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Re: CO2 measurements

Bob Dixon writes:

> Steve writes:
> > It seems to me that we should be able to make a much better estimate of
> >  CO2 concentration in tanks which contain other alkaline buffers aside
> >  from carbonate-bicarbonate by measuring the *change* in pH by simply
> >  boiling the water to elminate dissolved CO2. I couldn't guess at how to
> >  apply the formula to this but maybe Roger or George can.
> The boiling may also cause changes in the carbonate present.  I hear that
> carbonate levels do not directly affect pH, but in this scenario, I wouldn't
> be sure.
> Bob Dixon

Bob's right.  At the temperature of boiling water the bicarbonate can
break down to CO2 + CO3-- plus water.  Some of the carbonate (CO3--) is
likely to precipitate as pot fur.  You might get better results just
warming and aerating the water to drive out the CO2.

You might do something by measuring alkalinity and pH before and after
driving out the CO2. Unfortunately, driving out the CO2 in and of itself
alters the pH and the alkalinity changes with it, including any part of
the alkalinity due to phosphates and organic acids; you can't really sort
out what part of the change is due to driving out CO2 and what part is
caused be changing the amount of alkalinity due to other compounds.

An alternative might be to remove the other buffers.  You could filter a
sample through activated carbon to remove organics (maybe that would work,
maybe not) and through a phosphate sponge or add ferrous sulfate to remove
phosphate.  That would have to be done in sealed equipment or CO2 would be
gained or lost to the atmosphere.  Then you would measure pH and
alkalinity in the filtered sample and use that to determine CO2.

I've never done either of these and I wouldn't advice anyone else to do
it.  Is it really that important to know how many ppm's of CO2 you have in
the tank?  Healthy, robust growth should be enough to indicate adequate
CO2 concentrations.  Never use more than it takes to meet that end and
your fish won't be endangered.

Roger Miller