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RE: Carbon Dioxide Solubility

Date: Wed, 19 Dec 2001 16:56:59 -0500
From: James Folsom
Subject: Re: Aquatic Plants Digest V4 #1480

> I think its because CO2 is more soluble in pure water,
> But I'm just guessing.  If this is the case it does
> have bearing on the situation in the sense that people
> with liquid rock would see less equilibrium CO2 than
> people like us who have to add minerals.

It's not the case, however. Just as when people believe that carbon dioxide
content can limit oxygen.

Concentration of a gas is limited only by temperature, pressure (or partial
pressure) or its maximum solubility in the particular solute. At equal
temperatures, such as your two samples side- by- side, then each would
contain the same carbon dioxide levels. The difference in pH values, whether
determined from one another or from "neutral", are derived from the ratios
of carbonic acid to bicarbonate ions.

The fluctuation you see in the deionized water is basically the same pH
"bounce" you get when trying to reduce alkalinity with mineral acid - it
follows the solubility and distribution rates of CO2 and thus presents the
same shift curve. The calculated pH of pure water in contact with the
atmosphere is around 4.65 at equilibrium, the difference being caused by the
dissociation of carbonic acid and release of hydronium. But since there are
five orders of magnitude separating the two equilibrium constants it's easy
to see that only a miniscule amount of carbonic is dissociated into
bicarbonates from simple gas absorption. So the bicarbonate- to- carbonic
acid ratio stays pretty low.

The higher pH value in the tap merely shows the higher concentration of
bicarbonates, shifting the ratio...


David A. Youngker
nestor10 at mindspring_com