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RE: C02 Measurements
I have also always wondered why the addition of peat would affect the pH/dKH/CO2
relationship. My understanding of the system is as follows (bear with me and
please correct me if I'm wrong...the important equation is at the end).
When CO2 is bubbled into the water column (or from atmospheric CO2 via
diffusion) it will dissolve according to the following equilibrium:
CO2 (gas) <----> CO2 (aqueous)
The dissolved CO2 will then hydrate to form carbonic acid:
CO2 (aqueous) + H2O <----> H2CO3 (aqueous)
Therefore, the total amount of dissolved CO2 is the sum of CO2(aq) and
H2CO3(aq). In a stable tank, this will result in an equilibrium that is
dependent on the bicarbonate concentration (and the a lesser extent, carbonate
concentration):
All Dissolved CO2 <----> HCO3(-) + H(+) Bicarbonate Equilibrium
HCO3(-) <----> CO3(-2) + H(+) Carbonate Equilibrium
The equilibrium constant for this system is ( I look up the numeric value in the
CRC HandbooK):
Ka = [H+][HCO3-] / [CO2(aq) + H2CO3(aq)] = 4.45E-7
Solving for 'all dissolved CO2':
==> All Dissolved CO2 = [H+][HCO3-] / 4.45E-7
This last equation is the important one. It is this equation which is used to
generate the CO2/dKH/pH tables we have all come to love. Note that all
concentrations must be in 'moles per litre' units and not 'parts per million'
for the equation to work.
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?
Peter
Ottawa, Canada