Re: pH<7

[Paul Sears <psears at nrn1_NRCan.gc.ca>]

> From: Thomas Barr <tcbiii at earthlink_net>
> Subject: Re: pH<7
> 
> > From: "Hopkins, Samuel" <Samuel.Hopkins at marconi_com>
> > Subject: PH <7.0 for plants?
> > 
	(snip)
> CO2 levels are determined by both pH and KH (refer to the table).

	This is backwards.  The pH is set by the CO2 concentration
and the KH.  You can find out what the CO2 concentration is by measuring
the pH and KH.  You can only adjust it by altering the CO2 system or
the plants or fish or lighting.

> Water with high KH values(HCO3) at equilibrium (like a glass of it sitting
> on the table for 24 hrs say) has less CO2 and more HCO3 than water with low
> KH values.

	It will have the _same_ amount of CO2.  The equilibrium concentration
of CO2 in the water is _not_ affected by KH.

> That *****equilibrium**** part is very important. Seldom is water EVER in
> equilibrium. Plants remove the CO2, this raises the pH, we add CO2 to
> counteract this.

	Yes.

> Some folks don't use CO2, some plants and all algae use HCO3 instead of CO2
> as a carbon source splitting it HCO3=> CO2+ OH. This raises the pH and also
> softens the water(Removes the KH).

	This can only remove the KH if CaCO3 is precipitated.  What we get
is:
	HCO3-  ->  CO2  +  OH-
	OH-  +  HCO3-  ->  CO3--  +  H20
	Ca++  +  CO3--  ->  CaCO3 (solid)

	If the precipitation does _not_ occur the KH is unaffected, since
it is the total of OH-, HCO3- and CO3--  (the last counts for two of
either of the others).  In practice, the OH- concentration is almost always
much lower than the others, and the CO3-- concentration is lower than the
HCO3- concentration if the pH is less than 10.3.  I can't recall hearing
of an aquarium where that was not true.

> Soft water(in terms of KH) has more CO2 than hard water does at
> ****equilibrium****.

	No, this is not true.

	The CO2 concentration is dependent on the CO2 system, the fish,
and the plants, but _not_ the KH.  The CO2 concentration and the KH set
the pH.

-- 
Paul Sears        Ottawa, Canada