduce the conductivity to very low levels. It is only a last-resort measure; it is recommended to use specialized online electrodes for measurement, with conductivity serving as a supplementary indicator, so as to ensure the purity of pure water.