Clarification of "tied up" vs "Free"?

[chem geek]

chlorioisocyanurate will test as FC from my understaning. They ARE a form of 'combined chlorine' (I have seen them referred to as 'combined chloramides'. They are not what we consider CC normally.

Evan is correct. The chlorine tests consume the free chlorine (HOCl) and this gets filled in immediatly from OCl- and quickly from the chlorinated cyanurates (the chlorine tied up in CYA) though the half-life for the release of chlorine from one of the cyanurates (HClCY-) is 4 seconds and is 1/4 second for another (ClCY(2-)) which means it's not instantaneous. This has implications for the OTO chlorine test since it relies on an immediate color measurement for free chlorine and waiting for a combined chlorine reading so the relatively slow release of chlorine from CYA could make that test even less reliable than it already is. The DPD and FAS-DPD tests do not have this problem since it probably takes about 10 seconds or so to add drops, do mixing, etc. before you start the measuring part of the test.

You can think of the Free Chlorine test as measuring chlorine that is "available" for disinfection even if most of it isn't in disinfecting form but is stored in other forms and will get converted "as needed". So when you measure Free Chlorine, you are mostly measuring the "reserve" of chlorine and not its absolute immediate disinfecting capability. The actual amount of disinfecting chlorine (HOCl) at various levels of Free Chlorine and CYA are shown in the following chart (for a pH of 7.5): HOCl.htm
Ignore the coloring -- these are topics for discussion in The China Shop and are not ready for official use yet.

The Combined Chlorine test will measure all forms of chlorine so you get the Free Chlorine amounts as noted above PLUS the chlorine that is found (has combined with) other chemicals including ammonia (to form chloramines) and organics (to form chlorinated organics). Though the classic approach to removing the chloramines and chlorinated organics is through high "shock" levels of chlorine, in practice it is very hard to get rid of such compounds and it is believed that they (especially monochloramine) are often broken down in outdoor pools through sunlight (UV).

Richard

[duraleigh]

Hi, Richard,

from the post above:

Though the classic approach to removing the chloramines and chlorinated organics is through high "shock" levels of chlorine, in practice it is very hard to get rid of such compounds and it is believed that they (especially monochloramine) are often broken down in outdoor pools through sunlight (UV).

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?

[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