pH electrodes will NOT give accurate pH values in distilled or deionized water. This is because distilled and deionized water do not have enough ions present for the electrode to function properly. The readings will drift and be essentially meaningless. pH buffers are the best solutions in which to test your pH electrodes. Tap water usually has enough ions present to allow a pH electrode to function properly. Because of this, tap water can be a good short term (~24 hours) solution for storage.
Also, keep in mind that water (distilled, deionized, or tap) is NOT “pure” (i.e., pH equal to 7). The moment it comes in contact with air, CO2 gas begins dissolving into it, forming carbonic acid. The actual pH, therefore, will often be slightly less than 7.
If you need to accurately measure the pH of a very pure sample of water, the ionic strength of the water can be adjusted without changing the pH of the water. These pH Ionic Strength Adjusters (pHISA), can be purchased from Thermo Scientific, www.thermo.com.