Date: Wed, 12 Nov 97 11:45:52 cst From: mark.fisher@tpwd.state.tx.us To: cp@opus.hpl.hp.com Message-Id: <aabcdefg4319$foo@default> Subject: Water pH
There have been a few postings regarding the pH of distilled/purified water
lately, including:
>I've been watering all my plants with distilled water (pH ~6). Out of
>curiosity, I just took the pH of the water in my cephalotus' dish and
>found it to be about 7.2.
and:
>However, it appears to only remove the chlorine and the hardness of the
>water. It does nothing to lower the pH of the water, and the tap water
>here in St. Louis is about 9.0!
It is actually very difficult to measure the pH of pure water. Even the
super-accurate pH probes will not work well, as there is not enough
conductivity in the water to get a reading across the probe. To do so, one
must add a neutral ion, like KCl, to increase the conductivity and get a
reading. Also, as pure water has no buffering capacity, simply dipping
one's finger in the water can alter the pH appreciably. Bottom line--don't
bother measuring the pH of pure water. It's difficult, and has no real
meaning anyway.
Also, pure water is slightly acidic, not neutral (pH 7.0), because of the
atmospheric carbon dioxide dissolved in it (CO2 dissolves in water to make
carbonic acid). A difference of only 1.0 ppm CO2 can result in different
pH values of pure water.
On a related note, tap water often has a high pH because municipal water
treatment typically drives off all the dissolved CO2, which raises the pH.
The best way to restore the pH of tap water is to simply bubble air through
it, or just let it stand exposed to air. CO2 will eventually dissolve back
into the water, and drop the pH to it's equilibrium value. Example: here
in Austin, TX, our tap water is pH 10, but after bubbling air through it
for 36-48 hours, it stabilizes at pH 7.4 (our water is moderately hard, and
well buffered, BTW).
Kind regards,
Mark
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