Chlorine upper limit for a given level of CYA?

[Water_man]

Based on the CYA-Cl chemistry, whose fine details I've forgotten, is there an upper limit of Cl for a given level of CYA, above which the Cl will stop being effective? I think that there should be an upper limit because after all, the "stabilization" of CYA is a result of the actual bond between the two.
If all the CYA is already bonded to Cl, any extra Cl will be free and unprotected from the sun light.
If indeed this is true, are there any numbers?

 

[JasonLion]

Your wording seems a little garbled. As the FC level goes up, for a constant CYA level, the FC level eventually gets high enough that CYA stops holding the excess FC in reserve and stops protecting it from sunlight. However, this happens at FC levels way way above anything you should ever be using in your pool.

 

[chem geek]

It's an equilibrium so it's a proportion. At a constant FC level, more CYA will have more FC bind to it but never all of it. Twice as much CYA will have a little more FC bound to it leaving about half as much active chlorine. Every doubling of CYA cuts the active chlorine in half. It never gets to zero. Of course, once the active chlorine level (roughly estimated by the FC/CYA ratio) gets below certain amounts algae can grow faster than chlorine can kill it. This is the basis for the Chlorine / CYA Chart. At some point with really high CYA levels, bacteria can grow faster than chlorine can kill them but most bacteria are fairly easy to kill so it's more about the rate of kill to prevent person-to-person transmission of disease, especially in commercial/public pools.

It sounds like you are thinking that more CYA linearly attaches to more chlorine, but that's not how chemical equilibrium works. It's ratios and proportion. Since most of the chlorine is bound to CYA, that amount doesn't change very much in absolute terms, but the amount that is left as active chlorine changes proportionately. As described in more technical detail in this post, the relationship is essentially the following:

[CYA] * [HOCl] / [Cl-CYA] = constant

so solving for [HOCl] which is the active chlorine concentration we have

[HOCl] = constant * [Cl-CYA] / [CYA]

Since most of the chlorine is bound to CYA, [Cl-CYA] is approximately FC so

[HOCl] = constant * FC / [CYA]

where you can clearly see that if the CYA concentration is doubled (for the same FC level) the active chlorine (HOCl) concentration is cut in half. There is no "limit" in terms of some kind of saturation point.

 

[ROB N]

It's an equilibrium so it's a proportion. At a constant FC level, more CYA will have more FC bind to it but never all of it. Twice as much CYA will have a little more FC bound to it leaving about half as much active chlorine. Every doubling of CYA cuts the active chlorine in half. It never gets to zero. Of course, once the FC/CYA ratio gets below certain amounts algae can grow faster than chlorine can kill it. This is the basis for the Chlorine / CYA Chart.

It sounds like you are thinking that more CYA linearly attaches to more chlorine, but that's not how chemical equilibrium works. It's ratios and proportion. Since most of the chlorine is bound to CYA, that amount doesn't change very much in absolute terms, but the amount that is left as active chlorine changes proportionately. As described in more technical detail in this post, the relationship is essentially the following:

[CYA] * [HOCl] / [Cl-CYA] = constant

so solving for [HOCl] which is the active chlorine concentration we have

[HOCl] = constant * [Cl-CYA] / [CYA]

Since most of the chlorine is bound to CYA, [Cl-CYA] is approximately FC so

[HOCl] = constant * FC / [CYA]

where you can clearly see that if the CYA concentration is doubled (for the same FC level) the active chlorine (HOCl) concentration is cut in half. There is no "limit" in terms of some kind of saturation point.

Yeah, what he said!!! :-D

 

[chem geek]

There's another point to keep in mind and that is that the chlorine bound to CYA is better protected from breakdown from sunlight, but is not completely protected. The rate of chlorine breakdown from sunlight is too high to be explained only by the amount of hypochlorous acid and hypochlorite ion unbound to CYA. The chlorine bound to CYA must also break down from sunlight though not as quickly.

At high CYA levels there is likely a UV shielding effect from CYA itself and this explains why even at higher FC and CYA levels with the same proportion the rate of chlorine loss actually drops somewhat, especially as one gets closer to 80 ppm or higher.

 

[JasonLion]

chem geek, but what happens when you go the other direction. If you hold CYA constant and keep doubling the FC level. Say FC is 1000 and CYA is 50.

 

[Water_man]

Hi chem geek, thank you for your reply.

1. I know this is an equilibrium. Maybe you know its pK. Also, unless CYA is a relatively strong acid you may have omitted its own association – dissociation equil.

2. I didn’t expect a linear relationship.

3. My question was : per a GIVEN level of CYA in the water, is there a chlorine level limit, above which the FC above that limit won’t be effective?

IMHO you yourself indicated that such a limit indeed exists.

In your own words: “most of the chlorine is bound to CYA” which is a fair statement. I noticed that JasonLion agrees with this.

Now suppose for a given amount of CYA we added enough chlorine so that all the molecules of CYA are bonded to HOCL. I assume that for a strict quantitative analysis we need the pKs.

If from this point, we add chlorine, one can expect that this added chlorine will not be bonded to
CYA and thus it will be more exposed to sunlight destruction. Of course it’s not exactly the same added HOCl molecules because we have a dynamic equilibrium.
If this is indeed the case, one can expect that the limit (in ppm) of effective HOCL level will be the fixed amount of CYA multiplied by the ratio of the molecular weights of HOCL
and CYA. All this is valid if the pKs are high enough.
Pease correct me if I’m wrong.
Regards,
Mike

 

[chem geek]

OK, now I understand what both of you are asking. I'm not sure why you're asking since such high FC levels relative to CYA are not normal since they are above even our recommended shock levels, but at any rate yes once one gets high in FC relative to CYA the simple ratio formula doesn't work anymore. The following table shows the amount of hypochlorous acid (HOCl) at 50 ppm CYA at various FC levels all at a pH of 7.5:

CYA = 50 ppm; pH = 7.5
FC .... HOCl
1 ...... 0.008
2 ...... 0.016
5 ...... 0.043 .... up to here, HOCl is roughly 0.4*FC/CYA
10 .... 0.102
20 .... 0.305 .... regular shock level
30 .... 0.741 .... yellow/mustard algae shock level
40 .... 1.649
50 .... 3.377
60 .... 6.169
80 .. 14.014 .... from here up, HOCl is roughly 0.47*(CYA-FC)
100 . 23.301
200 . 70.716
500 . 215.072
1000 . 453.267

So you can see that once one gets to an FC that is higher than the CYA level, then the CYA is close to saturated with chlorine attached to it and any additional FC results in unbound chlorine. Such unbound chlorine breaks down faster in sunlight, but also note that the chlorine bound to CYA also breaks down though more slowly but also probably absorbs some UV in sunlight (without breaking down, just as CYA does) so may protect some lower depths.

Again, I'm not sure why this question is being asked since we don't operate our pools with an FC anywhere near the CYA level itself or higher than the CYA level.

 

[Water_man]

Again, I'm not sure why this question is being asked since we don't operate our pools with an FC anywhere near the CYA level itself or higher than the CYA level.

The question relates to clearing an algae infested pool.
It's simple. Suppose higher Cl level doesn't damage the equipment.
Is there any reason why we don't operate at higher CL level than shock?
Why not blast the bejesus out of this algae with high concentration of Cl? I mean what's stronger than lethal force? A: A more lethal force.

 

[carlscan26]

I'm curious about this too. I was shocking my pool based on a CYA of 60 when actually it's closer to 30 or lower. I was having a terrible time keeping FC above 20. But when I let it drop to 15 for shock level for cya of 30 my CL usage slowed way down.

 

[chem geek]

Unless the CYA level is low, it would take huge amounts of chlorine to get significantly above shock level which is normally 40% of the CYA level. Nevertheless, yellow/mustard shock level is 60% of the CYA level so we do get pretty high sometimes. Yes, you could go even higher, but the algae gets killed pretty quickly, well within a day at these levels. That's why the pool usually turns from green to gray/cloudy. After that, the clearing is more about getting the algae to the filter to be physically removed (backwashed/cleaned). For a small amount of algae or cloudiness, the chlorine can oxidize it, but when there is a lot then physical removal is generally faster.

High chlorine levels can be damaging so it's not a good idea to go much above 5 ppm FC with no CYA equivalent for very long so roughly an FC that is 100% of the CYA level, but you can see how rapidly the active chlorine level climbs in that region so with test errors you really can't hone in very accurately and could easily overshoot by a lot. Also, adding a large amount of chlorine has the pH rise if you are using a hypochlorite source of chlorine. At a TA of 80 ppm with CYA of 30 ppm, starting with a pH of 7.5 10 ppm FC raises the pH to 8.1, 20 ppm FC raises it to 8.5, 30 ppm FC raises it to 8.7. If you added 50 ppm FC when the CYA was 50 ppm, the pH would go to 9.0. If one has 50 ppm Borates in the water, then this rise in pH is far less so otherwise you'd need to lower the pH first to not get so high, but then when the chlorine got used up the pH would drop down a lot.

Things just become very difficult to manage if you start adding extraordinary amounts of chlorine to the water. If money is not an object (or if you are a pool service in a hurry), then you can use a different sort of oxidizer approach such as ProTeam® System Support which is sodium percarbonate so results in a combination of sodium carbonate and hydrogen peroxide. It's bubbling effect from the oxygen produced when hydrogen peroxide reacts raises a lot of debris to the surface where it can be more easily skimmed and the hydrogen peroxide oxidizes a lot of smaller material. So for really seriously fouled pools, it can be reasonable to use if one can't easily physically remove what is at the bottom or middle of the pool. After its use, then one shocks with chlorine to finish off the process. This approach isn't for regular algae alone, however, but more for leaves and other organic debris.

Again, I'm still not sure why you are asking this -- do you have a pool that is severely fouled or has a lot of algae?

 

[chem geek]

I'm curious about this too. I was shocking my pool based on a CYA of 60 when actually it's closer to 30 or lower. I was having a terrible time keeping FC above 20. But when I let it drop to 15 for shock level for cya of 30 my CL usage slowed way down.

If the rapid chlorine drop was during the day but not at night, then yes that is likely explained by having much more chlorine being unbound to CYA. If the drop was rapid at night as well, then that's just due to having a higher active chlorine level which will react more quickly if there are things to oxidize. So this is another reason why one wouldn't want to use an FC much above our recommended shock level -- it will tend to get used up too quickly, especially if that's during the day (i.e. from sunlight). If it were just lost from reactions to clear the pool more quickly, then that wouldn't be bad (i.e. it would just be working faster).

 

[carlscan26]

I'm curious about this too. I was shocking my pool based on a CYA of 60 when actually it's closer to 30 or lower. I was having a terrible time keeping FC above 20. But when I let it drop to 15 for shock level for cya of 30 my CL usage slowed way down.

If the rapid chlorine drop was during the day but not at night, then yes that is likely explained by having much more chlorine being unbound to CYA. If the drop was rapid at night as well, then that's just due to having a higher active chlorine level which will react more quickly if there are things to oxidize. So this is another reason why one wouldn't want to use an FC much above our recommended shock level -- it will tend to get used up too quickly, especially if that's during the day (i.e. from sunlight). If it were just lost from reactions to clear the pool more quickly, then that wouldn't be bad (i.e. it would just be working faster).

Thanks Chem Geek! Yes it was during the day in my case. This makes me feel a lot better about what I've been doing. Other than the wasted CL that is.

 

[Water_man]

Again, I'm still not sure why you are asking this -- do you have a pool that is severely fouled or has a lot of algae?

Yup. As I mentioned, the question was related to my algae infested pool or better call it a green swamp.
You were right about saying that the algae dies fast. It did, but I didn't know. I didn't see totally white water. Now I've realized that most of the process is filtering, filtering and more filtering because I finally can see the bottom at the shallow end. And bottom cleaning. As long as the water is kept on shock level of Cl, I don't see algae growing back.

So the conclusion is - if you can get your enemy with a gun shot, who needs a canon? Sorry for the military analogy.