Responding to this post.
Brad,
You can first check the first post in this thread on this forum that I created to explore the relationship between CYA and FC (check out the link near the end of the post where I have analysis about ORP as well). There's a lot of info there, but I also list the original source document that determined the equilibrium constants between the chlorinated cyanurates, cyanuric acid (CYA) and hypochlorous acid (HOCl). These constants are also found in this EPA document. There is also this article written by Kent Williams for the Professional Pool Operators Assocation (PPOA), but several people have said that Kent had an agenda as he was promoting ORP sensors, but my independent spreadsheet calculations (the spreadsheet is also found at the end of my post linked above) show that his data was essentially correct. Finally, this research paper indicates that the chlorinated cyanurates do not have disinfecting capability and in fact actually lower the effectiveness of hypochlorous acid -- in other words, not only does CYA significantly lower hypochlorous acid concentration (i.e. chemical equilibrium), but that small remaining hypochlorous acid is less effective in the presence of chlorinated cyanurates (but this is probably something unique to cysts that get some sort of sensitization from the presence of the chlorinated cyanurates). If you look at the table on page 1160 of the paper (click on the button for that page on the link), you can see reductions of "titrable Cl2" which is FC and that for CYA levels that are about the same (in ppm units -- the table shows Cy as moles/liter x10^5) the HOCl level is cut down by over a factor of 5. The situation is far worse when CYA concentrations are much higher than chlorine concentrations. By the way, I ran some of the numbers in the table in my spreadsheet and there is some error -- for example, the table at pH 7, Cy 7.0, Cl 14 shows HOCl as 1.88 while I calculate 2.18, but that's not a terrible error (though I wonder if the older equilibrium constants are off a small amount or if this paper's methodology is off or different...at any rate, it's not as if there is anything terribly wrong).
I am most proud of the fact that my results from theory match what Ben had discovered through observation in creating his Best Guess CYA Chart which I compare with calculated hypochlorous acid concentrations in a link in my post. If the calculations weren't borne out in the real world, they would be pretty worthless. The correlation with Ben's chart is pretty amazing for the Min/Max levels, above about 30 ppm, and the difference below 30 ppm is mostly due to Ben requiring minimum levels of chlorine so you won't run out, not just for disinfection. The shock levels don't track as well, but recent experience with nasty mustard/yellow algae on this forum has borne out the theory as well (some day, Ben's chart may be adjusted, but we'll need more validation before that -- and besides, it's his chart and his forum).
The bottom line is that the chemistry involved has been known for a long time and though a bit complex, it isn't quantum physics. I was able to do an accurate calculation in a spreadsheet. It is also possible to approximate the effects just looking at dominant species and only 2 of the 9 chemical equations (6 of which are independent). From my contact with various manufacturers, I would say that at least some are aware of these effects (one had done a spreadsheet themselves in order to create some of the data for the EPA link above), but that this information does not get out to the industry as a whole, probably for economic reasons. If people truly understood the relationship of CYA and FC to disinfection/oxidation, they would not use Trichlor as their sole source of chlorine (unless they backwash or drain/refill frequently) nor Dichlor as a frequent source in a spa. If people used chlorinating liquid or bleach (or an SWG) and did so properly so as not to get algae, then who would buy the expensive premium products (Trichlor, Dichlor, algaecide, clarifiers, flocculants, degreasers, stain removers)?
As for your 42 ppm FC example that was without CYA, that's a whole heck of a lot of chlorine so there is no wonder that corrosion occurred. By the way, for your indoor pool installations with no CYA, what is the typical FC level? Also, how do you shock the pool to rid combined chlorine (CC) and do you find that CC buildup to be a problem? Or do you use a weekly maintenance dose of non-chlorine shock (potassium monopersulfate, KMPS)? Since the only reports of people getting sick from pools is from indoor pools (presumably without CYA) with asthma and other repiratory ills from competitive swimmers and small children, I propose actually using some CYA (10-20 ppm) in indoor pools to significantly cut down HOCl levels since that would cut down the rate of production of disinfection by-products (DPBs). Also, using KMPS would eliminate such DPBs by avoiding chlorinated organics altogether. Radical ideas, but I don't think anyone has tried them to prevent the reported problems. (I know I'm getting off the topic of corrosion and if we get into this further, we should start a separate thread or go offline).
Richard
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