chem geek
07-07-2008, 11:20 AM
This thread is in response to the question posed in this thread (http://www.poolforum.com/pf2/showthread.php?t=8720) as it is a technical response (so belongs in The Deep End).
The short answer to why a lower pH with higher CH for the same saturation index results in a lower pH rise is "I don't know" since in theory it should not be any better and could be worse. The analysis below explains why a lower TA with higher CH might be beneficial.
The Calcite Saturation Index will determine the tendency to form scale (if positive) or to dissolve scale/plaster (if negative), but specifically for calcium carbonate via the following equilibrium reaction:
CaCO3(s) <--> Ca(2+) + CO3(2-)
The above reaction would say that water that is saturated with calcium carbonate would inhibit dissolving of plaster regardless of the level of calcium.
However, pool plaster itself is not only composed of a smaller amount of calcium carbonate, but is mostly composed of hydrated calcium silicate which is formed during curing as follows:
2 3CaO•SiO2 + 7 H2O ---> 3CaO•2SiO2•4H2O + 3 Ca(OH)2 + 173.6kJ
The Calcium Oxide is tightly bound to the silicate crystal structure, but can be dislodged via the following which is very strongly thermodynamically favored (were it not for the silicate structure):
CaO + CO2 --> Ca(2+) + CO3(2-) + heat
In the above reacion, saturation of calcium carbonate does not prevent the reaction from occurring though it reduces its likelihood. Reducing the carbon dioxide content further reduces the likelihood of the reaction and that means lowering the TA. If the TA is lowered, then either the pH or the Calcium Hardness (CH) must be increased to keep the calcium carbonate at saturation. Since one generally wants to keep the pH near 7.5 to be best for the eyes, this implies lower TA with higher CH could reduce the rate of dissolving of plaster. There is anecdotal evidence in extreme cases where calcium carbonate saturation in water very low in calcium results in pitting of plaster.
The result of having calcium carbonate dissolve or of having calcium oxide dissolve may be seen by giving an example starting with water at pH 7.5, TA 100, CYA 30, CH 300, TDS 525, Temp 80:
If one pound of CaCO3 as ions in 10,000 gallons is produced from dissolving Calcium Carbonate, this would raise the pH to 7.99 and the CH by 12ppm. If one pound of CaCO3 as ions in 10,000 gallons is produced from dissolving Calcium Oxide, this would raise the pH to 8.50 and the CH by 12 ppm. So the latter process raises the pH more for the same CH rise than the former.
So, in theory, having a lower TA and a higher CH could lead to a slower dissolving of pool plaster and a slower resulting rise in pH and CH over time.
Note that a lower pH instead of a lower TA with a higher CH to compensate (for saturation of calcium carbonate) does not produce the same effect. Instead the amount of carbon dioxide in this scenario is actually higher, not lower, so could have a faster dissolving of plaster. Therefore, if you see that a higher CH with lower pH has a lower pH rise, then it may be a different mechanism than described above or could just be a coincidence with something else happening in your situation.
As for the other questions in the original post, a lower pH of 7.4 would have slightly more carbon dioxide outgassing, but if the plaster dissolving were the main factor then that could overwhelm the outgassing effect. A lower TA instead of a lower pH would have the same effect on the dissolving of plaster, but would lower the outgassing as well so might result in an even lower pH rise and acid usage. As for eye comfort, I doubt there will be much of a noticeable difference between 7.4 and 7.5. The saturation index of -0.1 is only slightly negative. At the surface of a heat exchanger, the temperature may be 30F higher which would be around 0.2 higher in saturation index. As for the SWG cell, it has a very high pH at the hydrogen gas generation plate so the tradeoff is in having a negative index dissolve plaster faster. But remember that many of the measurements are approximate and have error so this isn't going to be perfect. Targeting -0.2 for the saturation index might be more reasonable, but it's not a big deal.
Richard
The short answer to why a lower pH with higher CH for the same saturation index results in a lower pH rise is "I don't know" since in theory it should not be any better and could be worse. The analysis below explains why a lower TA with higher CH might be beneficial.
The Calcite Saturation Index will determine the tendency to form scale (if positive) or to dissolve scale/plaster (if negative), but specifically for calcium carbonate via the following equilibrium reaction:
CaCO3(s) <--> Ca(2+) + CO3(2-)
The above reaction would say that water that is saturated with calcium carbonate would inhibit dissolving of plaster regardless of the level of calcium.
However, pool plaster itself is not only composed of a smaller amount of calcium carbonate, but is mostly composed of hydrated calcium silicate which is formed during curing as follows:
2 3CaO•SiO2 + 7 H2O ---> 3CaO•2SiO2•4H2O + 3 Ca(OH)2 + 173.6kJ
The Calcium Oxide is tightly bound to the silicate crystal structure, but can be dislodged via the following which is very strongly thermodynamically favored (were it not for the silicate structure):
CaO + CO2 --> Ca(2+) + CO3(2-) + heat
In the above reacion, saturation of calcium carbonate does not prevent the reaction from occurring though it reduces its likelihood. Reducing the carbon dioxide content further reduces the likelihood of the reaction and that means lowering the TA. If the TA is lowered, then either the pH or the Calcium Hardness (CH) must be increased to keep the calcium carbonate at saturation. Since one generally wants to keep the pH near 7.5 to be best for the eyes, this implies lower TA with higher CH could reduce the rate of dissolving of plaster. There is anecdotal evidence in extreme cases where calcium carbonate saturation in water very low in calcium results in pitting of plaster.
The result of having calcium carbonate dissolve or of having calcium oxide dissolve may be seen by giving an example starting with water at pH 7.5, TA 100, CYA 30, CH 300, TDS 525, Temp 80:
If one pound of CaCO3 as ions in 10,000 gallons is produced from dissolving Calcium Carbonate, this would raise the pH to 7.99 and the CH by 12ppm. If one pound of CaCO3 as ions in 10,000 gallons is produced from dissolving Calcium Oxide, this would raise the pH to 8.50 and the CH by 12 ppm. So the latter process raises the pH more for the same CH rise than the former.
So, in theory, having a lower TA and a higher CH could lead to a slower dissolving of pool plaster and a slower resulting rise in pH and CH over time.
Note that a lower pH instead of a lower TA with a higher CH to compensate (for saturation of calcium carbonate) does not produce the same effect. Instead the amount of carbon dioxide in this scenario is actually higher, not lower, so could have a faster dissolving of plaster. Therefore, if you see that a higher CH with lower pH has a lower pH rise, then it may be a different mechanism than described above or could just be a coincidence with something else happening in your situation.
As for the other questions in the original post, a lower pH of 7.4 would have slightly more carbon dioxide outgassing, but if the plaster dissolving were the main factor then that could overwhelm the outgassing effect. A lower TA instead of a lower pH would have the same effect on the dissolving of plaster, but would lower the outgassing as well so might result in an even lower pH rise and acid usage. As for eye comfort, I doubt there will be much of a noticeable difference between 7.4 and 7.5. The saturation index of -0.1 is only slightly negative. At the surface of a heat exchanger, the temperature may be 30F higher which would be around 0.2 higher in saturation index. As for the SWG cell, it has a very high pH at the hydrogen gas generation plate so the tradeoff is in having a negative index dissolve plaster faster. But remember that many of the measurements are approximate and have error so this isn't going to be perfect. Targeting -0.2 for the saturation index might be more reasonable, but it's not a big deal.
Richard