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Re: CO2 thoughts...

> From: George Booth <booth at hpmtlgb1_lvld.hp.com>
> Date: Thu, 30 Nov 1995 10:54:35 -0700
> Subject: CO2 thoughts ...
> 
> I was discussing CO2 with Olga yesterday and thought some of the stuff
> might be of interest to others.
> 
> [Olga wanted to increase the KH in her tanks]
> 
> > Well, my reasoning was to prevent pH swings. If I test in the morning
> > the pH is about 6.5 - 6.6 when I test at night it's about 7. I assume
> > this is because the plants don't use the CO2 at night -- but hardening
> > up the water by adding more baking soda or some calcium carbonate
> > would hold it steadier -- no?
> 
> Yes
>
	If the water is already buffered by the bicarbonate/CO2 system,
then adding more KH will make no difference to the size of the pH swings,
just to the range in which they occur.  For the same CO2 concentration
range, KH 1 will provide as much buffering as KH 10, but at a pH 1.0
lower.
	Where the higher KH _does_ provide useful buffering is when
other acids are being added (e.g. nitric as the ammonia produced by
the fish is oxidised).  The pH will stay in a (relatively) narrow range
until just about all the bicarbonate is gone, and the more you have,
the more acid addition you can take before the pH crashes.  At that
point the KH is just about zero.
 
	The pH for a HCO3-/CO2 buffered solution can be calculated from:

	pH = 7.58(depends a little on temp)  +  log(KH)  -  log(ppm CO2)




Paul Sears       Ottawa, Canada.