Algae, TA or what?

[chem geek]

[ A couple of questions:
1. Have you done the calculations for peroxide / chlorine interactions (dechlorination) and the interaction between monopersulfate and chlorine?
2. It may be there, but do you have the calculation for the carbonate fraction of the TA, given a specific value for TA, CYA, & borates? Do you have an 'adjusted' SI, based on the presumptive carbonate ppm?
3. Do you have a version with references to equation sources, etc.? ]
:
I want to understand them better, and consider what the implications are on CONCRETE pools (and what Richard has to say) before jumping on them for concrete pools.
:
Years ago, I found myself puzzled by the fact that, although most of the chemistry Richard is working with is not new (open to correction here) no one seemed to have put it "all together". Richard appears to have gone a very long way toward that.

1. I don't have hydrogen peroxide in the spreadsheet, but I've done the calculations for using it as a dechlorinator since people have asked about that. Roughly speaking, it takes the same volume of 3% hydrogen peroxide to neutralize a volume of 6% bleach (detailed calculations are in this post). As for monopersulfate, I do have the equivalent of that in the spreadsheet at line 359 "[HSO5-] assumed to be equal to [HSO4-]" which is a field that gets calculated when you enter in a quantity into the "NON-CHLORINE SHOCK" section. You can then adjust FC so that line 293 "Free Chlorine" matches in amount to 359 and that will give you the chlorine equivalent for the MPS. A pain, yes, but this question doesn't come up very much. Roughly speaking 3-1/2 teaspoons of Dichlor is equivalent to 5 fluid ounces of 6% bleach is equivalent to 7 teaspoons of non-chlorine shock (43% MPS) and is 7 ppm FC in 350 gallons and is the amount of oxidizer needed for every person-hour of soaking in a hot (104ºF) tub.

2. Yes, the Carbonate Alkalinity is in the first section on line 16. The S.I. calculations all use this adjusted TA (i.e. the carbonate alkalinity). I do not have any raw unadjusted S.I. using TA alone (that is, ignoring CYA and Borates). Note that my S.I. calculation comes from direct calculation of calcium and carbonate ion concentrations relative to the solubility product of calcium carbonate. I also derive the LSI formula in lines 436 through 481 which shows that it comes directly from the equilibrium of calcium and carbonate ions with calcium carbonate and has nothing to do with boiler systems, open vs. closed, etc.

3. I don't have a references chart, but I list a few in the spreadsheet. The constants for the chlorinated isocyanurates and for CYA come from the 1974 O'Brien paper. The most flaky, yet critically important, constant in the spreadsheet is for the solubility product of calcium carbonate where I ended up using consistent data from the CRC Handbook that resulted in closely matching the Taylor watergram. I also have constants from Wojtowicz that are used when you set 223, 224 and 225 (last three lines in that block) to TRUE in columns B and C. The ANSI/APSP-11 settled on S.I. tables that are a little closer to Wojtowicz are are generally 0.1 higher than my calculation (Wojtowicz is generally 0.15 higher than my calculation). The ion pair dissociation constants come from formation or stability constants mostly in a spreadsheet source I saved but would have to look up where that came from -- they aren't critical and only affect the saturation index by about 0.02 (unless sulfates are very high).

Using a lower TA in a concrete ("pool plaster") pool is fine so long as you compensate other parameters such as pH and/or Calcium Hardness (CH) to get the saturation index to near zero. This will saturate the water with calcium carbonate to protect such surfaces. Basically, the lower TA lowers carbonate ion but a higher pH does the opposite while CH increases calcium. Theoretically, a low TA and high CH combination is better in terms of calcium oxide since calcium carbonate saturation implies a constant right-hand side product while lower TA means a lower carbon dioxide amount on the left-hand side:

CaO + CO2 <---> Ca(2+) + CO3(2-) + heat

The calcium oxide is bound tightly in pool plaster mostly as 3CaO•2SiO2•4H2O while the above reaction is very much thermodynamically favored to the right (the reverse reaction starting from calcium carbonate is how cement is made in a kiln along with silicates).

The chemistry I've been working on is not new. I've just been putting together existing pieces The latest breakpoint chlorination model (chlorine oxidation of ammonia) is from 1992 and 2000. I also follow current disinfection by-product research, mostly by Ernest "Chip" Blatchley, from 2007, 2009 to present-day.

Richard