Nitrogen Reactions in a swimming pool?

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TreeFiter

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Jul 2, 2012
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Saugerties, NY
In the past I've asked about Ammonia in pools, and as I understand it, it is usually the result of bacteria feeding on CYA. Within the last few days, I have encountered several pools that are behaving as if there is Ammonia present. When chlorine is added, there is fizzing, and FC is eliminated almost instantly. I have also noticed that if I add pH increaser (Sodium Carbonate) there is a vigorous reaction as well.

Does anyone know what that reaction might be? I'm assuming it has something to do with the Ammonia in the pools, as I don't see it happen in other pools, but in all of the pools with ammonia issues.
 
duraleigh said:
In 8-9 years on the forum, I cannot recall anyone reporting "fizzing" during chemistry additions.
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They tend to be fairly extreme circumstances. In over ten years maintaining pools, I've only seen it two or three times, but in the last week, I've seen 4 pools all behaving this way. The fizzing is usually pretty fine and looks like cloudy water until you look real close. In the worst of these pools, you can actually hear the fizzing as the chlorine gas escapes. You can smell it too. The reaction with the pH is more of a cloudiness, but the plume of cloudiness expands rapidly outward from where the Sodium Carbonate was added, and then dissipates within about a minute. The expansion I describe actually pushes the water to the extent that when I finish adding pH increaser to some of these pools, all of the debris on the surface has been pushed to the middle of the pool, away from where the reaction was happening.

To demonstrate just how extreme these pools are; In one of these pools, I added about 240 ppm FC to the pool in the morning, and when I returned 6 hours later, there was 0ppm FC remaining. While I added the 240 ppm, I would pull samples from directly across from the return jets having allowed enough time for the chlorine to move across the pool, and even after a few seconds there would be 0ppm FC.
 
Fizzing from adding a carbonate probably means the pH is very low. Did you test the pH before you added the carbonate? Acid plus carbonate (or bicarbonate) shifts to more carbon dioxide where in the extreme this can cause bubbles to form.

The ammonia is a separate issue. One can have the bacterial conversion of CYA into ammonia without having very low pH. I'm guessing that these extreme pools you saw were on Trichlor, had the TA exhausted and the pH crash, and also had a bacterial conversion of CYA into ammonia as well though I didn't think the soil bacteria would like that low a pH. Or perhaps it's just that you are adding so much sodium carbonate that it is producing carbon dioxide gas locally, but I assume you are adding it because the pH is low so in that situation fizzing is not uncommon.

You mention chlorine gas escaping. Do you mean you smell chlorine? What you are probably smelling is high concentrations of monochloramine since chlorine reacts quickly with the ammonia to produce monochloramine. That's more smelly than chlorine. As you continue to add chlorine to the pool, it reacts with the monochloramine to produce even stinkier dichloramine and nitrogen trichloride (i.e. "bad pool smell") until it all gets oxidized.
 
chem geek said:
Fizzing from adding a carbonate probably means the pH is very low. Did you test the pH before you added the carbonate? Acid plus carbonate (or bicarbonate) shifts to more carbon dioxide where in the extreme this can cause bubbles to form.

The ammonia is a separate issue. One can have the bacterial conversion of CYA into ammonia without having very low pH. I'm guessing that these extreme pools you saw were on Trichlor, had the TA exhausted and the pH crash, and also had a bacterial conversion of CYA into ammonia as well though I didn't think the soil bacteria would like that low a pH. Or perhaps it's just that you are adding so much sodium carbonate that it is producing carbon dioxide gas locally, but I assume you are adding it because the pH is low so in that situation fizzing is not uncommon.

You mention chlorine gas escaping. Do you mean you smell chlorine? What you are probably smelling is high concentrations of monochloramine since chlorine reacts quickly with the ammonia to produce monochloramine. That's more smelly than chlorine. As you continue to add chlorine to the pool, it reacts with the monochloramine to produce even stinkier dichloramine and nitrogen trichloride (i.e. "bad pool smell") until it all gets oxidized.
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The pH was not exceptionally low. I was only adding pH increaser because the trichlor shock was bringing it down. I have seen the vigorous reaction from the pH+ in the past, but never knew what was causing it. It just so happens that in the last few days I have seen several pools with ammonia, and I made the connection that they all reacted the same way when I added pH+.

As far as the chlorine gas, yes I was referring to the smell. In some of these pools, the offgassing is so rapid that I can see and hear the bubbles fizzing, almost like carbonated water.
 
I assume you are using Trichlor shock because the CYA is at 0 and you wanted to raise it, but of course you'll want to limit how much of that you add and switch to chlorinating liquid or bleach to continue to add FC until it holds. Also, Dichlor may be less expensive than the combination of Trichlor and pH Up you are using and pH Up may raise the TA too high (unless it was already low). Technically, balancing the pH drop from Trichlor should be done using lye (if there is no carbon dioxide outgassing), but of course no one in the pool industry uses that. Technically, a separately added combination of Trichlor and lye would be equivalent to adding Dichlor, but might be less expensive.

The oxidation of monochloramine from chlorine is slowed down with CYA so you might consider just adding hypochlorite chlorine quickly to the pool with no CYA even though it is in sunlight. While half the FC would get lost in sunlight in an hour, with no CYA in the water you can have 90+% oxidation of monochloramine from additional chlorine in under 10 minutes. With CYA in the water, it's more like 4 hours. So while CYA will slow down FC loss, it will also take longer to get rid of the CC (monochloramine) and have FC hold. Finally, in the early stages when you are forming monochloramine, that happens in seconds with no CYA in the water so you should not lose any FC at all in that situation (and monochloramine does not break down quickly in sunlight).

I suppose it depends if you are there on-site to be able to add chlorine quickly vs. having to go away and come back to it.
 
chem geek said:
I assume you are using Trichlor shock because the CYA is at 0 and you wanted to raise it, but of course you'll want to limit how much of that you add and switch to chlorinating liquid or bleach to continue to add FC until it holds. Also, Dichlor may be less expensive than the combination of Trichlor and pH Up you are using and pH Up may raise the TA too high (unless it was already low). Technically, balancing the pH drop from Trichlor should be done using lye (if there is no carbon dioxide outgassing), but of course no one in the pool industry uses that. Technically, a separately added combination of Trichlor and lye would be equivalent to adding Dichlor, but might be less expensive.

The oxidation of monochloramine from chlorine is slowed down with CYA so you might consider just adding hypochlorite chlorine quickly to the pool with no CYA even though it is in sunlight. While half the FC would get lost in sunlight in an hour, with no CYA in the water you can have 90+% oxidation of monochloramine from additional chlorine in under 10 minutes. With CYA in the water, it's more like 4 hours. So while CYA will slow down FC loss, it will also take longer to get rid of the CC (monochloramine) and have FC hold. Finally, in the early stages when you are forming monochloramine, that happens in seconds with no CYA in the water so you should not lose any FC at all in that situation (and monochloramine does not break down quickly in sunlight).

I suppose it depends if you are there on-site to be able to add chlorine quickly vs. having to go away and come back to it.
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The main reason for using Trichlor shock is because it is what we have readily available. I do use Cal-Hypo when I can to avoid adding CYA, especially when I know I'll be adding a lot of chlorine. If I have liquid available, I'll try to use that as well, but a 5 gallon carboy takes up far more room than 6lb of trichlor.

Why wouldn't dichlor lower the pH as well? I thought it was the CYA in trichlor that brought down the pH. Since Dichlor has more CYA, I assumed it would lower pH even more.

With regard to what you said about the early stages of forming monochloramine, and how there will be little FC loss. This is not what I am seeing in these pools. When I add chlorine via the skimmer; testing the water in front of the return jet seconds later, there will be zero FC. I have observed the same when dumping 5 gallons of liquid into a pool, and testing right where I dumped it.

Typically, when I see this happen, I keep adding chlorine until I start to get a FC reading at the returns and then I keep testing further from the returns until I'm reasonably confident that the FC is not being consumed instantly. When FC levels are testing above shock levels, I'll usually add another dose of chlorine to make sure there will be a residual, because chlorine is still being consumed, just at a slower rate. Often even with this extra dose, the pool will be back to 0ppm FC by the next day, and will require additional chlorine before it will hold a residual. Like I mentioned above, one of these pools took 24lb of chlorine on the first visit (a combination of trichlor and Cal-hypo). Six hours later the FC was 0ppm again. I added more chlorine and came back the next morning, and again 0ppm.
 
Nope, the Dichlor doesn't lower pH on addition because when it reacts with water it produces a molecule of lye that counteracts the acidity of the CYA and the hypochlorous acid. So upon addition, Dichlor is only very slightly acidic, but of course when the chlorine gets used/consumed that is acidic. The net is that for every 10 ppm FC added by Trichlor it lowers TA (after chlorine usage/consumption) by 7 ppm while with Dichlor it lowers it by 3.5 ppm so is roughly half as acidic.

When I wrote there will be little FC loss when first adding it to form monochloramine, I meant little lost by sunlight. The chlorine reacts with ammonia to form monochloramine in seconds when there is no CYA in the water or in a minute or two when there is CYA in the water. So yes, the FC is consumed right away, but you don't need to worry about sunlight breaking it down -- that is what I meant.

So what you are describing is perfectly consistent with the chemistry, but if you were to not use stabilized chlorine you could make the process to faster. The stage where the FC doesn't go away right away but goes away more slowly could be sped up. That's where with CYA it would take around 4 hours to get rid of the monochloramine, if you kept the FC up, whereas it would only take around 10 minutes if there were no CYA in the water. So if you have the time to stick around you might consider just using a hypochlorite source of chlorine and try to get the pool back in shape on the same day. That's all I was suggesting, though would only work if there were really zero or very near zero CYA.
 
Thanks for the clarification chem geek. As always, you have been very helpful. I'm still curious about what I'm seeing with the pH+ reaction, but I'll have to pay closer attention to exactly what conditions it is happening under. It very well could be as you suggested. It can be difficult to keep track of everything in these situations, once I start throwing chlorine in the pool, so if the pH is lowered from adding significant amounts of trichlor, that could be the cause.

One more thought with regard to the type of chlorine used; if the bacteria is still present in the pool, is it possible that adding stabilized chlorine will be counter productive, because it will essentially feed the bacteria? Or is the interaction between the bacteria too slow to be a concern once FC is introduced into the pool?
 

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I don't know, and I don't want to derail this thread, but it confirms what I've thought about Chlorinating (SLAMing) ammonia laden pools. We should treat these a little differently for quicker CC turnaround IMO.
 
TreeFiter said:
One more thought with regard to the type of chlorine used; if the bacteria is still present in the pool, is it possible that adding stabilized chlorine will be counter productive, because it will essentially feed the bacteria? Or is the interaction between the bacteria too slow to be a concern once FC is introduced into the pool?
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Adding CYA would feed the bacteria, but at most they double in population every 15-60 minutes so if you kill them faster than that then the fact that you give them food isn't that important. Also note that monochloramine will also kill them, though more slowly, so even using stabilized chlorine you should be preventing the bacteria from growing and therefore converting much of the CYA you are adding. Of course, if you were to use unstabilized chlorine, then you avoid that issue entirely assuming there is no CYA in the water at all.

The kill time for monochloramine is roughly 70 times longer than chlorine at comparable levels, but in a pool with ammonia one will have rather high monochloramine levels so the bacteria kill time with even 1 ppm CC would be about 7 times longer than chlorine with a 10% FC/CYA ratio. So basically, the bacteria should be killed off when you add the chlorine regardless of whether it is stabilized or unstabilized.
 
Today I came across a pool with very low pH, and I also had to add Trichlor shock. When I added pH increaser, I was thinking about this thread, and the idea that the reaction I had seen in the Ammonia pools could be due to low pH. There was no fizzing, at least nothing close to what I saw in the Ammonia pools. So it seems that the reaction isn't due to low pH or the presence of trichlor.

Could there be a reaction between something like monochloramine and sodium carbonate?
 
It would be more likely that adding chlorine to monochloramine at low pH would produce more nitrogen trichloride and that might outgas, but would smell awful. I don't see how monochloramine and sodium carbonate would react in a way to produce a gas.

Is it possible that the more vigorous reaction you saw in the earlier pools was because their pH was lower? You aren't testing exactly how low the pH is are you? Were the earlier pools and this one both reporting zero TA so turning red immediately indicating a pH of 4.5 or below? Perhaps the earlier pool had an even lower pH in the 2's or so while the one you are looking at now might only be in the 4's. That would be a factor of 100 difference and could account for why one is more vigorous than the other with regard to bubbles from sodium carbonate addition. Basically, if the pH is lower, then the sodium carbonate doesn't raise it as much and the resulting bicarbonate will go more to carbon dioxide and may produce bubbles. If the pH starts higher, then the pH is raised by the sodium carbonate so stops more at bicarbonate and not producing enough carbon dioxide to form bubbles.
 
I don't know exactly how low the pH was in any of these pools. I'm working with test strips typically, so when its low, its hard to know just how low. I don't remember the Ammonia pools being all that low to start (maybe 6.8-7.0), but after adding 8-12lb of trichlor, I would add pH increaser to bring it back up.

The pool I tested yesterday was off the charts low. The test strips stop at 6.8, which is a light orange color. I was seeing something more yellow. If I were to try to interpolate the result, I would guess a low 6, maybe mid to high 5. So I don't think I'm dealing with extremely low pH as you describe.

Would nitrogen trichloride also have a chlorine odor? I have noticed that sometimes after adding pH increaser, I will smell chlorine. With the ammonia pools, the odor is already present from neutralizing the ammonia, so it might go unnoticed as a new odor.
 
Nitrogen trichloride is "bad pool smell" odor, not the fresher bleach-like smell. Adding pH increaser to a pool without ammonia might just aerate it more if you get some bubbling and that probably just outgases some chlorine which will smell more like bleach.
 
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