What Is the Conductivity of Ultrapure Water?

Electrical conductivity is the ability of a substance to conduct electric current; it is the reciprocal of resistivity. In liquids, conductivity, the reciprocal of resistance, is often used to measure its electrical conductivity. The conductivity of water is a very important indicator of water quality, and therefore, it is frequently used in ultrapure water equipment. Customers often ask about the conductivity of ultrapure water, so today we will explain in detail what the conductivity of ultrapure water is:

The unit of conductivity is the mol/L, also known as the Siemens. It is represented by S. Because the unit S is too large, millisiemens and microsiemens are commonly used. 1 S = 10³ mS = 10⁶ μS. Even in pure water, H+ and OH- ions exist. It's often said that pure water is a poor conductor of electricity, but strictly speaking, water is still a very weak electrolyte, existing in the following ionization equilibrium: H2O ⇌ H+ + OH- or 2H2O ⇌ H3+ + O+ + OH-.

The conductivity of ultrapure water is less than 0.10 μS/cm; the conductivity of natural water is mostly between 50 and 500 μS/cm, while that of mineralized water can reach 500 to 1000 μS/cm.

Based on the ion product of water being 10⁻¹⁴, the theoretical limiting conductivity of high-purity water can be calculated to be 0.0547 μS·cm⁻¹, and its resistivity to be 18.25 MΩ·cm (25℃). Therefore, the 18.25 MΩ resistivity often seen in the technical parameters of ultrapure water equipment comes from this, and it is also an indicator of whether the water quality meets the requirements.