Hi all,
The question I am wondering is why are the H+ exchanged for Ca++. Is that due to the reduction oxidation potential and isn't it just another indicator of lack of positive calcium ions and possibly even other reducers in the tank being exhausted out of the environment and can't keep up
Normally it is back to the CO2 ~ HCO3- equilibrium, so you don't actually lose any Ca++ ions, they just go into solution (although they may then form other insoluble compounds, for example if I added sulphuric acid (H2SO4) rather than HCl, insoluble CaSO4 will be precipitated).
All acids are "H+ ion donors", and as you add H+ ions, the H2CO3 is formed (from CaCO3 going into solution). Any excess CO2 is evolved as gas.
I'll use hydrochloric (HCl) as my acid, (but the process is the same for any H+ ion donor): HCl makes it simple to understand, because you can just ignore the chlorine ion (Cl-):
You need "2H+" to make the equation balance due to the different valencies of the ions. (s = "solid", g = "gas", l = "liquid", aq = "in solution")
2H+(aq) + 2 Cl-(aq) + CaCO3(s) --> Ca++(aq) + 2 Cl-(aq) + H2O(l) + CO2(g)
discard the balanced chlorine ions and you get:
2 H+(aq) + CaCO3(s) --> Ca++(aq) + H2O(l) + CO2(g)
This is exactly the same process that happens when we add CO2 (or remove CO2) from water. If we add CO2, more HCO3- goes into solution from the CaCO3 "buffer" (in equilibrium with H2CO3), if we heat the water CO2 becomes less soluble as the temperature rises, and CaCO3 is deposited as "lime scale".
CaCO3 is deposited because it is the less soluble than the carbonates of the monovalent metals (K+, Na+). If we had lots of Mg++ ions in solution they would be deposited first, quickly followed by the Ca++ ions.
cheers Darrel