onBalance said:
Chem geek, consider the following chemical reaction; calcium hydroxide and carbonic acid - Ca(OH)2 + H2CO3 after the first month in new plaster pools. How do you see that affecting outcomes?
There is no difference whether it is carbonic acid or bicarbonate ion that reacts with the calcium hydroxide to form calcium carbonate because the carbonates are in equilibrium with each other. The reaction with bicarbonate is the dominant one, but since the pH rises then technically there is some shift from carbonic acid (and therefore dissolved carbon dioxide) to bicarbonate ion and from bicarbonate ion to carbonate ion.
Ca(OH)
2 + H
2CO
3 --> CaCO
3(s) + 2H
2O
Calcium Hydroxide + Carbonic Acid --> Calcium Carbonate solid + Water
However, when carbonic acid is removed from water at typical pool pH, bicarbonate ion produces more of it as follows and that makes the pH rise:
HCO
3- + H
2O --> H
2CO
3 + OH
-
so the pH rises as I had shown before. Multiple equations are all occurring at the same time with multiple equilibria being achieved. I was only giving the simplified and dominant equation in the discussion. I've added these other processes, such as bicarb startup and plaster hardening (calcium hydroxide to calcium carbonate) to my PoolEquations spreadsheet. The following table shows the effect from the process indicated assuming that 10 ppm calcium carbonate is affected (assumptions are a starting pH of 7.5, TA of 80 ppm and the CYA is 30 ppm).
Process ...............................
pH .......
TA .....
CH
Adding CaCl
2 ....................... 7.50 ....... 0 ... +10 .... increasing CH levels by adding calcium chloride
Dissolving CaCO
3 ................ 7.99 ... +10 ... +10 .... plaster etching and degradation from low CSI
Dissolving CaO from CO
2 .... 8.51 ... +10 ... +10 .... speculative and may not occur in practice
Curing Plaster (traditional) ... 8.51 ... +10 .. +10 .... assumes calcium hydroxide dissolves in water
Curing Plaster (bicarb) ......... 8.00 ....... 0 ....... 0 .... assumes calcium hydroxide combines with carbonates to form calcium carbonate
Hardening Plaster ................ 8.00 ....... 0 ....... 0 .... assumes calcium hydroxide combines with carbonates to form calcium carbonate
Though I refer to calcium hydroxide combining with bicarbonate to form calcium carbonate, it can combine with carbonic acid or carbonate with the same result because of the equilibrium between the carbonates that subsequently occurs. Note that the curing of plaster with the bicarb procedure has the same result as subsequent hardening of plaster because the primary process in both cases is the conversion of calcium hydroxide to calcium carbonate via the carbonates in the water.