Re: Ph, KH, CO2 etc.
To: Aquatic-Plants at actwin_com
Subject: Re: Ph, KH, CO2 etc.
From: psears at NRCan_gc.ca (Paul Sears)
Date: Fri, 20 Dec 1996 09:04:29 -0500 (EST)
In-Reply-To: <199612200839.DAA06942 at looney_actwin.com> from "Aquatic-Plants-Owner at ActWin_com" at Dec 20, 96 03:39:01 am
> 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)
> > >
> > > 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.
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-
Paul Sears Ottawa, Canada
Finger ap626 at freenet_carleton.ca for PGP public key.