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Re: KH questions


On Wed, 9 May 2001, Ole Larsen wrote:

> >Any form of carbonate dissolving into the water would cause KH to rise.
> >Other buffers -- those that you might add yourself, like "pH Down" -- will
> >also cause KH to rise, but it's unlikely that any other buffer will
> >appear in your tank in any significant amounts without you adding it
> >yourself.
>
>
> I do not understand this. (bi)carbonates will rise KH, yes.  But "pH
> down" as far as I know is acid and will, being that, usually lower the
> KH too.  Unless the "pH down" actually is a primary phosphate ( of
> potassium, that buffers at ~4,5) and the starting point is lower than
> that. Am I misunderstanding something here, please ?

I think "pH Down" used to be phosphoric acid.  Perhaps they changed the
product.  It's been about 15 years since I last used it.  Whether or not
the product is acidic makes little difference, because (for common
conditions) lowering the pH does little to the change KH.

Whether or not the product is still phosphoric acid makes a difference
here only in my example; other acidic buffers will behave the same.  Over
the range of pH that we keep in aquariums the phosphate will exist as
HPO4-- and H2PO4-.  These two species are present in equal amounts at a pH
of about 7.2; at lower pH H2PO4- predominates and at higher pH HPO4--
predominates.

A KH test is an acid titration that measures the amount of acid needed to
drop the pH of the sample to a level of about 4.5.  Usually the reaction
from bicarbonate to carbonic acid is the only reaction that works
against the titration, so only the concentration of bicarbonate is measured by
the KH test.  If phosphate is present then the KH test will also measure
the amount of HPO4-- that reacts to H2PO4-; so some of the phosphate added
with the products gets measured as part of the KH.

The only acids that can be added to an aquarium to lower pH without also
raising the KH are relatively strong acids, including hydrochloric acid,
sulfuric acid and nitric acid -- acids that remain completely dissociated
above a pH of 4.5.

*Any* base that associates (gains a hydrogen atom) in the range of pH
between the starting pH of the sample and the endpoint pH of about 4.5 is
measured as part of the KH.  That includes not only HCO3- and HPO4--, but
also NH3, HS-, HSO3-, other inorganic complexes and a host of organic
molecules.  HCO3- is the only base on the list that commonly occurs in
signficant concentrations in natural water.  If there's something other
than HCO3- that changed the KH in an aquarium then probably the aquarist
added it.


Roger Miller