Re: CO2 Bends

To: "Olive K. Charlsey" <achaudh at emory_edu>
Subject: Re: CO2 Bends
From: Bill Warner <lww at ictech_com>
Date: Wed, 29 May 1996 21:50:54 -0500
cc: Aquatic Plants Digest <Aquatic-Plants at actwin_com>
In-reply-to: Your message of "Sun, 26 May 1996 16:04:56 -0400."

"Olive K. Charlsey" writes:
>
>CO2 does dissolve in water to form H+HCO3-. There is no other way that it 
>dissolves, no matter what pressure, no matter what the concentrations. 
>
Actually, pressure has a significant effect on the solubility of CO2.  This
fact is clearly demonstrated every time you open a bottle of soda.

>When the "CO2 laden water" comes out of the filter, it has a higher 
>concentration of carbonic acid (H+ HCO3-) since there is no difference 
>between dissolved CO2 and carbonic acid.
>	
This is a fairly common misconception which is perpetuated, IMO, by the over
simplifications used by many introductory chemistry texts.

There is a difference between "dissolved CO2" and carbonic acid.  When CO2 is
added to water it initially forms a loosely hydrated species denoted CO2(aq).
This dissolved molecular CO2 reacts _slowly_ with water to form carbonic acid,
H2CO3(aq).  

			CO2(aq) + H2O = H2CO3(aq)

Furthermore, this kinetically slow reaction does not go to completion.  At
equilibrium, only a small fraction (ca. 0.2%) is actually converted to
carbonic acid.  Most of the CO2 remains as solvated molecular CO2.  In fact,
the pKa most often reported for carbonic acid (pK1 = 6.38) is not really the
true pKa of carbonic acid.  Rather, it is the pKa of the equilibrium mixture
of CO2(aq) and carbonic acid.  Carbonic acid itself is actually a much
stronger acid than this, with a true pK1 value of 3.58.

As for this business about "CO2 bends"...  I think that if one actually thinks
about how an organism gets the bends, it's pretty clear that it ain't gonna
happen.

Bill