Re: CO2 tables - or how to save money..

[Paul Sears <psears at nrn1_NRCan.gc.ca>]

	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):
> 
> 12.839*KH*10^(6.37-pH)
> 
> 
> 15.664*KH*10^(6.37-pH)
> 

	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
0	6.58
5	6.52
10	6.47
15	6.42
20	6.38
25	6.35
30	6.33

	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