[APD] Re: RO/pH question

["James Purchase" <jppurchase at rogers_com>]

Russ asked:
"Somehow the water still has a pH of 9! I don't understand how
almost pure water can still have such a high pH."

RO/DI water doesn't really HAVE a measurable pH value, other than the
"theoretical" value of 7.0. 

Water is chemically H2O. A very small percentage of those H2O molecules will
break apart, becoming H+ ions and OH- ions. The pH value is the ratio of
these 2 ions, anything which adds H+ ions will shift the pH value in one
direction (more acidic), while anything that adds OH- ions will shift it in
the opposite direction (more basic).The ratio of [H+] ions to [OH-] ions is
constant in PURE water. Think about the definition of pH, pH = -log[H+]. 

Because it is so pure, even minute amounts of other substances can cause the
measured pH value to swing widely. This topic has been discussed (beaten to
death actually) a few times on the APD - check the archives of the list. You
can't trust the reading from a probe or a test kit in water this pure.

Be that as it may, remember that nothing will live or grow in RO/DI water
unless it is either cut with plain tap water or you use an electrolyte
replacement salt. Both fish and plants need some minerals in the water in
order to survive. 

Don't try to add CO2 to pure RO/DI water, or your fish will be swimming in
vinegar. Both RO and DI processes remove Alkalinity (Buffering Capacity),
and CO2 acts as an acid when dissolved in water, driving the pH value down.
With no Alkalinity, the drop in pH can be both sudden and deadly. Even
atmospheric CO2, dissolving into pure RO/DI water, can cause the measured pH
to plummet.

Remember as well that even your 6 stage unit isn't capable of taking
everying out of the input water. There will always be some pass-through.
100% pure water is only found in a laboratory, and it never stays "pure" for
long.

James Purchase
Toronto


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