Chlorine/CYA and Combined Chlorine

[chem geek]

This is a continuation of a discussion from this post.

First, for information about the chlorine/CYA relationship, you can look at this thread. The spreadsheet linked to at the end of the first post uses the equilibrium constants for the chlorinated cyanurates definitively determined in 1973. CYA just affects the rate of chlorine reactions, but because chlorine (hypochlorous acid) is released from CYA, it does not affect stochiometry -- that is, the amount of chlorine available to react (even if it's concentration is lower so that it reacts more slowly). FC measures the amount of active plus inactive chlorine; that is, it also measures the amount of chlorine bound to CYA so in effect measures chlorine capacity. CYA is essentially a chlorine (hypochlorous acid) buffer. The FC/CYA ratio is a rough proxy for the hypochlorous acid concentration so determines the rate of disinfection and oxidation. When this rate exceeds the generation (doubling) rate of bacteria and algae, then these are killed faster than they can reproduce.

As for shock level, this is somewhat arbitrary as a higher chlorine level will kill algae faster. The Minimum column in Ben's chart corresponds to killing algae faster than it can reproduce for nearly all pools and seems to work up to phosphate levels of around 3000-4000 ppb above which the algae may reproduce faster and need higher chlorine levels (and may not be worth it), but these are extremely high phosphate levels not normally found in most pools. The shock level is roughly 10 times higher in hypochlorous acid concentration in the table I use while Ben's numbers (in his table here) tend to be higher at lower CYA and lower at higher CYA. In theory, Ben's shock levels will take longer to clear a pool of algae at high CYA than at low CYA whereas in my table calculated from the chlorine/CYA relationship shown here has a more consistent active chlorine level. One uses a higher shock level not only to kill and oxidize algae faster, but also to ensure that the chlorine level inside a bloom is high enough to prevent further growth (circulation can be poor and kill rates slower where algae is clumped or forming biofilms which is why brushing helps).

Second, and not related to the above (i.e. CYA isn't involved except in affecting the rate of reaction, but not the amount of chlorine needed), the breakpoint of ammonia proceeds through a series of reactions, but the net is the following:

3HOCl + 2NH3 --> 2N2(g) + 3H+ + 3Cl- + 3H2O

So the ratio of chlorine to ammonia is 3:2 or 1.5. However, this is a molar (or molecular) ratio whereas chlorine is measured in parts-per-million (ppm) units using a chlorine gas (Cl2) equivalent weight (ppm is a weight proportion, not a mole-to-volume proportion) and ammonia is measured in ppm Nitrogen (atomic, not molecular nitrogen gas). So the ppm ratio of chlorine to ammonia is 1.5 * (35.453*2 / 14.007) = 7.593 (usually quoted as 7.6). In practice, for the reaction to fully complete it takes a little more chlorine due to intermediate steps so usually a full breakpoint needs 8-10 times as much chlorine as ammonia (both in their respective ppm units).

Combined Chlorine (CC) is typically monochloramine (NH2Cl) and this is measured in ppm units similar to chlorine as ppm chlorine gas equivalent, NOT ppm nitrogen as with ammonia. The reaction to break monochloramine is a subset of the one shown above and is as follows:

HOCl + NH2Cl --> NHCl2 + H2O
NHCl2 + NH2Cl --> N2(g) + 3H+ + 3Cl-
----------------------------------------------
HOCl + 2NH2Cl --> N2(g) + 3H+ + 3Cl- + H2O

So the ratio of chlorine to monochloramine is 1:2 or 0.5. Since the ppm are in the same units, it takes half the amount of FC to break CC, not 7.6 times as is the case with ammonia (measured as ppm nitrogen).

Breaking monochloramine is easy. When one has persistent CC, it is usually something else such as a chlorinated organic compound. These may not get more fully oxidized easily, but usually they aren't in large quantities and they might get broken down in sunlight which would explain why they aren't usually seen in outdoor pools (the other reason might be that they form more when the chlorine level is much higher as is the case in most indoor pools that are essentially over-chlorinated due to not having any CYA to slow down all chlorine reactions).

Richard