Chlorine and pH relationships. Chem experts knowledge?

[Coonspool]

OK, so I am a newbie. But, I’m ignorant of pH within Chlorine relationships.
I can find very much data here on Chlorine/CYA, but what about pH?
Basically without getting too technical……haha, (I’ve read Chem Geeks articles) I am
trying to figure out the effectiveness of Chlorine at different pH levels.

I am at the understanding that pH has relatively no affect on Chlorines ability to
kill algae and organisms, is this correct?
Weather a pool has a pH of 6 or a pH of 8, it should not make any difference on how well a maintained Chlorine level of 10 should be at destroying organisms, is this correct?

I know about TA and pH relationships, and I am not asking about that.

So, I guess the only real difference is that Chlorine out gasses at a higher pH than a lower pH? But it is not negligible to affect during a slam? Would a person use more Chlorine at a pH of 8 than a pH of 6 during a slam?

I am only asking to gain knowledge, not argue. Sorry for earlier comments in someone else help thread, I thought I had learned stuff here but am only learning I need to learn more.

 

[svenpup]

In a pool with zero CYA, chlorine is more effective at lower pH, but this relationship is much less with CYA in the water to point of being irrelevant.

Recommended pH range is based on preventing damage to the pool and equipment and eye comfort.

 

[Coonspool]

I am aware (I think by looking at ChemGeeks charts) with the CYA and Chlorine linearity with pH. I know the suggesting for pool equipment pH destruction also, when I made a comment in that other thread some one stated that it is “absolutely” important of pH’s ability to make chlorine kill, and then they quoted me a CDC reference.
Another comment was that they would use less bleach with a lower pH level, so I did not
know what I was missing? I do want to learn, because I do not want to comment on stuff when it seems like I must have a failing grade in PoolSchool?
I am only asking to gain knowledge, not argue.

 

[svenpup]

Whoever was saying that low pH was needed for chlorine to kill is ignoring the effect of CYA in the water (and exaggerating). This is just one more aspect of CYA that the pool "professional" community fails to accept.

In general you should maintain FC based on the the FC/CYA chart and maintain pH at 7.5-7.8.

For a SLAM, the instructions state to initially adjust PH to between 7.2 and 7.5.

 

[karmabiker]

My understanding from Chem Geeks posts is that the CL molecule in play changes as CL rises and the particular CL molecule in play at lower pH is significantly better at oxidizing than the molecule in play at higher pH. Some chemistry that is more advanced than my understanding starts to happen above pH 7.8 IIRC that greatly reduces the efficacy of CL as some flavor of reaction happens.

 

[chem geek]

I am at the understanding that pH has relatively no affect on Chlorines ability to
kill algae and organisms, is this correct?
Weather a pool has a pH of 6 or a pH of 8, it should not make any difference on how well a maintained Chlorine level of 10 should be at destroying organisms, is this correct?

What you wrote is not quite true or certainly needs more explanation. pH does affect the active chlorine (hypochlorous acid) level that is responsible for most of the disinfection of the water. However, when Cyanuric Acid (CYA) is in the water, it buffers this active chlorine so that pH has far less of an effect on it. To be specific, with no CYA in the water, going from a pH of 7.5 to 8.0 has the active chlorine level drop by 50%. With CYA in the water, it drops by only 15%. Going the other direction, going from a pH of 7.5 to 7.0 the active chlorine level with no CYA increases by a little more than 50% but with CYA in the water it increases by 35%. So we generally ignore the pH effect on chlorine except for a SLAM because the larger amounts of chlorine have the pH rise significantly so we recommend lowering the pH first before a SLAM, especially when it's a lot of FC due to high CYA levels.

As for chlorine outgassing, this will be mostly from the active chlorine and it's a very low amount and not changing much with pH so that is not an issue.

On the other hand, hypochlorite ion (OCl^-) breaks down from sunlight faster than hypochlorous acid (HOCl). With CYA in the water, while it buffers the active chlorine (HOCl), this makes the hypochlorite ion concentration vary more than if there were no CYA in the water. What this means is that the rate of chlorine loss from sunlight is higher at higher pH. HOCl breaks down in sunlight at a rate of around 32% per hour (half-life of 2 hours 10 minutes) while OCl^- breaks down in sunlight at a rate of around 204% per hour (half-life of 20 minutes). So the following table shows what happens with 3 ppm FC with 40 ppm CYA just looking at these two forms of chlorine at different pH:

[EDIT] NOTE: The following tables do not take into account any CYA shielding effect protecting chlorine from breakdown from UV in sunlight at higher CYA levels. It only accounts for the direct unshielded loss from the amount of chlorine unbound to CYA and assumes no breakdown of chlorine bound to CYA. Also, the per hour and total losses assume the FC level is maintained, not a one-time dose. I have added another column for one dose 8 hour loss and 8 hours is 8 noontime-equivalent hours so effectively a full day of sunlight. [END-EDIT]

3 ppm FC with 40 ppm CYA
pH ... HOCl ... per hour loss... OCl^- ... per hour loss ... Total 8 hour loss ... One-time Dose Loss
7.0 ... 0.042 ...... 0.013 ......... 0.014 ...... 0.029 ..................... 0.34 ..................... 0.32
7.5 ... 0.033 ...... 0.011 ......... 0.031 ...... 0.063 ..................... 0.59 ..................... 0.54
7.8 ... 0.028 ...... 0.009 ......... 0.059 ...... 0.120 ..................... 1.03 ..................... 0.87
8.0 ... 0.026 ...... 0.008 ......... 0.089 ...... 0.182 ..................... 1.52 ..................... 1.19
8.5 ... 0.023 ...... 0.007 ......... 0.248 ...... 0.506 ..................... 4.10 ..................... 2.24

Now there are additional chlorine losses not related to sunlight but to temperature and there may also be chlorine breakdown from chlorine that is bound to CYA but I don't have any definitive numbers for that. What the above shows, though, is that at a pH of 8.0 one would expect 0.9 ppm FC per day higher chlorine loss than at pH 7.5. Note that a pH of 7.8 would have only 0.44 ppm FC per day higher chlorine loss than pH 7.5 so the loss accelerates more from 7.8 to 8.0 and even faster above that. This is, in part, why one wants to lower the pH before a SLAM -- not only to have more active chlorine and reduce the risk of metal staining but also to lessen the amount of chlorine loss from sunlight during a SLAM (of course, the loss is much higher than I show above which only has 3 ppm FC). I show below what happens during a SLAM with 16 ppm FC with 40 ppm CYA.

16 ppm FC with 40 ppm CYA
pH ... HOCl ... per hour loss... OCl^- ... per hour loss ... Total 8 hour loss ... One-time Dose Loss
7.0 ... 0.382 ...... 0.122 ......... 0.129 ...... 0.263 ....................... 3.1 ....................... 2.8
7.5 ... 0.301 ...... 0.096 ......... 0.320 ...... 0.653 ....................... 6.0 ....................... 5.0
7.8 ... 0.273 ...... 0.087 ......... 0.578 ...... 1.179 ..................... 10.1 ....................... 7.5
8.0 ... 0.256 ...... 0.082 ......... 0.860 ...... 1.754 ..................... 14.7 ....................... 9.6
8.5 ... 0.206 ...... 0.066 ......... 2.188 ...... 4.464 ..................... 36.2 ..................... 14.3

So this shows clearly that letting the pH get high during a SLAM will result in much faster chlorine losses from sunlight. In the above example, if I use chlorinating liquid or bleach (or any other hypochlorite source of chlorine such as Cal-Hypo or lithium hypochlorite), then if I start at a pH of 7.5 and TA of 80 ppm, I end up at 8.31 where the loss/usage of chlorine to sunlight is around 26 ppm FC (if one were to maintain the 16 ppm FC level during the SLAM) while if I start at a pH of 7.2 I end up at 7.67 then the loss of chlorine to sunlight in one day during the SLAM is only 8 ppm FC so considerably less.

 

[Coonspool]

Thank you for explaining it without chemical equations, you've probably posted this before, I just couldn't find it.
I now move my pool school grade from an F to a F- !!

 

[Coonspool]

I did find this chart, which shows it to me also (I guess.) This of course is in normal tap water and doesn't take into consideration of the effects of CYA on water.

The normal pH of water supplies is within range where chlorine may exist as both hypochlorous acid and hypochlorite ion.
HOCl is a stronger oxidant and disinfectant than OCL-, which is why disinfection is more effective at lower pHs.

This is indicated in the following figure

 

[chem geek]

That chart does not apply when there is CYA in the water. A comparison of that chart and the correct chart when CYA is present is shown in this post. As I wrote, when CYA is present, you can largely ignore the pH effect on HOCl level in the normal pH range of pools (usually 7.2 to 7.8).