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Re: [APD] GH vs. PPM

Scott wrote:

> For those of us that didn't take organic chemistry because
> we thought it was about clocks that dripped over the edge
> of tables, how do the organic acids come into play?

In exactly the same way that inorganic acids may come into play.  Any 
acid that
is disassociated at the ambient pH and becomes more associated at a pH of 4.5
or so adds to the buffer capacity of the water.  It will be measured as 
part of
the KH.

Unlike the relatively small number of simple inorganic acids there are
approximated a gazillionkabillion different organic acids -- well, more than
two -- that can have an effect.

Some of the organic acids are large and can be massively 
polyprotonated. "Polyprotonated" means that each molecule can associate 
with more than one
hydrogen ion.  Phosphate is the most polyprotonated inorganic acid that we
usually deal with -- it can associate with up to three hydrogen ions.  Some
especially large organic acids may be able to associate with thousands of
hydrogen ions.  All that associating give some of those organic acids rather
poorly defined acid-base characteristics.

To make the situation even more difficult, some of those acids are not very
soluble and will remain in a solid state while still behaving like 
acids.  Part
of the organic fraction in soils and peat behaves that way.  The insoluble
organic acids will not be measured as part of the buffer capacity of the water
but can still have darkly mysterious effects on the water chemistry. They can
destroy the bicarbonate content of the water; they create an odd pH dependence
in the cation exchange capacity of soils.

Other organic acids are small and about as simple as inorganic acids.  Their
simplicity is offset by the fact that we can't readily identify the acids in
question and by the fact that there are a number of possible 
candidates, all of
which may be present at any one time.

The other complicating factor about organic acids is that -- unlike inorganic
acids which usually won't be there unless the you add them yourself -- 
they can
be generated by activity in the aquarium.  Some may be generated directly by
plants or fish and released into the water.  Some might be secondary products
from fungal or bacterial breakdown of organic material in the tank or 
filter. Probably the most difficult to understand are those that might 
be generated by
fungal activity in the substrate or in decorative wood in the tank  Wood (or
more precisely, the organisms breaking the wood down) may produce acids from
the time it goes into your aquarium, or it may be perfectly benign for months
or years, then start producing acids only after a population of acid-producing
fungi -- probably growing in an inobvious, non-fruiting stage deep inside the
wood -- grows to a signficant size.

The organic chemistry of aquariums is something that we know relatively little
about.  While it may be responsible for a lot of different effects in an
aquarium I doubt that the organics have much at all to do with melting watchs
or with hard-to-remember experiences during our youths.

Roger Miller

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