Why does deionised water give a different reading on my pH meter compared to pH paper?

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pickles116 | 14:14 Fri 18th May 2012 | Science
6 Answers
Measuring deionised water on my pH meter I get a reading of pH8, using pH paper the result is pH6. I checked the pH meter using buffers of pH4, 7 and 10 and the results matched with the pH paper. I tried using a different pH meter to measure the deionised water and I still got the same outcome. Why??


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Are you cleaning the PH meters with tap water between tests? I've found that can alter the reading if it remains on the probe.
Question Author
I'm cleaning it with deionised water between readings and then wiping it
It was just a thought!
I supect that pH paper works fairly well in a solution that is buffered but since deionised water has virtually no buffering capacity the pH paper itself may be affecting the pH. Have you tried distilled water? It is preferable not to wipe the pH probe, rinse and shake dry in 3 changes of deionised water should be perfectly OK.
That has reminded me of that riddle on AB about litmus paper that I still don't understand

F30, I agree with you re, the puzzle question, there are so many valid answers that litmus paper barely makes it into the top 100 :-)

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