Re: Aquatic Plants Digest V1 #232

> From: stevensj at calshp_cals.wisc.edu
> What exactly is your CO2 concentration & KH?  You should generally aim
> for about 15 ppm CO2, pH 7.0-7.6, KH 4-10.  High pH & KH makes it
> difficult for the plants to obtain any CO2 due to the carbonate 
> equilibrium.  Using distilled water for water changes helps lower GH &
> KH also.  Also adding peat can help too.
	There seems to be a slight misunderstanding here of the
relationship between CO2 concentration in the water and water hardness.
Water hardness will not affect the CO2 concentration at equilibrium,
but will change the pH at equilibrium.

	We have two equilibria:

	CO2 (gas)  <-->  CO2 (dissolved)  <-->  HCO3-  +  H+

	Each of these has its own equilibrium constant.  The equation
for each must be satisfied at equilibrium.

	On the left, we have the Henry's law equation for the solubility
of gases.  The gas side of this equilibrium is, in the simplest case, the
gas in the atmosphere, about 330 ppm (volume).  The solubility of CO2
in water will then result in an equilibrium concentration of CO2 in the 
water of about 0.5 ppm (weight).  Because the atmosphere is an essentially
infinite supply of CO2 at 330 ppm, the right hand reaction can have no effect
on the concentration of CO2 in the water at equilibrium.  It may affect
the time taken to get there.  If you have a CO2 injection system, the
effective CO2 concentration in the gas will be higher, and so will the 
concentration in the water. 

	The right side of the reaction scheme above is an acid dissociation,
which has its own constant.  This is:

          [H+][HCO3-] / [CO2(aq)]  =  4.16 E-7    gram-moles/litre

	Where [something] means the concentration of "something" in
gram-moles per litre.

	The harder the water is, the higher [HCO3-] is.  
The charge on the HCO3- must
balance that on the positive ions in the solution, taking into account
any other negative ions around as well (e.g. sulphate).

	So, for a given CO2 concentration, the hydrogen ion concentration
must be halved every time the carbonate hardness is doubled, and the pH
goes up by about 0.3 (log(2)).

	In summary, the carbonate equilibrium does not affect the CO2
concentration in the water.  It does affect the pH. 

	I hope this makes things clear!

	Perhaps someone who knows about plant physiology could comment
on the effects of water pH and hardness on the processes _within_ the plants.

Paul Sears