Have you ever wondered why you can’t get a consistent pH reading on your distilled or deionised water? Here is why…
The electrodes of a pH meter will not give accurate values in pure water because distilled and deionised water do not have enough ions for the electrode to function accurately. The readings will most likely fluctuate and be meaningless. For calibration purposes, pH buffer solution is the best way to test your pH electrodes because it has a defined and accurate pH. General tap water normally has enough ions present to allow a pH electrode to function correctly. Because of this, tap water is a good short term 24hr solution for storage.
You must also keep in mind that water (Deionised Water, or Distilled Water) do not have a pH of 7. As soon as water comes in contact with air, CO2 gas starts to dissolve it, forming carbonic acid (H2CO3). This results in the pH being lower than seven.
To measure the pH of water accurately you will require pH Ionic Strength Adjusters. These adjust the ionic strength of the water without changing the pH of the water making it possible to determine the accurate pH.
The purpose of a deioniser and distillation unit is to purify the water by substantially reducing the conductivity. This means you should really be measuring the conductivity, not the pH.