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.
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.
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