HOCL residual

[xDom]

Hi,
New poster here. There's an aspect of the CYA topic that I can't quite understand.
If there is an amount of CYA in the pool, say 30 ppm, then from what I gather, most of the HOCL in the pool will bond to the CYA.
There will be a small residual of HOCL that remains that stays fairly consistent across the pH range up until about 8.3.. ish (?).
My question is, if there is say, 30 ppm of CYA and only say, 3 ppm of FC then why does this residual exist at all? Why doesn't ALL the HOCL bond with the CYA?

I know this doesn't really have too much to do with the day to day running of my pool but I'm just curious.

Thanks

 

[mgtfp]

Both reactions, the "bonding" of chlorine to CYA and the "debonding" happen constantly, all the time. But they happen at different rates, and these rates are each dependant on how many of the involved species are present. After a "while" (this "while" is pretty short) you reach a state where the amounts of chlorine bound to CYA and of chlorine present as HOCl are just right so that the same number of chlorine atoms bond with CYA as chlorine atoms debond from CYA per second. That's what we call equilibrium, which is not a static equilibrium, but a dynamic one. Both reactions are still happening, but without a view into the micro-cosmos it looks to the outside observer that everything is constant.

These equilibrium reactions are pretty common in chemistry.

 

[mgtfp]

And welcome to TFP. Good to see another Aussie here.

 

[PoolStored]

More than you might want, but maybe this will help...

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

 

[xDom]

More than you might want, but maybe this will help...

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

Thanks, that poster is Richard Falk? I read ( or attempted to read and understand ) his paper on the FC CYA pH relationship. The guys at Orenda Chemicals speak of him often as well. I'm a bit of an Orenda fan.
I came to this forum because I heard Richad was on here as Chem Geek. I started reading through his posts, I will continue. It's heavy going for someone who's not actually a scientist. I need to take it in small clumps!

 

[xDom]

And welcome to TFP. Good to see another Aussie here.

Thankyou for the welcome. This is more of an explanation than anyone else has been able to offer me!

The graphs that I use as an example are in this link below, I get the feeling these two charts are quite well known amongst pool chem people?
I understand your point on a constant equilibrium being kept between the attached CYA-Chlorine and the free, active HOCL.
I just can't wrap my head around why there is that free HOCL quantity at all.

Chlorine, pH and Cyanuric Acid Relationships

pH controls chlorine strength (% HOCl), but NOT when Cyanuric Acid (CYA) is in the pool. CYA takes over control of chlorine strength. Did you know that? Chlorine and cyanuric acid.

blog.orendatech.com

 

[mgtfp]

Yes, Chem Geek is Richard.

When reading these threads you have to keep in mind that they are old, and were sometimes a work in progress at the time. It sometimes needs some putting into today's context. A good example is the sticky thread about adding borates as a pH buffer. When just reading the first few posts in that thread one might end up using Borax and muriatic acid, but when going through the whole thread you'll realise that TFP preferred method is adding boric acid.

You have to be a bit careful with Orenda, they tend to cherry pick the information they present.

 

[xDom]

Yes, Chem Geek is Richard.

When reading these threads you have to keep in mind that they are old, and were sometimes a work in progress at the time. It sometimes needs some putting into today's context. A good example is the sticky thread about adding borates as a pH buffer. When just reading the first few posts in that thread one might end up using Borax and muriatic acid, but when going through the whole thread you'll realise that TFP preferred method is adding boric acid.

You have to be a bit careful with Orenda, they tend to cherry pick the information they present.

OK thanks. that's interesting about Orenda.
Their basic point that is repeated throughout their blogs ( the one that no one who works in a pool water testing shop seems to know about! ) that pH DOESN'T need to be between 7.2 and 7.6 for CL to be effective once CYA is introduced.. you agree with that one?
I've only had my pool for since August and when I go to get my water tested ( on occasion, I mainly do my own testing ) I have to bite my tongue while the people in the shop tell me my pH is too high at 7.8, about how the chlorine won't be working at that pH.
All the while my pool is crystal clear.

 

[ajw22]

that pH DOESN'T need to be between 7.2 and 7.6 for CL to be effective once CYA is introduced.. you agree with that one?

Yes, we say any pH in the 7's is ok.

I've only had my pool for since August and when I go to get my water tested ( on occasion, I mainly do my own testing ) I have to bite my tongue while the people in the shop tell me my pH is too high at 7.8, about how the chlorine won't be working at that pH.

Many pools have a more stable pH/TA equilibrium with TA of 60 and pH around 7.8. Managing pH between 7.6 and 8.2 is often the best as long as CSI is within a good range.

 

[mgtfp]

I just can't wrap my head around why there is that free HOCL quantity at all.

It's because the bonding reaction is still happening, even though the debonding reaction is also happening.

Think of it this way:

Two properties with a paling fence in-between. Owner A has lots of balls on his side he starts throwing them over the fence. Owner B throws the balls back. Because the balls land scattered around his back yard, he has to find the balls and run between them back and forth so initially he will be slower at returning them.

But as he throws them back the balls get scattered around owner A's property and he also has to start running between balls to throw them back, but while there are more balls on his property, he will have to run less and will throw the balls at a faster rate.

If we first assume that A and B are equally fast runners and good throwers, they will eventually end up having at each time about the same number of balls on their sides, and will throw balls at the same rate. We have reached a symmetrical equilibrium. This is a dynamic equilibrium, balls are still flying back and forth, but at the same rate.

Now we assume that B is a faster runner and better thrower. This will lead to a different equilibrium where less balls will be on his side, so that the time he wins by running and throwing faster gets compensated by having to run on average longer distances between balls. We will reach a different equilibrium with less balls on side B where balls will again be flying back and forth at the same rate.

That's how there will be HOCl left. Even though debonding of chlorine from CYA is slower than the bonding, it is still happening, and will lead to a situation of constant numbers of bound and unbound chlorine, where the reaction rates in both directions are the same.

 

[xDom]

Yes, we say any pH in the 7's is ok.



Many pools have a more stable pH/TA equilibrium with TA of 60 and pH around 7.8. Managing pH between 7.6 and 8.2 is often the best as long as CSI is within a good range.

OK, Do you use the terms CSI and LSI interchangeably? I use the LSI calculator a lot. This is what actually put me onto running at a higher pH. As we move into the cooler months the pH seems to be the easy way of bringing my LSI into check.

 

[PoolStored]

OK, Do you use the terms CSI and LSI interchangeably? I use the LSI calculator a lot. This is what actually put me onto running at a higher pH. As we move into the cooler months the pH seems to be the easy way of bringing my LSI into check.

Pool Math (link--> PoolMath) uses CSI.

CSI and LSI - Further Reading

www.troublefreepool.com

 

[xDom]

It's because the bonding reaction is still happening, even though the debonding reaction is still happening.

Think of it this way:

Two properties with a paling fence in-between. Owner A has lots of balls on his side he starts throwing them over the fence. Owner B throws the balls back. Because the balls land scattered around his back yard, he has to find the balls and run between them back and forth do initially he will be slower at returning them.

But as he throws them back the balls get scattered around owner A's property and he also has to start running between balls to throw them back, but while there are more balls on his property, he will have to run less and will throw the balls at a faster rate.

If we first assume that A and B are equally fast runners and good throwers, they will eventually end up having at each time about the same number of balls on their sides, and will throw balls at the same rate. We have reached a symmetrical equilibrium. This is a dynamic equilibrium, balls are still flying back and forth, but at the same rate.

Now we assume that B is a faster runner and better thrower. This will lead to a different equilibrium where less balls will be on his side, so that the time he wins by running and throwing faster gets compensated by having to run on average longer distances between balls. We will reach a different equilibrium with less balls on side B where balls will again be flying back and forth at the same rate.

That's how there will be HOCl left. Even though debonding of chlorine from CYA is slower than the bonding, it is still happening, and will lead to a situation of constant numbers of bound and unbound chlorine, where the reaction rates I'm both directions are the same.

Ok! Thankyou! This is great!

I've posted this question on a few different platforms and have never received any real responses.
Can you confirm one more thing?
While the CL is bonded to the CYA, those molecules ( isocyanurates? ) are not responsible for any sanitizing? It's only the HOCL that is the active sanitizer?

 

[ajw22]

OK, Do you use the terms CSI and LSI interchangeably? I use the LSI calculator a lot. This is what actually put me onto running at a higher pH. As we move into the cooler months the pH seems to be the easy way of bringing my LSI into check.

CSI and LSI - Further Reading

www.troublefreepool.com

 

[mgtfp]

OK thanks. that's interesting about Orenda.
Their basic point that is repeated throughout their blogs ( the one that no one who works in a pool water testing shop seems to know about! ) that pH DOESN'T need to be between 7.2 and 7.6 for CL to be effective once CYA is introduced.. you agree with that one?
I've only had my pool for since August and when I go to get my water tested ( on occasion, I mainly do my own testing ) I have to bite my tongue while the people in the shop tell me my pH is too high at 7.8, about how the chlorine won't be working at that pH.
All the while my pool is crystal clear.

With CYA there is less HOCl, but the pH-dependancy is not as steep. The recommended FC/CYA Levels consider this and are chosen so that pretty much up to pH 8 there will be enough HOCl. At pH 7 there will be a little bit more, but it's not like all or nothing as it is between pH 7 and 8 without CYA.

The core of the problem is that the chlorine bound to CYA still shows up as FC in the available tests that are manageable outside of a science lab environment.

Without CYA you have:

FC = HOCl + OCl^-

This is again an equilibrium reaction which is dependant on pH. Around pH 7.5, you have both species in equal quantities, below pH there will be mainly HOCl, above pH 8 mainly OCl^-.

Both of these are oxidisers, but only HOCl is a good at killing germs and algae (I think mainly because it is electrically neutral do it is easier to get into biological cells).
That's where the classical rule of maintaining pH below 7.6 comes from.
With CYA you get:

FC = HOCl + OCl^- + CYA-Cl

Best would be to simply have a test that is only sensitive for HOCl. Then we could just run this test and don't care about pH at all (at least in terms of HOCl, there are still other reasons to keep pH in the 7s). Unfortunately, that doesn't exist outside lab environments, and we only have tests that are sensitive to all of the three species above.

The next best thing is to calculate all the equilibrium equations, test FC, CYA and pH and then calculate the amount of HOCl.

This has been done by O'Brien et al.:

Equilibria in Aqueous Solutions of Chlorinated Isocyanurate

The basic learning is that same FC/CYA ratios yield same amounts of HOCl. With a relatively flat pH dependency.

 

[mgtfp]

While the CL is bonded to the CYA, those molecules ( isocyanurates? ) are not responsible for any sanitizing? It's only the HOCL that is the active sanitizer?

I answered that before I saw you question iny previous.

Short answer: Yes

 

[mgtfp]

OK, Do you use the terms CSI and LSI interchangeably?

The LSI is also a CSI (calcite saturation index), but how Langlier first presented it. The concept is always the same, but in the original work had been some oversimplified assumptions in regards to temperature and TDS influences.

What is used as CSI by TFP in PoolMath is based on a more modern calculation of the saturation index going as much as possible back to first principles.

We prefer to use the term CSI to make clear that it is not Lsnglier's original formula.

I have seen many implementations. Some call it LSI, even though they seem to use the same formula for what we call CSI. Others call a calculation with a different formula (might even be Lsnglier's original formula) CSI.

For me, the CSI calculation in PoolMath is the benchmark. I understand what it's based on (while you are stumbling your way through Chem Geek's sticky thread, you will find his pool equations spreadsheet - at the bottom of this spreadsheet he explains the CSI calculation).

 

[mgtfp]

It's only the HOCL that is the active sanitizer?

Technically, some of the species that test as CC also have some sanitizing powers, which are for example used in drinking water. But because of the health risks that some of the CCs bring, and because HOCl is simply the best, this is not an advisable way of pool care.

But it is unfortunately used by some of the pool stores' magical potions, that they sell to poor buggers that have sky high CYA because they followed the store's advice in the first place. Now they have a green pool because there is not enough HOCl left with their CYA of 200 and their FC of 2 (as per pool store advice). Now the store tells them to basically add ammonia (as the main ingredient of a brew usually called something with green in it). The ammonia will react violently with any FC and form CC which can indeed help to clear a green pool. But you also end up with unhealthy CC species. It will take now huge amounts of chlorine to decompose the ammonia basically into nitrogen gas and oxidise all the CCs through before any FC will show again. But this FC will be mainly bound to CYA, so the cycle starts again. Great business model.

 

[xDom]

Technically, some of the species that test as CC also have some sanitizing powers, which are for example used in drinking water. But because of the health risks that some of the CCs bring, and because HOCl is simply the best, this is not an advisable way of pool care.

But it is unfortunately used by some of the pool stores' magical potions, that they sell to poor buggers that have sky high CYA because they followed the store's advice in the first place. Now they have a green pool because there is not enough HOCl left with their CYA of 200 and their FC of 2 (as per pool store advice). Now the store tells them to basically add ammonia (as the main ingredient of a brew usually called something with green in it). The ammonia will react violently with any FC and form CC which can indeed help to clear a green pool. But you also end up with unhealthy CC species. It will take now huge amounts of chlorine to decompose the ammonia basically into nitrogen gas and oxidise all the CCs through before any FC will show again. But this FC will be mainly bound to CYA, so the cycle starts again. Great business model.

So what CYA do you feel is ideal? 20-30 ppm?
I was reading in Falk's paper that even indoor pools should have CYA, to moderate the amount of HOCL.

 

[ajw22]

So what CYA do you feel is ideal? 20-30 ppm?
I was reading in Falk's paper that even indoor pools should have CYA, to moderate the amount of HOCL.

Depends how you chlorinate the pool.

What Are My Ideal Pool Levels?

Use our tool to find ideal chemical levels for any swimming pool. Check TFP recommended levels on pH, free chlorine, cyanuric acid, and more.

www.troublefreepool.com

What Are My Ideal Pool Levels?

Use our tool to find ideal chemical levels for any swimming pool. Check TFP recommended levels on pH, free chlorine, cyanuric acid, and more.

www.troublefreepool.com