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White Paper: The Fundamental Conductivity And Resistivity Of Water
By Truman S. Light, Stuart Licht, Anthony C. Bevilacqua, and Kenneth R. Morash
Pure water has a very low, but not quite zero, electrical conductivity. This conductivity provides a probe into fundamental properties of water, including the electrochemical mobility of the hydrogen and hydroxide ions. Deviation from this value is a measure of trace ionic impurities. Ultrapure water (UPW), with impurities at or below the sub-parts-per-billion range, is used extensively in many critical applications. Applications include chip fabrication for semiconductors, intravenous solutions for pharmaceuticals, and in high-pressure boilers for power generation.
We report here correction to a considerable error in the values for water conductivity and hydroxide mobility. In 1987, data was collected to establish the conductivity of pure water over a wide temperature range. In 1989, Thornton and Light measured the intrinsic resistivity of ultrapure water from 0 to 100°C. The results exhibited agreement within 0.25% to other values below 30°C, but showed the uncertainty rose to almost 3% at temperatures approaching 100°C. This discrepancy implied that at elevated temperatures, the resistivity change for impurity levels below about 1 mg/L (or part-per-billion, ppb) could not be calculated. This is an unacceptable limitation for modern conductivity instrumentation, necessitating a new study with detailed attention to the known issues that affect conductivity measurement and calculation accuracy.
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