Why Does the Resistivity of Ultrapure Water Plateau at 18.25 MΩ·cm?
Chemically, perfectly pure water cannot exist. Water molecules spontaneously and reversibly self-ionize via the equilibrium:
Even in the total absence of external impurities, water naturally contains hydrogen and hydroxide ions generated from its own dissociation. As long as ions are present, water conducts electricity weakly, so its resistivity can never be infinite.
How the theoretical limit of 18.25 MΩ·cm is derived
Resistivity is the reciprocal of conductivity. The residual conductivity of pure water relies on two fixed constants:
1. Ion product constant of water
2. Limiting molar ionic conductivities:
Calculation process
After precise calibration of physical constants, the universally recognized theoretical maximum is 18.248 MΩ·cm, simplified to 18.25 for daily use.
Even if ion-exchange resins completely remove sodium ions, chloride ions, silica and total organic carbon (TOC), the 0.0548 μS/cm conductivity from water’s self-ionization can never be eliminated. The value 18.25 MΩ·cm is the absolute theoretical "vacuum limit" for pure water resistivity.
Why ion-exchange resins cannot push resistivity higher than this threshold
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