For example, a sample of deionized water exposed to air can quickly adsorb C02 and form carbonic acid (H2CO3) which can alter the pH of neutral water at 7.0 to drop as low as 5.6. Remember that the pH scale is logarithmic and this represents a very large change in chemistry!

Does deionized water affect pH?

pH electrodes will NOT give accurate pH values in distilled or deionized water 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.

Does pH change if you add water?

Adding water to an acid or base will change its pH. Water is mostly water molecules so adding water to an acid or base reduces the concentration of ions in the solution. When an acidic solution is diluted with water the concentration of H ⁺ ions decreases and the pH of the solution increases towards 7.