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Re:Gas Permeable but not liquid Permeable?

>Date: Wed, 13 Sep 2000 15:15:05 EDT
>From: PacosBilly at aol_com
>Subject: Gas Permeable but not liquid Permeable?
>Anyone care to explain how those Breathable bags work? specifically the
>chemistry of the plastic? Any organic chemists work with plastics care to
>enlighten? What about the permeable plastic for those bromethyl blue CO2
>visual tests?  Best, Bill
That was just a guess on my part as to how these pH indicators might work.
James Purchase, who actually has some of them, explained how they really

He said:

".....Dupla, Eheim (I think), ADA, and Aqarium Landscapes all make a version of
this type of  in-tank CO2 indicator device. I have both the Dupla and the
ADA versions. They all work by diffusion, but it is thru an air/water
interface rather than diffusion through plastic.

The ADA is hand blown glass and very attractive - it looks like a little
drop of crystal. The Aquarium Landscape model appears to be an exact replica
(at least from the photos), but it should be available in North America
(Robert H. or Dwight can probably get them as they both sell Aquarium
Landscape products) and considerably less expensive that the ADA model I

In operation, I prefer the Dupla version as there is a white plastic section
against which the color change due to pH flucuation is easy to see (the ADA
model is just clear glass and depending upon the background against which
you look at it, you might have problems figuring out just what color the
liquid inside is)................"

 I figure that the CO2 diffuses from the tank water through air to the
indicator solution.  My guess is that the indicator is in a kind of
upside-down U-tube, where one end of the tube is closed.  It is the closed
end that has the indicator solution.  The open end is in the aquarium
water, and there is an air bubble at the upper part of the upside-down U
separating the tank water from the indicator solution.  It seems to me that
this kind of a device should work quite well, although it might take
several hours to catch up with a large, sudden change in the tank pH.

My original idea of a semi-permeable plastic container might work.  I don't
know if it has ever been tried.  The glass electrodes of pH meters work
because the hydrogen ions diffuse through the very thin glass, but other
ions can not.

Paul Krombholz, in central Mississippi where we got some rain recently, but
still have a long way to go to catch up.