Chlorine/CYA and Combined Chlorine

[chem geek]

(I was out of town overseas and unable to access this site which is why I took so long to respond.)

The FC numbers in the shock table are not at a level so much for getting rid of CC as for killing algae quite a bit faster than it can grow after an algae bloom has formed. Ben's original numbers were based on a combination of experience and theory. My numbers were based on Ben's as a starting point, but made more consistent based on theory. It's roughly an FC that is 40% of the CYA level and that results roughly in 0.3 ppm hypochlorous acid concentration (where "ppm" is in standard chlorine Cl2 units). So I wouldn't think about the shock column numbers in terms of CC or the 10x traditional industry rule. The shock table is essentially "rate-based", not based on stoichiometry (reaction completion).

As for CC and the 10x, the stoichiometry which says how much chemical is needed for the reaction to complete is an FC that is 0.5x the CC level. The 10x clearly came from the breakpoint chlorination of ammonia (which is measured in different units of ppm Nitrogen), not of monochloramine (which is measured in ppm Cl2). My comment with regard to 10x not being bad is not based on stoichiometry, which is still 0.5x (so perhaps with some amount of excess to ensure completion it's still less than 1x), but rather based on reaction rates since CYA reduces the effective "instantaneous" chlorine concentration while holding a lot more chlorine in reserve that is released reasonably quickly. Stoichiometry is based on total amount of chlorine available including that in reserve while reaction rates are based on the "instantaneous" concentrations of relevant chemicals (in this case, hypochlorous acid). These are two separate concepts -- the 10x rule is incorrect for CC for stoichiometry (i.e. reaction completion), but I was just saying that for reasonable reaction rates a 10x rule is OK to use when CYA is present -- not necessary, but it makes the reaction go faster. I was just trying to put some "reason" into an industry rule.

A properly maintained outdoor residential pool usually measures little or no CC so the need for shocking the pool is rare. Higher bather load can temporarily lead to an increase i CC, but this should drop on its own if the normal FC level is maintained and the bather load returns to a normal lower level.

By the way, I've been asked about more detail about the chemistry behind the chlorine/CYA relationship in a couple of posts so I created a new post with that info here.

Regarding the time and effort that many have put into this and other pool forums, many of us are not professionals in this industry. Speaking for myself, I do not work in the pool/spa industry (or anything related to it) and am just a pool owner like most who post here. It is true that we don't get paid for this, but I'd much rather see the industry shift towards full disclosure of basic information (the chemistry of the chlorine/CYA relationship has been known since at least 1974, for example). These forums can only help those who look at them and are a great "workaround" until the integrity of the pool/spa industry improves. It's the CPO and TECH courses that need to get updated (I've written to the NSPF and APSP organizations about that) as well as training materials for pool store employees (except for waterbear/Evan who already knows this stuff because he's the smartest most honest pool store employee you'll likely ever meet!).

Richard