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**To**:**Aquatic-Plants at actwin_com****Subject**:**Re: CO2 tables .....****From**:**Paul Sears <psears at nrn1_NRCan.gc.ca>**- Date: Tue, 9 Jul 2002 08:33:33 -0400 (EDT)
- In-reply-to: <200207090748.g697m3s03507 at acme_actwin.com> from "Aquatic Plants Digest" at Jul 09, 2002 03:48:04 AM

> From: "S. Hieber" <shieber at yahoo_com> > Subject: Re: CO2 tables - or how to save money.. > > 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) > > (snip) > > > > 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 your pKa. (snip) > > 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. :-) Agreed. One should get things right. I do think, though, that people are worrying far too much about the precise concentrations of things in their tanks when it doesn't really matter that much, and our test kits aren't very good. > 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)? > :-) To add to the motes and nits, if you want the factor at the start to lots of decimal places, I make it 15.696 to three places. :) That is using accurate atomic weights. Now to start reading my KH test kit to four significant figures.... 16*KH*10(6.34-pH) looks good to me for about 27 C. -- Paul Sears Ottawa, Canada

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