IC Controls Ltd.

Conductivity Measurement in high purity water samples below 10 microsiems/cm


Courtesy of IC Controls Ltd.

High Purity Water Resistivity/ Conductivity Measurement
Ultra-Pure Water without any chemical impurities will still have a conductivity because of the presence of H* and OH' ions due to the self-ionization of water. Ultra-pure water has a conductivity of 0.055 HSiemens'cm or a resistivity of 18.18 mQ-cm at 25 °C. In order to know the conductivity or resistivity of ultra-pure water it is required to do accurate temperature compensation at very low conductivities, e.g. conductivities below 10 |iSiemens/cm. AnIC Controls 210-C analyzer in High Purity Water configuration (HPW) is intended for use with a 402 side stream sample sensor, or 403 direct insertion-retractable sensor, with a 0.01/cm cell constant. It will accurately measure the conductivity (or electrical conductivity) or resistivity (electrical resistivity) of pure water samples below 10 uS/cm in continuous flowing samples.

The self-ionization of water is strongly ternperature-dependent. For accurate temperature compensation the conductivity of the pure solvent must be subtracted from mat of the solution to determine the conductivity of the electrolyte. Simply applying a linear per-degree-Celsius temperature adjustment will not give accurate temperature compensation in high purity waters.
Illustration 1 shows the conductivity of ultra-pure water in |iSiemens/cm over the range 0 °C to 100 °C.

Since pure water conductivity measurement is detecting trace amounts of ionic contaminants in the already temperature dependent self-ionizing water, the installation and calibration setup must make special provisions to eliminate or reduce any trace contaminants. Further, the trace contaminants themselves exhibit variable temperature coefficients in pure water mat can rise as high as 7 % per °C. Some such as NaOH for instance can cause conductivity at trace levels to go negative by suppressing the conductive H+ions with the less conductive OH'ions resulting in a reading below that of theoretically pure water. The IC Controls 210-C analyzer has different temperature coefficient algorithms to allow the user to select for neutral salts, acidic, or basic samples (as well as the traditional linear % per °C).

Since high-purity water contains little dissolved material it is like a dry sponge ready to soak up any contaminant it meets. Absorption of carbon dioxide on exposure to air will result in carbonic acid formation and cause a real change in conductivity of up to 2 to 3 uS-'cm. This fact can be readily demonstrated by taking a beaker of fresh demin water from a laboratory column, with conductivity electrode inserted and showing less than 0.5 ^iS/cm. then bubbling compressed air through it and observing the conductivity reading quickly rise to between 2 and 3 nS/cm. Also if dissolved gases are in the sample, on exposure to air they may escape (similar to opening a bottle of pop). Similarly air exposure will allow CO: and other soluble gases to dissolve into the sample raising the conductivity. Clearly, calibrations using open containers of high purity water will have problems that the same samples in continuously flowing enclosed tubing should not encounter.

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