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Re: CO2, blackwater
Paul Sears wrote:
> > 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+
Fair point Paul, but I suspect there's another CO2 'reservoir' at play
here complicating the calculation.
Yes, an equilibrium constant is afterall *constant* :). But I find
without CO2 injection the peat extract and low hardness hardly buffer at
all. With it the pH is rock steady and doesn't depend much on the amount
of extract used. I assume perhaps incorrectly that this buffer system
must involve CO2, but the hardness alone cannot fully account for it. At
first guess it involves the phenoxy groups:
H2CO3 + ArO- -> ArOH + HCO3-
Not sure if this would be reversible though, the Ka's of most phenols
are *much* lower than H2CO3. So it implies a steady change with
continual CO2 injection rather than the CO2 reservoir such a buffer
> 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.
fully agree with this.