Very high CYA levels, HELP!

[chem geek]

If you need technical references, there is the original O'Brien paper from 1974:

Equilibria in Aqueous Solutions of Chlorinated Isocyanurate

and there is my analysis of the equilibrium constants from that paper to derive why the FC/CYA ratio is a decent proportional proxy for the active chlorine (hypochlorous acid) level:

http://www.troublefreepool.com/pool-...28.html#p50548

The bottom line is basic equilibrium chemistry where the primary chemical components at typical pool pH are the following (the ppm assume the following is the only reaction, but there are actually others that are less dominant):

HClCY- + H2O <<<---> H2CY- + HOCl
"Chlorine Bound to CYA (Inactive)" + Water <<<---> CYA + "Unbound Chlorine (Active)"
........ abt. 9 ppm Cl2 .................................... 150 ppm CYA .... abt. 0.024 ppm Cl2

Most of the chlorine is bound to CYA and the total CYA level is typically much higher than that so if the CYA level rises then you need to proportionately raise the Free Chlorine (FC) level to maintain the same amount of active chlorine. The FC measured in tests measures both the unbound and bound chlorine since the reaction above moves to the right quickly as active chlorine gets used up in the test (technically, half gets released every 0.25 seconds -- see tihs paper and this paper). Equilibrium chemistry says that the following is a constant (at a given temperature):

[CYA]*[Unbound Chlorine (Active)]/[Chlorine Bound to CYA (Inactive)]

So as CYA rises, you need to increase "Chlorine Bound to CYA (Inactive)" by the same proportionate amount to keep the active chlorine level the same. Since most of the chlorine is bound to CYA and is measured as FC, it's easiest to just say to raise the FC proportionately when the CYA rises -- which is the same thing as saying to keep the FC/CYA ratio constant.

I know, I know, TMI.