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Re: [APD] GH vs. PPM

On 9/27/05, S. Hieber <shieber at yahoo_com> wrote:

> Carbonic is only one acid that might be present in an
> aquarium. If the KH and pH is low , that doesn't
> necessarily mean the CO2 is high.

If the pH is X and the KH is Y then the CO2 concentration is always Z
regardless of other acids present if you have a steady state that
looks like an equilibrium.  That's what an aquarium is if we set it up
right.  Shifting the pH with another acid, say phosphoric, changes the
equilibrium between gaseous CO2, and dissolved carbonate and
bicarbonate ions.  You will measure a change in KH if you shift the pH
significantly with phosphoric acid.  If you have enough dissolved CO2
--indicated by KH measurements -- you can watch a solution fizz by
adding enough acid (that is strong enough) to it.  You see this every
time you open a can of soda.

"Carbonic acid" is a whole other can of worms.  The substance people
think that is -- H2CO3 -- doesn't exist in water except, possibly, as
a transient species with a lifetime of less than a picosecond.  That
isn't what freshman chemistry books, or the internet, will say.  If
you use "carbonic acid" to mean "whatever the heck you get when you
dissolve CO2 in water" you are okay.



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