Clarification of "tied up" vs "Free"?

[chem geek]

Hi, Richard,

from the post above:I'm a little confused by that. It seems to me that breakpoint (shock) DOES remove these on a practical basis and results in CC's testing at zero or close to that usually within 12 hours of shocking...day or night. Am I misinterpreting what you have written?

No, you interpreted what I said correctly. Achieving breakpoint through super-chlorination (shock) works great in theory and controlled experimental conditions and may work reasonably well when you mostly just have monochloramines (not trichloramines or chlorinated organics) and have good air circulation above your pool, but experience has shown (Ben can tell you more and you can also hear this from several other sources including PPOA articles such as Part1 and Part2) that it is often hard to achieve.

In an indoor pool, it is very hard to get good air circulation while in an outdoor pool you are usually using CYA so that getting a high enough concentration of chlorine to achieve breakpoint is difficult. I do not know enough about the detailed chemistry to know if the 10:1 ratios normally quoted are only referring to the stochiometry (in which case CYA wouldn't matter) or if they are at least partly referring to reaction kinetics and the lifetime of complex intermediate chemical species (in which case CYA can make a difference by causing the actual concentration of HOCl to be quite low). [EDIT]From the PPOA article and from other articles I've seen written by water treatment districts, there are less disinfection by-products (DPBs), such as chlorinated organics, formed when the chlorination process is done at a higher pH. But again, I haven't seen the detailed chemistry for these reactions so can only go by such qualititive information. The use of a maintenance level of non-chlorine shock (such as potassium monopersulfate - MPS - trade-name Oxone from DuPoint and sold as many different brands) is supposed to oxidize organics (and ammonia) before chlorine gets a chance to do so and therefore may avoid the creation of DPBs. So it may make sense to use a maintenance level of MPS for indoor pools, though it's not as clear if this is needed in outdoor pools where sunlight may break down DPBs (I don't know if this is true).[END-EDIT]

Richard