The Mystery of Ultrapure Water (UP) - Why Can't We Measure the Resistivity of Ultrapure Water Offline?

The resistivity/conductivity of ultrapure water cannot be accurately measured offline.

The formula for calculating conductivity is as follows:

X is conductivity, F is the Faraday constant, Ci is the concentration of a certain ion, Zi is the charge of the ion, and Ui refers to the ion's migration ability.

X = FΣ CiZiui

Theoretically, absolutely pure water contains only two types of ions: H+ and OH- produced by the decomposition of water (H2O). Completely ion-free ultrapure water has a conductivity of 0.055 μS/cm at 25°C.

Because the conductivity of ultrapure water is very small, it is difficult to measure. Therefore, its purity is often expressed using resistivity (MΩ·cm). Resistivity is the reciprocal of conductivity. The resistivity of ultrapure water at 25°C is 18.248 MΩ·cm.

When ultrapure water is exposed to the external environment, it can be contaminated. Laboratory air contains a large number of ions (such as CO2), which can enter the ultrapure water. In addition, the instruments used to measure conductivity (beakers, resistivity meters/probes) can also introduce ionic contaminants. As shown in the conductivity calculation formula, any increase in the number of ions will affect the conductivity value, because conductivity is directly proportional to the type of ion and its charge.

Therefore, to accurately measure the resistivity of ultrapure water, it must be measured using a dedicated electrode with temperature compensation in a closed system with the ultrapure water in motion; otherwise, an accurate resistivity value will not be obtained.

Strictly speaking, the conductivity of ultrapure water cannot be accurately measured!

This is because conductivity is a measure of the ion content in water. In ultrapure water, strictly speaking, there are no ions present, and because the ion concentration is extremely low, the measurement error will be particularly large. Therefore, the measurement results are inaccurate and can only be used as a reference. Therefore, do not attempt to measure the conductivity of water produced by a water purifier, especially offline conductivity measurements.

If you attempt to measure the conductivity of pure water using a laboratory offline benchtop conductivity meter, the following points should be noted in particular! And the measurement results are for reference only and should not be used as an accurate basis for water quality assessment.

1. Electrode constant of the conductivity electrode (sensor): It must be an electrode with C=0.1, or even 0.01; otherwise, the measurement accuracy will be severely reduced.

2. The conductivity of pure water in an open container will increase sharply due to the absorption of CO2 from the air. Therefore, a rapid reading is required, ideally within 120 seconds.

3. Use a flow-through cell or conductivity cell for measurement. If neither is available, thoroughly clean a 250ml polyethylene cup (do not use glass). Insert the pure water outlet tube near the bottom of the cup, adjust the pure water flow rate, and slowly fill the cup until it overflows, ensuring no air bubbles appear. While the water is continuously overflowing, insert the conductivity electrode near the outlet tube for measurement. Only in this way can a result closer to the true conductivity value be obtained.