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CaCO3-calcium carbonate, will dissolve in water quite readily and produce carbon dioxide gas and calcium ion
(in form of bicarbonate i think)

calcium ion remains in the water and drives the hardness of the water up, while the carbon dioxide will lower the
pH of the water by some mechanism called something i can't remember.

i've always thought that pH and hardness will increase or decrease proportionally.



[span class="edited"][Edited by JQW 2004-07-20 06:55][/span]

I think you missed the first -HCO3 part which buffers the pH and assumed it all turns into CO2 and H2O which has no buffer(making pH determination nearly impossible).

Carbon and hardness chemistry gets folks into trouble easily and is certainly ther most difficult part of water chemistry for most people.

The cation balance is Ca++ and the anion balance is CO3-- but water reacts to give -HCO3 at the pH's normally found in aquariums.

See the equations in the links below.

The -HCO3 can be removed by adding a strong acid like HCl, H2SO4 etc which will destroy the HCO3 and rip of the OH forming water with the strong acid's H+'s or by plants/algae when they are CO2 limited in the water column under high light, but under normal conditions this does not happen.

Some reading for you and also look at the figures, they are useful:

http://www.seachem.com/support/Articles/UGCPA02.html
http://www.seachem.com/support/Articles/MarinepH.html

Regards,
Tom Barr