Resistivity of Very Pure Water and Its Maximum Value

Theoretical calculations have been made for the resistivity (and its reciprocal, the conductivity) of pure water in the vicinity of the neutral point at 25°C. A surprising result is that the maximum resistivity is not found at pH 6.998, which corresponds to absolute water, but at pH 7.039, which corresponds to water with approximately 0.8 μg/litre, (parts per billion) of sodium hydroxide added. The resistivities corresponding to these two points are 18.18 and 18.28 MΘ · cm respectively. Although the effect is slight, it will be of interest to users of high-purity water since resistivity is widely used as a criterion for water purity. The temperature coefficient of resistivity is also evaluated.