Ultra-pure water machine water quality testing

Water quality monitoring standards for ultrapure water machines Generally, pure water is measured by electrical conductivity, and ultrapure water is measured by electrical resistivity. The formula for calculating the conductivity is as follows: X is the conductivity, F is the Faraday constant, Ci is a certain ion concentration, Zi is the charge number of the ion, Ui refers to the ion mobility, the formula: X = FΣ CiZiUi Theoretically, it contains no ions at all The ultrapure water has a conductivity of 0.055μS / cm at 25 ℃. Resistance value is the reciprocal of conductivity. Then R = 1 / X, 1 / 0.055 = 18.1818 is about 18.2 MΩ.cm

At 25oC, the relationship between NaCl concentration and conductivity / resistivity value:

Resistivity (MΩ.cm) 0 0.055 18.18 1 0.057 17.60 5 0.066 15.20 10 0.076 13.10 20 0.098 10.20 50 0.16 6.15 100 0.27 3.70 300 0.70 1.43 500 1.13 0.88 1000 2.21 0.45 5000 10.80 0.093 20000 42.70 0.023

In general, we cannot accurately measure the resistivity or conductivity of ultrapure water offline. Because when ultrapure water is exposed to the outside environment, it will be polluted by the outside environment. The air in the laboratory contains a lot of ions, which can enter the ultrapure water, thereby affecting the water quality of the ultrapure water. In addition, instruments used to measure conductivity (beakers, resistivity meters / probes) can also introduce ionic contaminants. As shown in the calculation formula of conductivity, any increase in ions will affect the conductivity value, because the type of conductivity ion is proportional to the charge it carries.

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