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Re: Using the carbonate ion to adjust KH, GH

CaCO3 is very insoluble in water. But remember, we are adding CO2 to the
water. This makes things more interesting:

CaCO3 + H2O + CO2 <-------> Ca+2 + 2HCO3-

This equation shows you that when carbon dioxide is added to water and
calcium carbonate you get a reaction that yields the calcium ion plus 2
bicarbonate ions. These two ions are the main contributors to GH and KH.

Furthermore:

CO2 + H2O <--------> H2CO3

This equation shows you that when carbon dioxide is dissolved in water you
get carbonic acid. 
Carbonic acid is the main contributor to the pH drop, due to the formation
of the hydronium ion (H+):

H2CO3 <---------> H+ + HCO3-

HCO3- <---------> H+ + CO3-2

All of the equations are two way:
Add more carbon dioxide (CO2) in the presence of excess calcium carbonate
and things shift to the right of the first equation. Add less and things
shift to the left. 
Add more carbon dioxide (CO2) in the presence of little calcium carbonate
and things shift to right of the last two equations. Add less and things
shift to the left.

The breakdown of the calcium carbonate in the presence of CO2 is slow and
gradual, which lead some people to conclude that it is ineffective. Also,
the use of the fine, lab grade calcium carbonate, dumped in the water column
covers everything in a white powder which sticks around for awhile. This
leads people to conclude that it is not only ineffective but also ugly.

I prefer using 5 or 6 larger pieces of calcium carbonate (referred to as
marble chips in some chemistry supply catalogs) in the filter. Every month
or so, I add more, leaving the CO2 set. It works very well, is extremely
inexpensive, and requires no fuss or bother.

 
Augustine Rodriguez
Rice Lake, WI USA

> I remember reading about someone using carbonate salts to adjust KH and GH in
> APD. I have a piece of information for you. According to what I learnt in
> Chemistry, only Potassium, Sodium and Ammonium carbonates are very soluble in
> water.(ph=7) 
> 
> The rest of the carbonate salts are partially or very insoluble(solubility
> cannot be observed by our eyes) in water. Eg.CaCO3 is very insoluble in water
> as mentioned by Gregory Morin.
> 
> Hope this helps. 
> Wayne Wah from Singapore.