# Re: CO2 formula question

```> From: Dennis8425 at aol_com
> Subject: CO2 formula question
>
> There are graphs to show relationship between GH, pH and CO2 concentration.
> But does anyone have an actual formula to do these calculations?

I posted it in March 1996, and the following posting was made in
December 1996:

>
> > From: "Loh Soon Hin" <lsh9933 at singnet_com.sg>
> > Subject: Re: KH,pH,CO2 table
> >
> > > > pH as a function of CO2 concentration and KH value:
> > > >  7.58+@LOG(KH)-@LOG(CO2)

This was the one I posted in March 1996.  The CO2 concentration
is in ppm.

> > > >
> > > > CO2 concentraction at a given pH and KH
> > > >  10^(7.58-pH+@LOG(KH))
> >
> > What is the standard unit for KH?
> >
> >  A note under the Optimum CO2 Content Table in Dupla brochure says :
> >   '' carbonate hardness in German degrees ( 1 degree dKH~ 10 ppm)''
> >
> > However I had read from other references that say 1 German Degree equals
> > 17.9 ppm .
>
> 	This is a mess.  One degree of _general_ hardness is defined as
> 10 ppm of calcium _oxide_ in the water.  Of course, there is no calcium
> oxide actually in the water, but that is how it began, from a definition
> point of view.  This corresponds to 17.9 ppm of calcium _carbonate_ in
> the water.  (The molecular weight of CaO is 56, and that of CaCO3 is 100)
> There is no calcium carbonate in the water either.
> 	Now, if you dissolve either the oxide or the carbonate in water
> the reaction with CO2 will give you the bicarbonate.  So, if you start
> from CaO, CaCO3 or even Ca(OH)2 and make a solution containing 1 degree
> of general hardness, you will end up with 1 degree of KH as well.  The
> connection is solid, but almost Byzantine in its derivation!  We are
> stating a bicarbonate concentration in terms of the equivalent amount
> (equal and opposite charge) of calcium, but stating that as either an
> oxide or carbonate amount.
> 	If we used millimolar concentrations of things, then this mess
> and a _lot_ of confusion would disappear.

Nothing changes!

>
> 	The equation relating pH, KH and CO2 concentration was originally
> posted by me on the APD in March.  It was derived from the equilibrium
> for the reaction:
>
> 	"H2CO3"  <->  H+  +  HCO3-
>
> 	The tables on the Krib give somewhat different answers, which
> can be reproduced (at least as far as I have checked) by using the numbers
> 7.48 (for George's table) or 7.46 (for the Finnish one).  The tables were
> derived using a different K for the reaction above.  K _is_ temperature-
> dependent.

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