Fundamental Conductivity and Resistivity of Water

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.2 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-perbillion (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.