Re: Aquatic Plants Digest V2 #101

> From: "Olive K. Charlsey" <achaudh at emory_edu>
> Date: Sun, 26 May 1996 16:04:56 -0400 (EDT)
> Subject: CO2 Bends
> If you have read the article at:
> http://www.cco.caltech.edu/~aquaria/Krib/Plants/CO2/co2-bends.html
> <snip>
> 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.
> <snip>
> Then you may be able to answer my question.
> 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?
> 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.
> Also, the water is not exactly "laden" compared to the rest of the 
> tank water when it exits the filter. Certainly it has a higher CO2 
> concentration, but really, not a whole lot more. 
> Any thoughts? Am I missing something here?
> 					Alok
The same problem could be encountered in either case, you are drastically 
increasing the partial pressure of CO2 in the reactor and the filter. 
This causes the water to saturate and perhaps supersaturate for 
dissolvable CO2. The water then comes out of the reactor etc and suddenly 
the partial pressure drops and some CO2 may come out of solution as 
bubbles. This would be worse if either was under extreme pressure as the 
pressure drop alone can cause this effect. 

There are two things which mitigate any concerns that I may have about 
this. The first is that this would only occur in the water from the 
outlet of either the reactor or the filter and would be sudden and 
localised. It would rapidly reach equilibrium again. The second is that I 
never bother to use aged water in my water changes. During the winter 
here the dissolved gas reaches high levels (gases dissolve mopre in cold 
water) and during a water change this all comes out because of the sudden 
change in temperature. It has not worried either my fish or plants , a 
single look at the health and colours of my fish is enough to see that. I 
keep mostly Australian rainbows with a few Corydoras and a hill stream 
loach, together with the obligatory SAE's. I have not had to treat the 
tanks for any disease now for 2yrs and have not had a death in custody, 
except for 2 fish that were damaged by the fish catchers at the store.

The problem would be nasty if the supersaturated water was inside the 
fish and someting caused the gas to come out then. I have a bit of 
trouble envisioning this situation in an aquarium, but I am sure that 
some of the inventive people on this list may think of something( eg 
suddenly deliberately applying a vacuum to the tank).

Hope this helps

Peter Hughes