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

["S. Hieber" <shieber at yahoo_com>]

Paul Sears gave a new formula (vis-a-vis the two I had noted) and
described a basis for using diferetn values at different temps:

> 	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!

Thanks, Paul.

Indeed.  It's duly noted that test kit readings will give variances far
beyond the differences in formulas.

Still, it's nice to have in a CO2/KH/pH table, either an accurate
formula (even if the accuracy is only for a cited temperature) *or*
numbers rounded to the nearest 5 or 10 units.  :-)  At least then,
there's a basis for reconciling the different formula or versions of
the chart.  Things are confusing enough without the math seeming to not
work out.  :-)

Picking a last shread of mote from the nit, it looks like the
"15.664*KH*10^(6.37-pH)" formula or "15.7*KH*10^(6.37-pH)" is a good
for about 70 degrees F.  A more likely *tropical* tank temp is about
78-80 degrees F, which would put the formula thus: 
15.664*KH*10^(6.34-pH).   Or should we just call it 16*KH*10(6.34-pH)? 
:-)

Scott H.

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