Re: CaCO3 dissolution

> Something Paul said about calcium carbonate requiring CO2 in solution
> in order to dissolve makes me think it might make a better pH buffer
> than sodium bicarbonate which dissolves easily and would cause an
> immediate increase in pH. Calcium carbonate would not dissolve as
> much, hence the pH increase should be less (?). As CO2 gas was added
> to the system, the pH changes should be less overall. Is this right
> thinking or all wet? ;-)
> Steve
	When you dissolve sodium bicarbonate, the carbonate hardness
will rise, so the pH will go up.  The CO2 concentration will remain
essentially constant and you can get the pH from the usual relationship.
If you put CaCO3 in, it will dissolve quite slowly, taking CO2 from
the water to do it.  The reaction is

CaCO3  +  CO2  +  H20  ->  Ca++  +  2(HCO3-)

	You are increasing the carbonate hardness, but also decreasing
the CO2 concentration.  If there is not much CO2 in the water to begin
with, and it is not replaced quickly, the pH will rise quite a bit!
Eventually the CO2 will be replaced, getting back to the original
concentration, assuming the CO2 injection mechanism remains the same.
	Once all the CaCO3 has been dissolved, and the CO2 concentration
has got back to normal, it makes no difference at all whether the
carbonate hardness is there as the calcium or sodium bicarbonate.
The ions are separate in solution.  The buffering effects of the
sodium and calcium salts are the same.
	For constant CO2 concentration, the pH will rise by 1 for
every 10-fold increase in carbonate hardness, or by about 0.3 for
every doubling of the carbonate hardness (log(2) = 0.3010).
For constant carbonate hardness, the pH will fall by 0.3 for every
doubling of the CO2 concentration.  At no pH of interest to us will
there be significant quantities of carbonate in solution, only bicarbonate
is there.
	I would suggest that if you want to increase general and
carbonate hardness at the same time, you dissolve the CaCO3 in a
relatively small amount of water by bubbling CO2 through it.  You
can then add that solution to the tank.  It may have quite a hefty amount
of CO2 in it just after it is made, so measure its pH and wait until
it decreases to about the value you want before you add it.  This 
should avoid major swings in pH.  If you dump CaCO3 straight into
the tank, the pH will certainly rise as it dissolves, and the transient
effect may take it into regions you do not want.  

Paul Sears,   Ottawa, Canada.