# Re: Aquatic Plants Digest V2 #839

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> Date: Thu, 17 Jul 1997 09:27:23 -0400
> From: Michael Eckardt <mike at odg_com>
> Subject: Re: ph-kh-co2
>
> >The formula is:
> >
> >	CO2 = 12.839 * KH * 10^(6.37-pH)
> >
> >With KH in German degrees, CO2 is calculated in mg/l (=ppm).
>
> As an accountant, my math skills don't extend much past debits and credits,
> so I need some help. The formula that I derived from a post by Paul Sears a
>
> 	CO2= 10^(7.58-pH+@LOG(KH))
>
> The formulas result in CO2 values that are about 10-20% different. How
> come?
>

You seem to be doing all right, despite the handicap :-).  I went back
through the archive, looking for Paul's original post.  I think I found it
dated 3/29/96.  Your derivation from Paul's post is fine.  Paul's formula
was also fine, but he didn't seem to describe where the 7.58 value comes
from, so I'll speculate.  I do that a lot.

It appears to me that Paul used a "pK" value of 6.38.  George used 6.37 (a
value I've seen elsewhere), the USGS reference used 6.34 and James
Plambeck (http://c.chem.ualberta.ca/~plambeck/che/data) lists 6.36.
Different sources of data give different values, and 6.38 is certainly
reasonable.

Most of the difference seems to be in the conversion between KH and molar
bicarbonate concentrations.  George multiplied KH by 17.8 to get
alkalinity in mg/l, then divided by 61 mg bicarbonate per mmole to get the
molar (or millimolar) concentration.  So, the molar concentration [HCO3]
is 0.292 * KH.  The ph-kh-co2 table posted by Pauli Hopea and shown at
the Krib gives pretty much identical values so it seems like the author of
that table must have used the same conversion.

I use a different conversion, and it looks to me like Paul used something
similar to what I used.  Neil Frank (posting on 4/4/97) gave the
definition of one degree KH as 10 mg of CaO in a liter of water.  CaO has
an equivalent weight of 28.04 grams/equivalent, so one degree KH is
10/28.04 or 0.3566 meq/liter.  Bicarbonate is singly charged, so for it
meq and millimoles are the same thing.  With that, the molar concentration
[HCO3-] is 0.3566 * KH.  This can also be gotten to more indirectly:  KH
times 17.84 gives mg/l alkalinity as CaCO3.  that divided by 50.04 (the
equivalent weight of CaCO3) gives the molar concentration of [HCO3-].

The difference in conversion would account for a 22% difference in the
results.

Anyone else want to weigh in on this?

Roger Miller
```