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Re: CO2, black water

> From: toado <toado at ihug_co.nz>
> Subject: Re: CO2, backwater & vermiculite.
> On CO2 & black water..
> Paul Sears wrote:
> > 
> >         If you have humic acids in the water, you are not likely to be
> > able to get much idea of the HCO3- concentration (KH).  The charts
> > are still all right, but that doesn't help much if you can't measure
> > the KH.  You won't be able to _measure_ the CO2 very well, either.
> > 
> As usual, I can only agree with Pauls comments. 'Semi' black-water tanks
> are my pet interest, so I'd like to expand on this a little. Hopefully
> without generating too much misunderstanding :).
> The humic acids thoroughly foul up the easy 'direct' CO2 estimation
> methods, at least those I'm familiar with. Worse, they'll invalidate a
> few assumptions used to calculate the chart by providing an alternate
> buffer system. Both the calculations based on the bicarbonate pKa and
> methods used to determine KH won't be accurate any longer. 

	The calculations _are_ O.K., since they depend on the equilibrium
constant for the reaction:

	H2CO3  <->  HCO3-  +  H+

	This will not change.  It is not possible, though, to get a value
for the HCO3- concentration by the usual method (titration with acid),
because the black water buffering material will also give the water some
"alkalinity", (base component of the buffer) which will show up as well.
The CO2 measurements will be screwed up by the acid component of the
black water buffer.

Paul Sears        Ottawa, Canada

Finger ap626 at freenet_carleton.ca for PGP public key.