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Conductivity & pH Technical Information

Measuring pH and conductivity properly in pure and ultrapure water is difficult. pH and Conductivity are critical for process control and for preventing corrosion for example. Thornton (Mettler-Toledo) are experts that we rely on for answers and guidance.

pH of Ultrapure water is 7.00 (actually between 6.95-7.05). All that is present is H+ and OH-. Don’t try to measure the pH of Ultrapure Water!

pH of Pure water (even RO permeate) is difficult and requires special probes, procedures, grounding, etc. The issue is that you need conductivity to make the pH probe work well. We use pHure which is a grounded, shielded vessel with a slipstream of sample with controlled flow.

The standard “Test Method” for On-Line pH Measurement of Water of Low Conductivity is ASTM D5128.

Conductivity and Resistivity are highly temperature sensitive. Almost all values are referenced to 25C. This makes temperature measurement and internal correction critical. It also means that when your temperature is much off of 25C (below 15C or above 35C), your reported measurement is suspect and possibly off true value. In high purity water (Ultrpure) the correction is between 4%/C and 7%/C, very large. This means that at 15C or 35C, the correction is in the range of 40-70%!

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Thornton on Water Purification