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Re: Dupla CO2 indicator vs. CO2 test kits

> Date: Wed, 27 Aug 1997 12:49:32 -1000
> From: Wade Shimoda <wshimoda at hei_com>
> Could someone please explain the difference between the Dupla CO2
> indicator (I think Eheim also has a similar device) and a CO2 test
> kit?  Does the Dupla indicator actually measure CO2 directly?  When it
> indicates sufficient CO2, how much CO2 is in the water?

I'm making an educated guess, so take this with a grain of NaHCO3.

The CO2 indicator is showing the pH of the water vapor entering the
body of the indicator.  The body contains a liquid indicator that
changes color around pH 7 (bromthymol blue ?).  As such, it is
sensitive to the alkalinity of the water.  I believe they recommend a
"KH" of around 3-5 degrees.  I would hope that under the right
conditions, "good" is about 15 mg/l.

A CO2 test kit will titrate the sample water with sodium hydroxide
until all the dissolved CO2 (as carbonic acid) is converted to Na2CO3
at a pH of 8.3.  Phenophthalein turns red at this pH, so it is used as
an indicator.

At least, I think so :-).