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Re: CO2, KH

> From: Thomas Barr <tcbiii at earthlink_net>
> Subject: Re: C02 & KH
> > Tom wrote:
> > 
> >> Another is to raise your KH up high. This is a bit more dicey. But at the
> >> higher KH your pH will not swing as much to big dumps.
> Chuck wrote: 
> > Isn't that a mis-statement?   Regardless of the KH, the size of the pH swing
> > would be the same, wouldn't it?   The starting and ending pH values would
> > be higher, but the size of the movement would be the same.
> Nope. Often folks with Discus tend to add very soft water with low KH.
> Add a small equal amount of acid into two equal volumes of  distilled water.
> One with a KH of 0 and another with a KH of 5 (add some baking soda). What
> happens to the pH? Do you still think the pH swing are going to be the same?
> Try it yourself and see. Now try 1:6 and 2:7 etc. I think you'll see a
> trend.
> The KH of 5 will resist the change more. So when a big dump occurs... it
> helps resist the shock.

	Here we go again.  If the pH of the water in your aquarium is 
controlled by the CO2/HCO3- buffer system, then the size of the pH
change for a given change in CO2 concentration, say 2 ppm to 20 ppm, 
is independent of the KH.  The KH must not be zero, because then
the buffer system is not the CO2/HCO3- one.

	In the experiments described above, the KH 0 case never was using
the CO2/HCO3- buffer.  What happens to the other one depends on the acid
you use.  If you use a strong acid (e.g. HCl), then you destroy KH with
it and make CO2.  The pH falls, but will recover to some extent as the
CO2 leaves, provided that you have not destroyed all the KH.  If the 
KH is zero and you have a strong acid in there, the pH will crash.

	Adding CO2 never destroys the KH.  If you have some KH, the size
of the pH swing _is_ independent of the KH.  Running with _no_ buffering
system in the tank is definitely asking for trouble, but KH 1 or 2 will
do just fine if you are only adding CO2.  You do then have to beware of
destruction of the KH by HNO3 produced in the tank, but with any luck
the plants will stop that.

Paul Sears        Ottawa, Canada