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How to Measure the pH of pure water
 | Yes you can test for pH in pure water.
We get many calls regarding the pH value of pure water. The pH of pure water (18.2 megohm) is always 7 but water does not stay pure if it comes in contact with the atmosphere. CO2 in the air quickly dissolves into pure water and forms carbonic acid.
H20 + CO2 = H2CO3
Stored pure water has a pH of as low as 5. The longer it sits, the lower the pH. The pH of pure water in open air changes within seconds.
also see : What is pH
Since pure water has the perfect balance of Hydrogen and hydroxyl, there is no buffer. Anything that touches the water can easily change pH.
In order to measure the true pH of pure water you need to add a buffer to freshly produced pure water. The buffer you need is a concentrated solution of potassium chloride.
The potassium chloride solution should be made by dissolving as much potassium chloride as you can into 50-75mls of pure water.
You then add 4 drops of concentrated potassium chloride in a clean empty beaker, add 50-100 ml's of freshly made pure water to that and then measure pH.
The potassium chloride does not shift pH and stops the CO2 from forming carbonic acid.
If all goes well, the pH should be 7.
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