Why does stabilized CL test as fc instead of cc?

[Themadczar]

This is just a curiosity question...
If cya locks away some CL by "combining" with it, then why doesn't it test as combined chlorine like CL that is "combined" with anything else? Since it isn't "free" why does it test as free?

[chem geek]

The reason is that there is an equilibrium between the chlorine bound to CYA which is essentially inactive and the unbound chlorine part of which is active (hypochlorous acid is active while hypochlorite ion is less active). The chlorine is going back and forth very quickly but for any single molecule the rate at which it attaches is much faster than the rate at which it detaches. That's why at equilibrium, which by definition means the rates forward and backward are equal, the concentration of chlorine bound to CYA is much higher than that which is unbound since the reaction rate is the product of the rate constant (related to an individual molecule's probability of reacting) times the concentration (relating to the number of molecules in a given volume). The following is the most dominant reaction seen in pool water:

HClCY(-) + H2O <<<---> H2CY(-) + HOCl
"Chlorine bound to CYA" + Water ---> Cyanurate Ion + Hypochlorous Acid

The above reaction is mostly towards the left (i.e. most chlorine is bound to CYA), but it is always going back and forth to maintain equilibrium (balance -- a fixed ratio of reactants to products).

The reaction rate going to the right is such that if all the hypochlorous acid on the right is used up instantly, then half of the chlorine bound to CYA can be released to replenish it in 4 seconds, but in practice there's another chemical pathway that I don't show above that takes only 1/4 second. So when you do the chlorine test, the active chlorine very quickly reacts with the chemical dye and then the above reaction moves to the right with the net effect that every 1/4 second half the bound chlorine is released so in practical terms within one second 94% has been released and within 2 seconds 99.6% has been released. So in the time it takes you to read the chlorine test, you are measuring the sum of the active chlorine that was originally there plus nearly all the chlorine that was bound to CYA. This same effect happens in a pool as chlorine gets used up oxidizing bather waste, killing pathogens, or getting broken down by sunlight; the chlorine bound to CYA is quickly released to replenish the chlorine that gets used up.

Chlorine that you measure that is Combined Chlorine (CC) is attached to molecules much more strongly and therefore gets released much more slowly (think days or months or years, depending on the chemical). It takes a reducing agent such as potassium iodide (R-0003 reagent) to be able to react with that bound chlorine in order to measure it in the test. The chlorine bound to these CC chemicals reacts with iodide to produce iodine, the chlorine becomes chloride and on the CC molecule gets its chlorine replaced with hydrogen. The reaction is like the following where I show the most common scenario where the chlorine is bound to a nitrogen and "R" is some organic chain or a hydrogen:

R2NCl + H+ + 3I- ---> R2NH + I3(-) + Cl-
"Combined Chlorine" + Hydrogen Ion + Iodide Ion ---> "Organic or Ammonia" + Iodine (triiodide ion) + Chloride Ion

The Iodine (triiodide ion) then reacts with the dye in the chlorine tests.

As for why the chlorine is called "free", the earlier term for it is "Free Available Chlorine (FAC)" which is more accurate. In other words, it is not the active level of chlorine but chlorine that is "available" as needed. It is the chlorine reserve or reservoir of chlorine.

Unfortunately, the pool industry has only focused on the active chlorine, hypochlorous acid, vs. hypochlorite ion (less active) as a function of pH and mostly ignores the chlorine/CYA relationship except in general vague terms. By the way, the equilibrium between hypochlorous acid and hypochlorite ion is similar to what we've been describing except that it is much more evenly balanced (close to 50/50 or evenly balanced at pH 7.5):

HOCl <---> H+ + OCl-
Hypochlorous Acid <---> Hydrogen Ion + Hypochlorite Ion

where again you are measuring both the active chlorine, hypochlorous acid, and the less active chlorine, hypochlorite ion, in the test, even when there is no CYA present. So even with no CYA in the water, the FC that gets measured is not all active chlorine and at pH 7.5 roughly half is in the most active form, hypochlorous acid, at least in terms of disinfection. In terms of oxidation of chemicals in general, it's more complicated since some reactions are with hypochlorous acid while others are with hypochlorite ion and this is dependent on the chemical being oxidized. This is one of those "lies" about ORP being a measure of oxidation power -- it's not really true. ORP is measuring a thermodynamic quantity that says what is possible, but doesn't measure reaction rates which says what actually happens and how quickly. Chemistry is not only about what is possible, but how quickly reactions occur. Otherwise our bodies would be oxidized by the oxygen in the air into carbon dioxide gas, nitrogen gas and water. That is in fact technically happening, but very, very slowly. So while thermodynamics are important to know if something CAN happen, reaction kinetics are what determines if something WILL happen. But I digress...

[Themadczar]

So, to summarize what I think I read, as FC reacts with the dye, the cya realeases CL so quickly that by the time we read test results, all FC (even that combined with the stabilizer) is made available and reacts with the dye. Correct (even if over simplified)?

[BigDave]

The CC test measures mostly chloramines (which are the result of chlorine destroying pool contaminants (body oils, lotion, ammonia). Chlorinated isocyanurates (chlorine on stabilizer) are different and show up in the FC and CC tests.

[Themadczar]

Follow up question: why then have I read on these forums that pools with higher cya levels need to keep higher than normal minimum fc levels to remain sanitized? Doesn't the stabilizer continue to release the CL as it's needed until there is no more CL in the pool? If it does so when reacting to dye, why not with contaminants?
While I am not a chemist, I can hold my own in the math/science departments. The details of the chemistry above were beyond me but I still find all of this very fascinating!

[chem geek]

So, to summarize what I think I read, as FC reacts with the dye, the cya realeases CL so quickly that by the time we read test results, all FC (even that combined with the stabilizer) is made available and reacts with the dye. Correct (even if over simplified)?

Yes, you've got it, except I wouldn't call it FC that reacts with the dye since all of it is called FC, but as "active chlorine" reacts with the dye, the CYA releases more of it so quickly...

[chem geek]

Follow up question: why then have I read on these forums that pools with higher cya levels need to keep higher than normal minimum fc levels to remain sanitized? Doesn't the stabilizer continue to release the CL as it's needed until there is no more CL in the pool? If it does so when reacting to dye, why not with contaminants?
While I am not a chemist, I can hold my own in the math/science departments. The details of the chemistry above were beyond me but I still find all of this very fascinating!

Another good question. It's because there is a difference between having a chlorine reserve or capacity (i.e. what is measured as FC) vs. the amount of chlorine that is active. Reaction rates that determine the rate of killing pathogens, preventing algae growth, and oxidizing bather waste are dependent on the concentration of active chlorine. The amount in reserve is not relevant to that reaction rate.

An analogy might be helpful. Think of fighting a war against an enemy and that the front-line soldiers are the ones doing the killing. As those front-line soldiers get killed, sacrificing their lives to kill the enemy, they are replaced by more soldiers from the reserve/reservoir of additional troops. The rate of killing has nothing to do with how many troops you have in reserve nor with how quickly front-line troops are replaced from the reserve. The number in reserve just tell you how long you can keep fighting, but not whether you are killing the enemy faster than they can reproduce.

The reaction of chlorine with dye is very, very fast. The reaction of chlorine to kill pathogens is slower, though still in seconds to a minute or so for bacteria (for a 99.9% kill), but algae are harder to kill due to thicker cell walls and slower permeability to water (and active chlorine). So with algae, you very much notice differences in the active chlorine level whereas with the dye the reaction is so fast that even a low active chlorine level still shows up as color in far less than a second.

Because of chemical equilibrium, the amount of active chlorine is proportional to the FC/CYA ratio (technical derivation in this post). 3 ppm FC with 30 ppm CYA has the same active chlorine level as 6 ppm FC with 60 ppm CYA and is the same as 9 ppm FC with 90 ppm CYA. The FC levels at each CYA level were determined by Ben (PoolDoc) years ago to be what was needed to inhibit algae growth regardless of algae nutrient (phosphate and nitrate) level and under ideal growth conditions of sunlight and temperature. That is, these are roughly worst-case so that nearly all pools can be managed from that chart.