Thank you , again, Richard for your comprehensive post.
Here are a few theoretical and practical questions.
A. Theoretical
1. The CYA/HOCL relationship
Let's see if I understood the crux of the matter.
There's a chemical equilibrium between CYA, HOCL ("active chlorine") and HOCL bound to CYA. The latter
is not a "potent" chlorinating agent, however, it protects the bound HOCL from UV degradation. By the simple law of the chemical equilibrium constant, the concentration of HOCL is inversely proportional to the concentration of CYA.
Did I get it right?
This is pretty close. There is a chemical equilibrium between a collection of chlorinated cyanurate species (1 to 3 chlorine bound to CYA) and CYA or other chlorinated cyanurates with one less chlorine and HOCl. For simplicity, you can just think of it as CYA-Cl (and water) in equilibrium with CYA and HOCl. The simplified chemical equilibrium constant is then K = [HOCl][CYA]/[CYA-Cl] so it's not as simple as an inverse relationship on first appearances. Under normal conditions when the CYA >> FC by a factor of 5 or more, most of the chlorine is bound to CYA so for practical purposes [CYA-Cl] is roughly proportional to FC. So the relationship is K = [HOCl][CYA]/"FC" or solving for [HOCl] = constant x FC / CYA so you can see that the hypochlorous acid concentration is roughly proportional to the ratio of FC to CYA. So for a constant FC you are correct that HOCl is inversely proportional to CYA.
As for the protection of chlorine from sunlight, this comes from two factors. The first is what you mentioned about CYA-Cl being more resistant to breakdown from sunlight. The second factor is that CYA itself absorbs UV light thereby shielding lower depths of water and remember that there is a lot of CYA in the water -- much more than CYA-Cl -- so this second factor is important. Nevertheless, in practical terms, the protection of chlorine from sunlight increases with increasing CYA, but it's not linear -- possibly due to this CYA absorption effect combined with imperfect circulation of chlorine especially near the surface.
2. You mentioned that HOCL acts on organic material by combining with ammonia. While this is easy for simple stoichiometric calculations, wouldn't it be fair to say that the active chlorine actually acts with almost "anything that walks", that is, a lot of organic compounds, especially those that contain amine groups? There’s no free ammonia on fabric stains! How would that affect all the calculations?
Chlorine doesn't react with [EDIT] everything. [END-EDIT] In fact, it mostly reacts with compounds that contain nitrogen and to a lesser extent it reacts with hydrocarbons that have double bonds, but it does not generally react with saturated (single-bond) hydrocarbons. In practice, sweat and urine are composed mostly (after water) of urea and then ammonia and then a much smaller amount of amino acids -- all of these contain nitrogen to which chlorine reacts at various rates. Urea is effectively like having two ammonia attached to a common carbon, but the actual reactions of chlorine with urea are not yet understood (this is currently being investigated via a grant from the NSPF as described here). The reaction of chlorine with ammonia, on the other hand, is fairly well understood. Though it is true that general Combined Chlorine may not have a 1:2 relationship to complete oxidation, it isn't going to be far from that. The main point was that it wasn't 10:1 or anything like that because the units are completely different measuring ammonia (as ppm nitrogen) vs. Combined Chlorine (as ppm chlorine gas).
B. Practical:
1. I just got my Tailor kit, and read their "Water Chem" brochure.
On one hand they advocate the 10X CC rule for shock, and on the other hand they admit that high CYA level has a negative effect on the affectivity of FC. Nevertheless, they have do not address this issue quantitatively. This is amazing. What's wrong with them? They seem to know their chemistry right.
There are a lot of people, including chemists, who just take rules on faith and don't question them. They also don't have time to find the original research and do the calculations which, though only requiring 1st year chemistry, are quite tedious (see my spreadsheet, for example). Of course, there are canned programs on the market where you can just plug in the equilibrium constants and ionic strength (salt levels) and out comes an answer, so there really is no excuse. Nevertheless, when there is CYA in the water, the 10x rule isn't bad as it does take a while to get rid of the non-ammonia-based (i.e. not monochloramine) CC so having a higher chlorine level can make that go faster, but sometimes the CC is persistent and no amount of chlorine will get rid of it. We've done bucket tests with some persistent CC from indoor pools and exposed it to very high chlorine levels and very high UV levels (from tanning beds) and long exposure to sunlight and nothing got rid of it. We don't even know if it's real or something that interferes with the test (though we eliminated obvious things like use of MPS).
2. I followed your post regarding shocking of algae-free pool and I'm still not sure if I understood you correctly.
I understood that if the minimum level of FC is maintained, and even if the CC momentarily rises above 0.5 you still don't need to shock. Then when do you?
If CC >0.5 for more than three days?
Suppose, based on a persistent CC > 0.5, you need to shock, why would the FC ppm would be that of an algae shock? In your table, with my 60 ppm CYA, I need 23 ppm FC to shock. According to Ben's table, it's 20 ppm.
Should this (20 - 23) be my shock level for CC >0.5 and no algae problem?
I don't believe there is any need to shock the pool if the pool is clear (not even the dull or cloudy beginnings of algae) and the CC is low (<= 0.5 ppm) and FC holds overnight (<= 1 ppm drop). Even if the CC is higher, if this is soon after a high bather load or organic matter dumped into the pool I'd still wait a day before worrying about it as normal chlorine levels (and sunlight) may take care of it.
"Traditional" pool procedures recommend shocking every week. From what I understood so far, as far as you are concerned, the only thing that matters is if CC >0.5.
So suppose, my pool goes on CC <0.5 for a long time, and there's no other visible organic contaminant as pollen or suntan lotion film, I don't need to shock?
I question everything so don't follow traditional procedures just because they are traditional. Many do make sense, but some don't. The only reason I can think of for periodically raising the FC level higher would be if there is a risk of some pathogens becoming resistant to chlorine as the higher level would wipe out such colonies before they get a foothold. However, most bacteria are very easy to kill and algae doesn't reproduce as quickly so I think the risk is low. In practice, from what I've seen on pool forums I don't think there is any problem with not shocking regularly and instead just maintaining consistent and appropriate FC levels relative to CYA.
Apropos pollen: It seems reasonable to shock the pool if there’s pollen on the surface, doesn't it?
A skimmer sock would probably take care of the problem more quickly. Shock levels of chlorine will break down the pollen, but generally it's a bit slow as pollen is 1) larger than bacteria and possibly some algae and 2) often have a hearty shell that is slow to oxidize. You generally notice a chlorine demand from a lot of pollen, but physical removal is usually preferred when possible.
