The pH of deionized water tends to be lower than normal water because
the dissolved salts (ions) in the normal water can somewhat neutralize
or "buffer" the carbon dioxide (CO2) that is also dissolved in the
water. There is always some CO2 dissolved in water that comes from the
air, and this CO2 exists in the water as H2CO3 (carbonic acid), as well
as the bicarbonate ion (HCO3-) and carbonate ion (CO3 --) in solution.
When there are no other ions in the water, the HCO3 has a stronger
effect on the pH, because without other ions to buffer the acid, the
water becomes very sensitive to small amounts of CO2 and the result is
that the pH is lower.
To make deionized water, the water is passed through ion-exchange resins
that remove ions like Na+ and Ca++ and replace them with H+, and remove
ions like Cl- and replace them with OH-. This process can also affect
the pH but the overall affect is still from the dissolved CO2 that will
still be present, or will re-dissolve in the water after deionization
from contact with air. Deionization does NOT sterilize the water unless
some sterilization method is also used.
Distilled water is somewhat like deionized water except that the water
is boiled to make water vapor and then condensed back to liquid water,
leaving the dissolved salts (ions) behind. This can have some
sterilization effect as well.
Sterilized water is typically heated and cooled, killing organisms, but this does not remove the salts or ions.
Reverse osmosis is a filtration process that can remove ions AND
sterilize the water to a large extent. There are home units available
that can be mounted under the sink but they are pretty expensive.
From a chemist working in the water treatment industry.
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