Re: What's an Oxidizer do?
PoolOwnerNumber9 said:
Richard, at your chart here
pool-water-chemistry-t628.html
you show that the half life of free chlorine at 50ppm CYA is nearly the same as it is at 100 ppm. You say “A Little CYA Goes A Long Way†and “The following is a graph showing that a large amount of the benefit of CYA protection of chlorine from UV (sunlight) is already there at around 20 ppmâ€. Your chart shows a logarithmically diminishing protection ending at 100ppm. It appears that your chart shows exponentially diminishing returns after 50ppm.
At the beginning of the section you are referring to, I have the following Note:
NOTE:
The mechanism of protection of chlorine from sunlight by CYA is currently under review in this thread. Higher CYA levels may protect even proportionately higher levels of chlorine more, especially in deeper pools.
The graphs were done BEFORE Mark's experiments and the slew of other data from pools that demonstrated that higher CYA levels in the 60-80 ppm range (especially 70+ ppm) had lower chlorine loss even at the same FC/CYA ratios. The graph was done fitting the PPOA data and other similar data using a half-life of hypochlorous acid and hypochlorite ion (50/50 mix at pH of 7.5) of 35 minutes with a half-life of the chlorinated isocyanurates (or the dominant species HClCY
-) of 8.4 hours. The problem is that this only takes into account ONE of TWO factors where CYA protects chlorine degradation.
The PPOA (and my) graph only accounts for the fact that most of the chlorine is bound to CYA. It does not account for the fact that CYA itself shields lower depths of water from UV through absorption and this is absolutely, positively an effect that needs to be considered.
Also, I have since made a model of chlorine breakdown using accurate absorption spectral measurements (from
here and Florida sunlight data (from
here) and have a spreadsheet that actually predicts a 35 minute half-life for a 50/50 mix, but that the half-life of hypochlorous acid is 2 hours and 10 minutes while that of hypochlorite ion is 20 minutes so there is a large pH dependence that is exacerbated by having CYA in the water. This is because CYA acts as a hypochlorous acid buffer so changes in pH make rather large changes in hypochlorite ion (since hypochlorous acid doesn't change much) as shown in the graphs on the right in
this post.
Data from Wojtowicz shows a roughly 14%/day drop in FC levels due to oxidation of the chlorinated isocyanurates by chlorine (actually, hypochlorite ion) that is temperature dependent (i.e. thermal decomposition). This drop is independent of sunlight and occurs day and night. So here, too, is a pH dependence where there would be faster breakdown at higher pH. Information is inconsistent, however, since Wojtowicz says that the chloroisocyanurates do not significantly absorb UV radiation in sunlight while data from O'Brien shows significant absorption of HClCY
- up to 230 nm which is as high as he measured. The UV from sunlight drops off rapidly below 320 nm and can essentially be ignored at 290 nm and below so this missing data is critical (but I can't find it anywhere). Also, the data from O'Brien shows that CYA itself does not absorb sunlight, but that HCY
-, which is the dominant species in pool water, does. In this case, higher pH provides more protection, but the increase is relatively small since most of the CYA is in the form of HCY
- already.
From the O'Brien data, it looks like the HClCY
- will have a higher molar absorptivity than HCY
- even accounting for the higher CYA concentration compared to FC (in molar terms, it's less -- around a factor of 3 when the FC is 10% of the CYA level). That would say that the CYA protection effect is even greater when the FC level is higher -- that is, raising CYA without raising FC isn't as good for this effect. One key unknown is whether the absorption of UV by HClCY
- results in its breakdown (for HOCl and OCl
- every UV photon absorbed results in its breakdown), but my guess is that it does not as the absorption may be more with the inner ring than at the N-C bond, but this is speculation on my part based on our not seeing high chlorine loss at the higher CYA levels.
So there is inconsistency in various data sources. The PPOA graph which has a fairly rapid breakdown with a half-life of 6-8 hours as a limit at high CYA is inconsistent with the fairly low loss of chlorine people experience in real pools at high CYA levels with no bather load and is also inconsistent with the Wojtowicz data. I would tend to lean towards what the O'Brien measurements of UV absorption imply followed by some of the Wojtowicz info and would regard the PPOA chart as incorrect because it is incomplete. However, even Wojtowicz says nothing about the effect we have seen which may be somewhat dependent on pool depth.
Richard