Thanks to numerous questions from listeners (to ruleyourpool@gmail.com), Eric explains the actual chemistry of why alkalinity rises in pools feeding CO2 for pH management. CO2 itself does not raise alkalinity. Eric shows what's really going on, and what a pool operator can do about this constant rise in total alkalinity.
[00:00] - Intro
[01:01] - How acid lowers pH
[03:14] - Hypochlorite chlorines
[04:20] - How hypochlorites are made
[06:20] - Chlorines leave behind muriatic acid in water
[07:28] - Excess hydroxides
[09:50] - What can be done to limit the alkalinity rise?
[12:01] - Closing
194. Why Alkalinity Rises in Pools using CO2 for pH Management
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Eric Knight-4: What's up everybody? Welcome back to the Rule Your Pool podcast. I'm your host Eric Knight, and this is episode 194. And after the last two episodes where we talked about burning through alkalinity and we talked with David down in Australia about injecting carbon dioxide, I got a lot of feedback at ruleyourpool@gmail.com.
A lot of questions. A lot of pool operators actually asking questions, which is great to see. Thank you for listening. Uh, If you are operating a commercial pool and you don't like spending a fortune on chemicals, yeah, I guess that is a pretty cool message to hear. So, um, I've been asked by more than one person to dive in a little bit more on the topic of why pools that are feeding CO2 or have an increase in alkalinity without adding sodium bicarbonate.
Now, I know when I was with Orenda, I wrote an article or two on this. I know we've talked about it a lot, and we made mention of it in the episode with David, that it's not the CO2 that's raising the alkalinity, it's actually coming from your chlorine. So I wanted to show you today what those reactions actually look like. So let's get into it, episode 194.
[00:01:01] How acid lowers pH
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Eric Knight-4: It seems to be a pretty good theme to show my formulas on video. And if you're watching this on video on the Facebook page for the Rule Your Pool podcast, you can find it there. We don't actually publish on YouTube. But if you're listening to this, I will talk through it as well. Hopefully, I don't lose you, Kathryn...
But everybody else, you know, this hopefully will not be a very long episode. It's a lot simpler than what we were talking about before. So the question is, why does alkalinity rise on these pools?
Let me back up for a second. If you don't feed CO2 on a pool, and you never have, that may not make a lot of sense to you because, you know, you're used to suppressing pH with manual additions of acid, or maybe you have an acid feeder. And that's fine. That does the job too. But the way that acid lowers pH, as we've discussed, I think two episodes ago, acid neutralizes against alkalinity. Specifically, it neutralizes against bicarbonate alkalinity, and that bicarbonate alkalinity converts into carbonic acid, which then dissociates into dissolved CO2 and water.
And when you have more CO2 in the water, that brings the pH down. Then as turbulence happens and time, you know, time and turbulence, you off-gas some of that CO2 because, quote-unquote, "The beer has to go flat again." Henry's Law kicks in. So you lose that CO2 that was just created by the acid sort of burning through alkalinity. That CO2 off-gases and your pH rises again.
So for pretty much every residential pool operator listening to this, that's what you're used to. So you actually probably have never seen this phenomenon. But if you're a commercial operator or perhaps you have a very high-end customer that has CO2 on their backyard pool, which is rare, you have probably noticed that the alkalinity rises without you adding bicarb. It just seems to naturally rise.
Now, the theory here has always been, well, the CO2 is raising the Mm, no, not exactly. And I'm going to explain exactly what's happening here, and I appreciate all the questions for encouraging me to look deeper into this.
[00:03:14] Hypochlorite Chlorines
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Eric Knight-4: Let's start with chlorines. Pools that feed carbon dioxide are almost never feeding chlorine from a salt generator. I don't think I've ever seen one in the field, and I've personally evaluated almost 400 indoor pools doing air quality studies in my career. I've seen a lot, but I have never seen a CO2 injector on a pool that had salt systems.
Typically, when you have those, they're going to be either on liquid chlorine or cal-hypo. And on a commercial scale, they're not using cal-hypo granular shock. It's all automated. So you have to be able to chlorinate 24/7 if needed. So it's an on-demand system. Those chlorines, both cal-hypo and liquid chlorine, are called hypochlorite chlorines.
So the actual name for liquid chlorine, as most of you probably know, is sodium hypochlorite. That's NaOCL. And the actual name for cal-hypo, that's short for calcium hypochlorite, which is Ca and then parentheses OCL close parentheses 2. If you see it, it'll make a lot more sense. So I'm going to share my screen here, and I'm going to go to this article that I recently wrote, and this actually shows how both of these were made.
[00:04:20] How Hypochlorites are Made
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Eric Knight-4: So, um, when hypochlorite chlorines are being manufactured, at a very basic level, what you're doing is you're interacting chlorine gas, elemental chlorine, with either sodium hydroxide for liquid chlorine or calcium hydroxide for cal-hypo.
Now, calcium hydroxide could be dry, so they actually make a slurry of it. It's called slaked lime, is kind of the casual name for it. And you interact these two substances together, and you get some byproducts. So in liquid chlorine, if we go down here, liquid chlorine, again, this is based on a, a molar ratio.
This is very simplified. The manufacturing process, like there's more to it than this, but this is the basic chemistry. So you get chlorine gas, you get two sodium hydroxides, and that yields sodium hypochlorite, salt, which is sodium chloride, NaCl, and water. So that's the process for liquid chlorine.
Now, what is not explained in this is you have a slight excess of that hydroxide when this is made. Part of the reason for that excess hydroxide, we'll get to in a second, is to keep the pH high to extend the shelf life of the product. So we're going to move down here, and we're going to look at cal-hypo.
How is cal-hypo made? Well, it's a very similar process. Here again, we have elemental chlorine gas with what's called slaked lime or a slurry of calcium hydroxide, and that yields calcium hypochlorite or cal-hypo. And then the calcium salt, which is calcium chloride. We buy this in flake or prills to increase calcium hardness in our pools, but that's technically a calcium salt. Sodium chloride. Well, hey, calcium chloride, here we are. And then you get water again.
So they're very similar in how they are made. Every single one of these chlorines is going to increase your TDS. In fact, the only chlorine product that you can use in a swimming pool that does not increase your TDS is on-site salt chlorination. And that's because you recycle the chlorides. You're not adding more. You're just reusing what's already in there.
But that's-- I don't want to, uh, segue too much.
So getting back to this, there is an excess hydroxide in how these chlorines are made. It slows the degradation or decomposition of these, so you get more shelf life basically.
[00:06:20] Chlorines leave behind muriatic acid in water
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Eric Knight-4: And why that matters is when you use these chlorines, after they are used, they create muriatic acid. So HOCl in water. I don't need to go through it all right now, but as you use it... Actually, I can just scroll down and show you. So as you use it, here's the example from Bob Lowry, And this is with liquid chlorine.
So you have sodium hypochlorite plus water yields hypochlorous acid and sodium hydroxide, which temporarily raises the pH. Well, um, after it does its thing, or gets broken down by sunlight, that hypochlorous acid breaks down into hydrochloric acid. It loses the oxygen. So let's say we got an HOCl, which is our active chlorine, and it oxidizes something. Well, it takes electrons from something, and it has to give it something, so it gives it its oxygen.
So HOCl becomes HCl, hydrochloric acid, also known in our industry as muriatic acid. Now, it's a very different concentration, but the point is it's an acid, and that brings the pH back down. That acid neutralizes almost to a net zero, but not quite. That slight excess of hydroxides that were introduced in the manufacturing process are going to accumulate.
[00:07:28] Excess Hydroxides
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Eric Knight-4: Now, the reason you don't notice this in a pool that is feeding with acid is because acid easily wipes out that excess hydroxide, so you never see it. But in a pool using carbon dioxide instead of acid, you don't actually neutralize that. You don't neutralize alkalinity to create the CO2, and you don't neutralize the hydroxides. CO2 just directly increases how carbonated your pool is and introduces more carbonic acid. It bypasses all of these steps. Which is great in some ways. But when you're using a hypochlorite chlorine, those hydroxides, they just build, and they build.
And the reason that they raise alkalinity, we're going to come back to the original paper here, is because those hydroxides directly interact with the CO2 in your water. So I have two formulas here. The first one is what's going to happen in the pool, but we're actually going to simplify it in the second one by subtracting water because we are in the pool. What does it look like without the water to have less complication? So let me read this to you.
The title here, if you're listening, is Why Alkalinity Rises in CO2-Fed Pools. Hypochlorite chlorines leave behind a slight excess of hydroxides, which are normally neutralized by acids used to suppress pH.
Note I said suppress pH, not control pH. But if the pool is using CO2 for pH suppression, these hydroxides are not neutralized. They then bind with carbon dioxide in the water to create bicarbonate ions. Here's the formula. You have a hydroxide plus carbonic acid, which if you remember, is H2O plus CO2, but together. And that is in equilibrium with bicarbonate and water.
So depending on how the pH goes, this is the reaction that will happen. So what you're actually doing is you are kind of converting the carbonic acid back into bicarbonate because you're raising the pH. So you move the hydrogen over. That's why that happens.
Now, if we subtract water from this, just for the concept of what's happening here, hydroxide plus CO2 in the water creates bicarbonate. Well, bicarbonate obviously contributes to your total alkalinity.
So when you inject CO2 It is directly interacting with those excess hydroxides from the chlorine that you used.
Either cal-hypo or liquid chlorine. Doesn't matter, they both leave behind hydroxides because they're both produced using hydroxides. And that excess is what interacts with it. So you build more alkalinity from it. And that is why on a commercial pool, let's say, that is using CO2, your alkalinity will climb.
[00:09:50] What can be done to limit the alkalinity rise?
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Eric Knight-4: So what do we do about it? It's very simple. Continue using CO2 for your pH suppression system or your pH management, but you're going to need to get on a schedule of manually adding acid. Maybe you have a feeder on it with a timer, something like that. You can automate this as well, but it's a separate system.
And what you want to do is you want to use acid not to control pH. You want to use acid to knock down that alkalinity again and get it out. As we discussed two episodes ago, when you use acid to air quote "burn through alkalinity," you're creating an elimination of that material because the CO2 actually off-gases.
So what you want to do when you're doing this, when you feed that acid, let's say it's on a manual feed. You're the one who turns it on. You're the one who turns it off. If you know how much you need, and you should. Um, if you don't, make sure you measure before you go. Turn on all the turbulence you can, all the water features, bubblers, sprayers, fountains, whatever you have, if you've got it. Make sure that you've got turbulence to get the carbon dioxide out as fast as possible.
Another way of thinking about this is to say, if you are burning through alkalinity, the smoke is carbon dioxide. Make sure that you help to get that smoke out.
Now, none of this is like an emergency, but what happens is if you let it get out of line, the alkalinity climbs so fast it, it compounds on itself. The pH ceiling rises, and so your pH climbs faster and faster and faster and faster because you're more and more carbonated. Which means you're going to be feeding more CO2 based on the pH probe, and this becomes a self-fulfilling prophecy.
It's a downward spiral, basically. You're going to feed a lot more chemicals, and it's going to make your problem get faster and faster and faster.
So you definitely want to make sure if you are using CO2, that you have acid on hand, separated of course, and you're going to be manually using that to bring down your alkalinity.
I always like to have alkalinity on hypochlorite chlorine somewhere between 60 and 70 parts per million. And for those of you who think you need 80 to 120, well, check my work. You'll find that you're going to spend a lot more money on chemicals if you have that higher alkalinity. In reality, 60 to 70 is pretty optimal for hypochlorite chlorines. Make sure in order to do that, that you have sufficient calcium hardness in your water so that you can maintain LSI balance when you have that lower alkalinity.
[00:12:01] Closing
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Eric Knight-4: And on that note, that is this episode. This has been episode 194. I hope this clears things up. To summarize, let's see if I can do it in one breath.
When cal hypo and liquid chlorine are produced, they are produced using chlorine gas into a hydroxide. And when they're manufactured, you have a little excess hydroxide. When that's in the water, it interacts with CO2 in the water to create bicarbonate, which increases your alkalinity. And the only reason you see it on a CO2 pool, not an acid-fed pool, is because acid neutralizes that along with alkalinity to create CO2, whereas injecting CO2 directly puts it in and it does not neutralize the hydroxides.
How'd we do? Episode 194. Thank you so much for your comments. They are heartwarming. It is amazing to see. We are getting our listenership back, and it's a really exciting thing to see. I'm glad you're getting some value out of this show. I hope it is helping you rule your pool. And if you are a pro listening to this, I hope it is helping you on a much bigger scale rule multiple pools at the same time.
It's fantastic. I really hope this helps. And if you have a commercial pool operator that needs to hear this, send it to them. It's pretty easy to share directly from your podcast app, whether it's Apple Podcasts or Spotify or whatever else. Happy to help. And please find us on Facebook. Search Rule Your Pool. You will find it, and we'll go from there. Thanks so much, everybody. Take care.