My first SLAM: Lessons Learned and questions about SLAM pH

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ChrisA76

ChrisA76

Silver Supporter
Dec 1, 2022
53
East Bay, California
Pool Size
14400
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
So, I did my first SLAM as discussed in this thread and I believe it was highly effective. Thanks to many this forum for their advice and guidance on many topics, such as my possible need for doing a SLAM, for getting an FAS/DPD test, and for advice on raising my CYA.

I passed my OCLT (lost less than 0.5 ppm FC overnight) and now my CC is less than 0.5. Water's also perfectly clear but it's been clear for weeks.

My CYA level is currently at < 30 ppm (probably about 20 ppm and I will be bringing it up, no need to comment) so I decided to bring up my FC to about 13 ppm just to be on the safe side even though 10 ppm was recommended. Because my pool is covered, I can keep most UV rays out of the pool and it's not that bright out now, so I lost almost no chlorine. It's actually still at 12 ppm as I write this and I started the SLAM about 4 days ago.

Lessons learned:
  • It took keeping the FC high for about 36 hours only before CC was unmeasurably low and I passed the OCLT the next night.
  • I did it with about $12.99 of chlorine. This was simple and cheaper than the $16 of L*****'s FreshAndClear, and I'm not even sure that the latter would have done the job. I had added it before and it didn't clear lower my CC for long.
  • My pH was a little on the high end when I started (about 7.6-7.7 pH) when I started but given that I had some amount of CYA I figured that the dominant factor was the CYA and not the pH. I didn't want to drop my pH only to have to add TA later and raise my pH again later. Starting the SLAM at this pH was intentional. I think if I had a serious algae problem before I started, I would have lowered pH first.
  • It smells slightly when I open the cover. (I guess these are the by products from reacting with whatever CCs were there before?) But the water doesn't smell at all and it's the clearest I've ever seen it. I guess this is because of the extremely low amount of CC (and algae) in the water. In fact my tap water smells more like chlorine than my pool water. (I actually tested my tap water and it has about 0.4 ppm chloramines).
Questions:

My pH raised to over 8.0 as others here have posted. Extrapolating roughly from an acid demand test, I think it's about 8.3 now but this obviously isn't accurate. I expected this since I've read in Bob Lowry's "Pool Chemistry for Service Pros" and other places is that adding liquid chlorine, will increase pH temporarily because Sodium Hypochlorite plus water creates hypochlorite (HOCl or OCl depending on the pH) plus sodium hydroxide, the latter of which increases pH. And that after the chlorine is hit by UV rays or is used in an oxidation reaction that it produces some muriatic acid (HCl) as a byproduct that combines with the NaOH to produce water and NaCl (salt) thus dropping the pH back.

I know that the TFP wisdom is "don't worry about this high pH during a SLAM" but I'm asking why should I not worry about the currently high pH? Is this because people typically only leave their FC high for short periods during a SLAM? I'm pretty convinced I could leave my FC high and pH high for over a week. But do I want to? Will this produce scaling eventually or is there something that keeps it from scaling at these high pH levels?
 
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ChrisA76 said:
but I'm asking why should I not worry about the currently high pH?
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Because as a long term concern, it won't matter in the short term. All in all, the PH is going to rise during the SLAM, but you won't have a trusty PH reading at 10+ FC. So you lower it before starting, and let it be for the week or two of SLAM. If/when it goes above 8, it won't be there long enough to care. Then you commence dropping it at your earliest convenience afterwards. Even if that takes a day or two more, no biggie.
 
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ChrisA76 said:
My pH raised to over 8.0 as others here have posted. Extrapolating roughly from an acid demand test, I think it's about 8.3 now but this obviously isn't accurate.
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I left my pool open all day and it dropped FC from 13 to 8 ppm and the pH went back down to about 8.0. I'll drop both down a little more and then just leave it be.
 
ChrisA76 said:
I left my pool open all day and it dropped FC from 13 to 8 ppm
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Likely from the low CYA. That's a huge loss this time of year.

Remember to only test PH less than 10 FC or you're likely getting a false high.


*edit. If you're using the colorQ,, I don't think that is affected by FC like the drop tests. Disregard.
 
Likely from the low CYA. That's a huge loss this time of year.
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Yes I know. Also, I'm going to raise CYA.

Yeah, I truly believe it's around 8.3 as evidenced by the ColorQ reading and the acid demand test I did on the Taylor kit. Therefore it's likely not a false high.
 
Newdude said:
Because as a long term concern, it won't matter in the short term. All in all, the PH is going to rise during the SLAM, but you won't have a trusty PH reading at 10+ FC. So you lower it before starting, and let it be for the week or two of SLAM. If/when it goes above 8, it won't be there long enough to care. Then you commence dropping it at your earliest convenience afterwards. Even if that takes a day or two more, no biggie.
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So, if I left pH at 8.3 for a long time with a CSI around 0.5 - 0.6 would it be a problem?
 
ChrisA76 said:
So, if I left pH at 8.3 for a long time with a CSI around 0.5 - 0.6 would it be a problem?
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Define long time ? A long SLAM, like was referenced..... would be fine. When the FC is in range after SLAM, then you can trust the PH reading and correct it accordingly. My point was that a couple weeks wasn't cause for concern. It takes many weeks or months to cause issues. (y)


ChrisA76 said:
Yes I know. Also, I'm going to raise CYA
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I SWEAR I wasn't rubbing it in. I was just pointing it out in case you didn't connect those dots. 😁
 
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Newdude said:
Define long time ? A long SLAM, like was referenced..... would be fine. When the FC is in range after SLAM, then you can trust the PH reading and correct it accordingly. My point was that a couple weeks wasn't cause for concern. It takes many weeks or months to cause issues. (y)

I SWEAR I wasn't rubbing it in. I was just pointing it out in case you didn't connect those dots. 😁
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No no, you don't need to worry about that, you've helped me a ton. :)

It's good information that I can fix it in a day or two or even a week and not lose sleep over it.
 
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ChrisA76 said:
It's good information that I can fix it in a day or two or even a week and not lose sleep over it.
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Watch the FC like a hawk. Ensure you never get close to minimum. Minimum is lava. Don't touch it, don't smell it, don't do the curly shuffle within 15 ft of it.

If you want some time not watching it like a hawk, add higher and you'll buy that time off as it drifts down. (Seeing how you know the recent daily loss from the frequent/accurate testing (y))

As long as you're above minimum, the other #s can wait until it's convenient.
 
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The effect on pH of adding bleach and maintaining the increased FC can be quite significant when adding a lot of chlorine to reach a high Slam FC at a high CYA (or even going further to mustard slam level). Hence the step in the slam process of reducing pH first.

In your case, the pH wasn't too bad, because your slam FC wasn't that high for CYA 30, but it can get close to pH 9. As long as TA is in the TFP recommended range, you would then usually be above the pH ceiling, and the water would actually start absorbing CO2 and pH would trend back towards the pH-ceiling. But for someone with very high TA, pH would stay high.

High pH during a slam has now a number of downsides:

1) High CSI:
In the short term this is usually not a huge problem, but for someone who's CSI was already high at normal pH, this can accelerate scaling. If someone decided to keep their SWG running during the Slam (against TFP recommendation) this will certainly create scaling in the cell.

2) Metal staining:
The combination of high pH with high FC oxidizes dissolved metals and lets them fall out of solution and create discolourations.

3) Increased chlorine demand:
High FC means a higher percentage of FC presents as OCl-. OCl- has a much shorter half life under UV compared to HOCl, which means that maintaining slam FC during the day requires adding more chlorine.

At normal target FC, this effect of pH on chlorine decay under UV is quite small, but it is more pronounced at higher FC levels. Here is an explanation by Chem Geek with example calculations (when viewing on a phone, then hold it landscape so that the tables are shown properly):

chem geek said:
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.
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Newdude said:
Watch the FC like a hawk. Ensure you never get close to minimum. Minimum is lava. Don't touch it, don't smell it, don't do the curly shuffle within 15 ft of it.

If you want some time not watching it like a hawk, add higher and you'll buy that time off as it drifts down. (Seeing how you know the recent daily loss from the frequent/accurate testing (y))

As long as you're above minimum, the other #s can wait until it's convenient.
Click to expand...
Yeah I plan on closing the pool in a couple hours, measuring the FC to see where it got and then keeping it closed and only checking it once a week or so, and keeping it at about 5-6 ppm (my Minimum is 2 ppm according to Chem Geek's chart). It's been a fun month of learning, capped off by knowledge I've found here.
 
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ChrisA76 said:
And that after the chlorine is hit by UV rays or is used in an oxidation reaction that it produces some muriatic acid (HCl) as a byproduct that combines with the NaOH to produce water and NaCl (salt) thus dropping the pH back.
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You got the principal, but there is no muriatic acid created, and there was also no NaOH.

When you add HCl to water, it dissociates completely into H+ and Cl-, that's why it's called a "strong" acid. The H+ is what reduces pH - pH is just a fancy way of counting H+ ions.

Same with NaOH, which in water dissociates into Na+ and OH-. Adding OH- is equivalent to removing H+, and increases pH.

That means on the other hand, that the OH- created in the cell doesn't combine with Na+ in water, both just hang around.

And "using" chlorine creates H+, which is not interested in the Cl-, because HCl is a strong acid and wants to dissociate in water, not recombine. But the H+ neutralises the OH- created in the beginning into H2O.
 
ChrisA76 said:
It smells slightly when I open the cover. (I guess these are the by products from reacting with whatever CCs were there before?) But the water doesn't smell at all and it's the clearest I've ever seen it. I guess this is because of the extremely low amount of CC (and algae) in the water. In fact my tap water smells more like chlorine than my pool water. (I actually tested my tap water and it has about 0.4 ppm chloramines).
Click to expand...

That's not unusual. The CCs created by chlorine oxidizing stuff outgass to a certain degree from the water. In an uncovered residential outdoor pool you don't really notice it, as it it just disappears quickly into the atmosphere. It is very noticeable in indoor pools.

When using a cover, the CCs can't escape. That means you get a big waft when removing the cover.

Trapping outgassing CCs also means that you slow down the outgassing process. That's why it's important to open the pool regularly and let it breath.
 
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mgtfp said:
You got the principal, but there is no muriatic acid created, and there was also no NaOH.

When you add HCl to water, it dissociates completely into H+ and Cl-, that's why it's called a "strong" acid. The H+ is what reduces pH - pH is just a fancy way of counting H+ ions.

Same with NaOH, which in water dissociates into Na+ and OH-. Adding OH- is equivalent to removing H+, and increases pH.

That means on the other hand, that the OH- created in the cell doesn't combine with Na+ in water, both just hang around.

And "using" chlorine creates H+, which is not interested in the Cl-, because HCl is a strong acid and wants to dissociate in water, not recombine. But the H+ neutralises the OH- created in the beginning into H2O.
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It wasn't from my cell, it was from liquid chlorine, NaOCl. I didn't add HCl either.
 
ChrisA76 said:
It wasn't from my cell, it was from liquid chlorine, NaOCl. I didn't add HCl either.
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I didn't want to imply that you added HCl, I just wanted to explain that HCl dissociates completely when added to water. And the other way round, that in the chlorine decay only H+ gets created, and the H+ is not interested in the Cl- in the water but goes straight to the OH-.

Same with liquid chlorine, any Na added via Sodium Hypochlorite or excess lye from the bottle hangs around as Na+, because NaOH is a strong base and dissociates completely in water.

This is something that often gets oversimplified in internet explanations.
 
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