I found this from (
http://www.stuart-equipment.com/adminimages/t11_001_ph_of_distilled_water.pdf):
pH of Distilled Water
Introduction
Pure water, whether from a still, deioniser or reverse osmosis system, is an excellent solvent and
will dissolve carbon dioxide from the atmosphere to form a very dilute solution of carbonic acid
with a pH below 7. The reason tap water is not similarly acidic is that the buffering effects of the
other contaminants in the water mask the effect of the dissolved carbon dioxide. In a water still,
this solution can form as the steam liquefies in the condenser, resulting in a distilled water output
with a pH which may be as low as 4, although pH 5.0 to 6.5 is more usual.
Acidity level
Many people are concerned about this seemingly highly acidic solution – but take a moment to
consider the actual level of acidity involved. A solution of pH 5 would be obtained by taking just
0.01ml of molar hydrochloric acid and diluting to 1 litre.
How much CO2 will cause a pH of 5 in distilled water?
pH is defined as the negative of log10 of the molar concentration of the hydrogen ion.
Therefore, if pH = 5, [H+
] = 0.00001g/l or 0.01mg/l
Since 1 molecule of CO2 releases 2 hydrogen ions, this concentration of H+
ions would be
produced by just 0.22mg/ml of CO2, i.e. 0.22ppm.
A pH of 4 would, therefore be caused by just 2.2ppm of carbon dioxide in solution.
An acidity of pH 4-5 does not mean that the water is grossly contaminated, as only very few
parts per million of dissolved carbon dioxide will result in a pH value in this range.
Neutralising or removing the acidity
In those applications where pH is critical, such as some microbiological and tissue culture
procedures, the effect of dissolved carbon dioxide can be overcome by the addition of buffer
solutions to the reaction mix but the effect on most laboratory procedures is negligible.
If absolutely necessary, the carbon dioxide may be removed by boiling the water but then great
care must be taken to protect the water from contact with the air until it has cooled, otherwise it will
very quickly redissolve.