Pool School Question

[wmarq]

Just throwing a question out there for anyone who cares to respond:

From Pool School: "As the HOCl and OCl- get used up, more chlorine is released from the CYA to maintain an equilibrium balance."

Question is how does this actually happen? how does the cya know that the active hoci just got used and its time to let go of its reserve to be used on the front line now ? For some reason when CYA gets to high it doesn't let go anymore? or am I thinking about that totally wrong ?

 

[JoyfulNoise]

You need to know equilibrium chemistry principals. Basically all of these chemical species - HOCl, OCl-, cyanuric acid/cyanurate, chlorinated cyanurate, etc, - are in equilibrium that is dependent on their individual reaction equations, rates (equilibrium constants) and pH. There’s also something called Le Chatelier’s Principle. Essentially, when you change the concentration of HOCl by it getting used up to sanitize or oxidize something, all the other chemicals in the equilibrium will change their concentrations to try to reform the HOCl that was lost and find a new equilibrium. The entire chemical ecosystem is attempting to maintain its equilibrium. So it’s not that the CYA “knows something”, it’s simply reacting to the change in the chemical environment it’s in. Reaction rates in this case are very fast so equilibrium is established very quickly.

 

[wmarq]

fantastic answer. thx for satisfying my curious mind !

 

[AUSpool]

A really good question, a question I’ve often asked myself. The process is quite complicated so we tend to look at an overview of the system in general without getting down to the nuts and bolts of it which would be way too much detail for most of us.

One thing I’d like to add is ‘how much’ is released. As FC is used the CyA doesn’t release the entire reserve, it only releases enough to stay in equilibrium.

As a small amount of FC is used a small amount of FC is released from the reserve FC bound the CyA to maintain equilibrium. But the small amount released is a little less than the small amount used because using that small amount of FC has changed the systems equilibrium in favor of the CyA. And without adding more FC to the system, when more FC is used a little less is released because the concentration of CyA is increasing in relation to the FC.

This is why we need to maintain the FC level as stable as possible according to our ideal FC/CyA ratio and why things go pear shaped when the FC gets too low or the CyA gets too high. And why FC and CyA should be treated as a ratio of FC/CyA as apposed to independent values.

And when we say “bound” it’s not a force of attraction but an actual reaction, or more specifically, an equilibrium reaction. There are several possible reactions but within the normal pool pH range it could be simplified as;

Active free chlorine + cyanurate ion <—> chlorinated cyanurate.


This is probably a step too far for this discussion but the reaction that I believe the most likely is;

HOCl + H2Cy- <—> HClCy- + H2O

Edit: Those reactions above are not the only possibility’s, the system is complicated with many reactions all happening simultaneously and attempting to stay in equilibrium.