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    Resistivity of Very Pure Water and Its Maximum Value

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    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.


    resistivity, power plants, conductivity, water, temperature coefficient, instrumentation, water quality

    Author Information:

    Light, TS
    Principal scientist, Research Center, The Foxboro Co., Burlington, Mass.

    Sawyer, PB
    director, Research and Development, Vaponics, Inc., Plymouth, Mass.

    Committee/Subcommittee: D19.95

    DOI: 10.1520/STP28286S