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CO2 Injection Gives Bends?

Contents:

  1. CO2 reaction in Canister Filters
    by gb-at-dixie.cs.unc.edu (Gary Bishop) (2 Jul 92)
  2. CO2 Bends
    by Bill Warner <lww-at-ictech.com> (Wed, 29 May 1996)
  3. CO2 Bends
    by George Booth <booth-at-hpmtlgb1.lvld.hp.com> (Tue, 28 May 1996)

CO2 reaction in Canister Filters

by gb-at-dixie.cs.unc.edu (Gary Bishop)
Date: 2 Jul 92

Neil Frank (the editor of The Aquatic Gardener) is having trouble
posting to *.aquaria news groups.  He asked me to bring up this topic
for him.  I have put his address <Neil.Frank-at-lambada.oit.unc.edu> in
the Reply-To field above but it may be best to post responses here.

His message is in response to my posting about the manual CO2 system
that I built.  I'm using vinyl tubing (the standard clear stuff) and a
Penn-Plax Hi-Pore airdiffuser in the strainer of my filter.

Neil's posting begins here
------------------------------------------------------------------
I have a few comments on your approach for the Co2 system.  I am
surprised that nylon tubing will last several years.  I thought it would
get brittle faster.  I use the silicon tubing.

I have played with some similar CO2 systems - they are not too efficient
which is good - less chance of overdosing.  Today, I was talking with
John Burleson (with Dupla).  I asked him about his reactors - too
efficient to use without a controller.  When I told him of the filter
idea (I am currently doing it with an eheim), he said there may be some
problem because of the change in partial pressure as the co2 laden
water exits from the filter.  He thought this caused the CO2 to dissolve
instead of converting to carbonic acid.  Neither one of us knew enough
to understand what is really happening, but he assured me that he
discussed this with reliable sources and thought this is discussed in
one of Spotte's books.  The problem is that the dissolved CO2 can cause
an embolism, like the bends for divers.  This may be a good topic for
the net.

Thanks


neil


CO2 Bends

by Bill Warner <lww-at-ictech.com>
Date: Wed, 29 May 1996

"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


CO2 Bends

by George Booth <booth-at-hpmtlgb1.lvld.hp.com>
Date: Tue, 28 May 1996

> From: "Olive K. Charlsey" <achaudh-at-emory.edu>
> Date: Sun, 26 May 1996 16:04:56 -0400 (EDT)
> 
> If you have read the article at:
> 
> [here - Editor]
> 
> > I have played with some similar CO2 systems - they are not too efficient
> > which is good - less chance of overdosing.  Today, I was talking with
> > John Burleson (with Dupla).  I asked him about his reactors - too
> > efficient to use without a controller.  When I told him of the filter
> > idea (I am currently doing it with an eheim), he said there may be some
> > problem because of the change in partial pressure as the co2 laden
> > water exits from the filter.  He thought this caused the CO2 to dissolve
> > instead of converting to carbonic acid.  Neither one of us knew enough
> > to understand what is really happening, but he assured me that he
> > discussed this with reliable sources and thought this is discussed in
> > one of Spotte's books.  The problem is that the dissolved CO2 can cause
> > an embolism, like the bends for divers.  This may be a good topic for
> > the net.
> 
> Any thoughts? Am I missing something here?

No much.

Yes, Dupla reactors are very efficient and should be used with a
controller.

> 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. 
> 
> So, what are they talking about in that message?

FUD.

In regards to asking a reactor vendor about NOT using his products:

  Uh-huh.  Clueless or trying to generate Fear, Uncertainty and Doubt
  (FUD)?  

For the CO2 to supersaturate (if that was possible), you would
need higher pressure in the filter.  The only significant extra
pressure you will see is due to the head of the water above the
canister filter, which may be 48" to the surface of the tank and maybe
24" from the bottom of the tank.  I don't think scuba divers worry
about the bends in 4' of water.  

"Discussed with reliable sources" indeed.  FUD, clear and simple.

George in Wet and Mucky Colorado


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