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Re: CO2 Measurements

On Fri, 26 Nov 1999, Peter_Bradley wrote:
> My question is this. In any closed equilibrium system there can only be ONE
> hydrogen ion concentration (i.e. pH) and ONE carbonate/bicarbonate concentration
> (i.e. dKH).  If you change the pH (i.e. add an acid), the equilibrium shifts
> accordingly. Therefore, by measuring the pH and dKH you can determine the CO2
> concentration regardless of any other factors in the tank. The presence of peat
> or anything else should not matter. Does this make sense? Am I missing
> something? Any comments from water chemist out there?

You make perfect sense, but what you're missing is that KH doesn't always
measure bicarbonate concentration.  KH measures alkalinity and other
inorganic and organic acids -- some of which might enter the water from
peat -- can contribute to the alkalinity.

Alkalinity (at least in unpolluted natural waters) is generaly due only to
bicarbonate.  There are cases where this isn't true.  When there are other
significant sources of alkalinity the pH-KH-CO2 relationship no longer
holds.  It isn't because the equations are wrong; it's because the KH is
no longer an accurate measure of bicarbonate.

Roger Miller