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Re: CO2 table & Phosphate

    * From: "Rachel Sandage" <rachelsor at hotmail_com>

what do I test for? pH/KH? Sorry to be so dense.

Paul K. said,
"One way you could get a pretty good estimate of the CO2 in your new tank
with its phosphate addition would be to take two samples of later.  Test the
first one immediately and let the second stand open to the air for 12 to 24
hours.  and then test it.  The difference in the results would be the
difference between equilibrium with atmospheric air, which has 0.03 %
CO2---very little---and the actual tank water.  You can assume that if your
tank water were in equilibrium with the air the growth of your plants would
be CO2 limited.  You want your tank water to have 15 to 20 mg/l CO2 above
what it would have if it were in equilibrium with the air."


I was thinking you would test directly for CO2 using a kit.  You can't use
the table if there is a lot of phosphate present, since the table was
worked out assuming bicarbonate is the only buffer present.  These CO2 kits
usually use phenolphthalein as a pH indicator.  It is colorless at pH's
below 8.2 or so, and then begins turning pink.  You titrate with a basic
solution, such as sodium hydroxide, until you get a faint persistant pink
color, and then calculate the amount of CO2 from the amount of sodium
hydroxide used.  Of course, the kit also assumes that bicarbonate is the
only buffer present, and so if you have a lot of phosphate present, that is
going to make the results inaccurate.  That is why I thought of titrating
two samples of tank water, one just drawn, and the other equilibrated with
the atmosphere. The difference would have to be due to the loss of CO2 in
the sample that is equilibrated with the atmosphere.

Actually, now that I think about it, you could also get the results by
testing for pH/KH in the two samples.  Use the table to calculate mg/l CO2
for both samples and then subtract.  Both readings from the table should be
inaccurate because of the phosphate present, but the difference should be
accurate because it would be due to loss of CO2.  Remember that both the pH
and the KH will change when the tank water loses CO2 to the atmosphere.
You have to measure both.

I havn't thought this out real carefully, so I am hoping one of you
chemists out there will correct me if I have gone astray.

Paul Krombholz, in cool central Mississippi, with only light snow forcast.
The heavier stuff is going to be to the south of us.  Rats!