CO2 thoughts .. episode 3

> From: Stephen.Pushak at saudan_HAC.COM
> Date: Mon, 4 Dec 95 15:30:55 PST
> Subject: Re: CO2 thoughts...
> > From: psears at emr_ca (Paul Sears)
> > 
> [snipped Olga, George & Paul's comments on CO2 pH buffering]
> > 	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.
> Does this oxidation of ammonia to nitric occur when ammonia/ammonium 
> is being metabolized by the plants or during biological consumption
> by bacteria? 

	The oxidation by bacteria changes the ammonia nitrogen to nitric
acid nitrogen.  If plants use it, it ends up as mainly as protein.

> Does this amount to a net consumption of carbonates? 

	Yes.  You go from carbonate hardness (HCO3- in solution) to
non-carbonate hardness (NO3- in solution).

> What 
> happens to the sodium or calcium ions? 

	They stay in solution.

> Does it mean that carbonates need
> to be replenished regularly through water changes and possibly additions
> of chemicals? 

	I would change the water.  The alternative is to keep adding either
calcium or sodium bicarbonate, but then the metal ion and the nitrate ion
concentrations will keep on climbing.

> In the absence of sufficient CO2, a planted aquarium will
> increase in pH due to biogenic decalcification (calcium precipitation).

	This is quite interesting.  The removal of CO2 by plants from
the HCO3- ion would only occur at very high pH (well over 9) if it were
not for the low solubility of calcium carbonate.  I have done some
calculations for a specific example (about KH 6), and it turns out that
CaCO3 will begin to precipitate at about pH 8.4, if I haven't made an
error in the calculation.  This will _not_ happen if the KH is due to
sodium bicarbonate; there will be no calcium to precipitate.

> Would there be a net pH increase without intervention? (assuming plants,
> fish, CO2, lights, food and no other chemical inputs other than distilled
> water to make up for evaporation)
	If you don't put CO2 in there will be an increase, all right.
If you did, but did no water changes, I would expect a very slow drop in the
hardness as the _calcium_ itself was used by the plants.  I'm assuming
that you would keep pruning the plants.

Paul Sears       Ottawa, Canada.