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The pH of Ultra-Pure Water

Published in Water Filtration
Written by Nicholas Papp

Creation Date Wednesday, 20 April 2011.

It is difficult to measure the pH of type I ultra-pure water. It rapidly picks up contaminants that affect its pH and it has a low conductance, which causes instability in most pH meters unless they are specifically designed to work in ultra-pure water.

Fortunately, since the concentration of hydrogen ions in the water affects both pH and resistivity, the pH must lie within certain limits for a given resistivity reading. For example, if the resistivity is 10 Megohm-cm the pH must lie between 6.6 and 7.6. The pH of ultra-pure water can drop to 4.5 as it absorbs carbon dioxide from the atmosphere, but this does not mean that the water is now grossly contaminated; just a fraction of a ppm of CO2 will cause the pH to fall.

About the Author

Nicholas Papp

Nicholas Papp

Nicholas Papp has a B.S. in Physics, with Minors in Chemistry and Math, from Baldwin-Wallace College. He is the Vice President and General Manager of AQUA SOLUTIONS, INC., and has worked in the water filtration/purification industry for more than 35 years.