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Re: CO2 concentrations in water

The equilibrium concentration of CO2 in water in contact
with outdoor air at about 23C is,
as various people have pointed out, about 0.5 ppm.  The outdoor air
CO2 concentration is about 350 ppm.  _Indoor_ air CO2 concentrations
are rather higher.  I think that below 1000 ppm CO2 is recommended,
but it isn't at all uncommon to see it higher.  That could easily
explain 2 ppm of CO2 in water.

We are trying to measure low concentrations of CO2 using very simple
equipment and methods.  If you leave a CO2 test kit lying around, the 
dilute sodium hydroxide solution will absorb atmospheric CO2, and your
measured CO2 number will be high.  If you are slow measuring the pH
of a sample from your tank, the CO2 will leave, the pH will be high
and your inferred CO2 concentration will be low.  The KH test is one 
of the better ones, from an analytical point of view.

I regard aquarium test kit results as a rough guide.  If I wanted accurate
numbers, I'd use better technique, to put it mildly.  I'm not offering 
this as a criticism of the kits - they do a good job, but one should
be aware of the limitations.

Paul Sears        Ottawa, Canada