Is chlorine more effective with a lower pH?

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G

GK

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Mar 21, 2011
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Orlando, FL
I have been trying to understand the more advanced concepts of what is happening to the chlorine in the water. I came across this article about "Water Treatment To Prevent Emitter Plugging"

http://fawn.ifas.ufl.edu/tools/irrigati ... ance/four/

It looks like it would be helpful to lower the pH to near the load end of the acceptable range in order to get the most effect from the chlorine when fighting algae, especially cleaning up a swamp.

Your comments please.

Gary
 
Could be why the pool school shocking information here says to lower ph to 7.0-7.2 (IIRC) while shocking. ???

Now, I'd like to know if standard min/target is less effective at higher ph levels (say, 7.7).
 
This is really only true when there isn't any CYA in the water. As soon as there is any CYA in the water, almost all of the chlorine effectiveness variation with PH goes away. For details, see this post.

frogabog, Pool School doesn't say to lower PH to 7.0 to 7.2 when shocking that I'm aware of. And if it does it is wrong.

All of our recommendations have already taken this kind of thing into account.
 
frogabog said:
Sorry, I got the numbers wrong, 7.2-7.4 (not 7.0-7.2). I should have checked before I posted. I am however referring to the url that TrippleB4me referenced above.
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frogabog, when I saw your post I too remembered reading that chlorine was more effective at lower pH levels, I wasn't picking at your numbers but agreeing that it has been said that chlorine is reported to work better at lower pH levels, which to me, contradicted Jason's response.
 

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I was playing with some numbers. If the pH rises from 7.5 to 7.8, then the HOCl concentration falls from (say) 10% to 9% of the overall free chlorine level (or thereabouts). It sounds like hardly any change. But to look at it another way, your chlorine is only 90% as effective at pH 7.8 as it was at pH 7.5.

When you're working with such tiny quantities to start with, perhaps it does matter. I find that pH does make a difference in how well my pool runs.

However, perhaps the pH has an influence on other things, like reaction rates of making combined chlorine.
 
All the pools I maintain (13 of them) have a PH of 7.6 to 7.8 and most are public pools and I don't have any problems with my HOCl levels. I also have the lower end ALK so a high PH really helps my balance. When considering what levels you are going to have never take them in isolation but consider what effect it will have overall when all other parameters are put into the boiling pot so to speak.
 
loop_pea said:
I was playing with some numbers. If the pH rises from 7.5 to 7.8, then the HOCl concentration falls from (say) 10% to 9% of the overall free chlorine level (or thereabouts).
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I don't know what you are using to play around with the numbers, but if you use my spreadsheet, then with an FC that is 3 and a CYA that is 30 ppm, at a pH of 7.5 the HOCl is 0.042 ppm or 1.40% of the FC level while at a pH of 7.8 it is 0.038 ppm or 1.27% of the FC level. Nevertheless, your point of the chlorine level at a pH of 7.8 being 90% of what it was at a pH of 7.5 is still true. This should not normally make any noticeable difference.

As for disinfection by-products, there is more nitrogen trichloride produced at lower pH while there are more trihalomethanes (such as chloroform) produced at higher pH.

loop_pea said:
I find that pH does make a difference in how well my pool runs.
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Can you be more specific? What kinds of differences do you notice? As Freelancer noted, a higher pH can be more stable with regard to carbon dioxide outgassing. This effect, as well as TA, are shown in this chart. Roughly speaking, the amount of carbon dioxide in the pool is about half as much at 7.8 as at 7.5.
 
chemgeek, i was using your spreadsheet :)

Apologies for my maths. It's up the spout at the moment, even with a calculator, but the 90% is the point I was trying to convey.

It feels like when you're working with HOCl levels of only a few percent, then the magnitude of the change is relatively large compared to where you started, and it might push you over a threshold where there isn't enough HOCl to deal with bather waste in the pool fast enough.

In the UK the advice is to keep CC at less than half FC because it influences the chemical reactions. we're told that if CC is too high relative to FC then it encourages the production of nitrogen trichloride.

Low pH never comes into it for us, as we start with a fresh fill of high TA water. After a bit of a battle on startup, the TA and pH have stabilised nicely at around 130 and 7.6 respectively (trichlor pool).

However, in my experience high pH also coincides with more nitrogen trichloride production, which is the opposite of what's generally said.

I've been trying to get my head around the complex relationship between all these things, FC, CC and pH and their influence on pool smell. I really hate pool smell. :grrrr:
 
I think you are mixing up your interpretation of what the FC and HOCl levels mean. Just because the HOCl level is very low does not necessarily mean that there isn't enough HOCl to deal with bather waste fast enough, at least in outdoor residential pools. There is plenty of capacity to handle the bather waste in the FC level as the FC converts to HOCl quickly (half of the FC can convert to HOCl in 1/4th of a second, if needed). The very low HOCl level is nevertheless enough to kill most pathogens quickly (see this link for some kill times) and is enough to prevent algae growth. It oxidizes some bather waste reasonably quickly where ammonia is mostly oxidized in a matter of a few hours, though in practice this is continual (i.e. ammonia isn't all dumped in at once). Urea is slow to oxidize by chlorine, but this and other organics most likely get handled when chlorine breaks down by the UV in sunlight forming hydroxyl (and chlorine) free radicals that are very powerful oxidizers. For this analysis I assume the FC is at least 7.5% of the CYA level (see the Chlorine / CYA Chart. It is true that if your FC/CYA ratio is too low, then oxidation rates may get too low as well.

The CC should not be half of the FC but should rather be fairly low, usually <= 0.5 ppm and most often in outdoor residential pools it is <= 0.2 ppm. You say you use Trichlor pucks/tabs, but for every 10 ppm FC added by Trichlor, it increases CYA by 6 ppm. So even with 2 ppm FC per day chlorine usage, Trichlor will increase CYA by over 35 ppm PER MONTH. The higher CYA level is likely what causes you problems unless you proportionately raise the FC level as the CYA level climbs.

High pH has less nitrogen trichloride, but more trichloromethanes such as chloroform. I suspect your high CYA level is causing a buildup of monochloramine and other combined chlorine. Do you know your CYA level? Are you talking about an indoor pool or an outdoor pool exposed to sunlight? Residential low bather-load pool or commercial/public high bather-load pool?

A properly managed outdoor residential pool won't have any bad smell. It will only have the faintest of bleach-like chlorine smell if you churn up the water a bit. You might smell some chloramines (mostly monochloramine) on your skin if you get out of the water since your sweat will continue to react with chlorine, but that's about it. So something is not right with your pool if you are having trouble with its smell. That was the problem you were having in this thread about a year ago when you first came to this forum.
 
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