[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: CO2 tables - or how to save money..
Scott Hieber wrote:
> Two formula are profferred for calculating CO2 levels from KH and pH
> values -- there might be more but I only know of two (not counting
> algebraically equivalent recastings):
By my reckoning, the formula should be:
CO2 (in ppm) = 15.7*KH*10^(pKa(CO2/HCO3 system) - pH)
The factor at the beginning effectively compensates for the
weird units used for the CO2 and bicarbonate concentrations. The
pKa requires molarities.
I have dug up a reference for the dependence of the pKa for
CO2/bicarbonate on temperature, and from it I get:
T (C) pKa
Pick you pKa.
I would point out that the 20% difference in the factors given
at the beginning can be compensated for by a change of only 0.09 in the
number used for the pKa, or an error of 0.09 in the pH measurement!
Can you read your pH that well? I could if I took a pH meter
home from work. Do I care that much? No. Do I trust my KH measurement
to 20%? Maybe.
This does not need to be, nor is it, precise analytical chemistry!
Paul Sears Ottawa, Canada