Does pH affect chlorine consumption?

[anonapersona]

I've begun testing chlorine daily for awhile, just trying to get to know the pool better. Current test info, FC 4 to 6, CC 0, TA was 90 lowered to 80, CH 240, CYA 50.

What seems weird, first when I was running the pool up to FC 8 it would take 2 days to go to FC 4. Fine, sort of high consumption but hey, it's hot and sunny. Then I started just going to FC 6.0 and in a day it was down to 4 or 4.5. Most of the time my pH is at 7.7 or 7.8 or even 7.9.

Since TA has been creeping up with water additions I lowered it rather aggressively from 7.9-ish to 7.0 ish then aerated. So, today the pH is 7.3 and what seems weird is that I see no chlorine consumption at all, or nearly so. I raised FC to 6 (or maybe 6.5 tops assuming some measurement error while pouring bleach) and this evening FC tests at 6.0.

I always assumed that the suggested range for pH of 7.5 to 7.8 had to do with eye comfort while swimming, as well as scaling issues. But, does pH have any effect on chlorine loss?

 

[MikeInTN]

AFAIK it doesn't, but then again, I'm not a "chem geek". Oh Richard?? Where art thou? :-D

 

[chem geek]

The breakdown of chlorine from sunlight for hypochlorous acid is slower with a half-life of 2 hours and 10 minutes compared to hypochlorite ion with a half-life of 20 minutes. At a pH of 7.5 with a 50/50 mixture of each, the half-life is 35 minutes which is from 1 / ( (1/130 + 1/20)/2 ). Because Cyanuric Acid (CYA) buffers hypochlorous acid in the water, changes in pH result in lower changes in hypochlorous acid but that results in larger changes in hypochlorite ion. So at a pH of 7.7 the overall half-life becomes 29 minutes. At a pH of 8.0, the overall half-life becomes 25 minutes. Going the other way, at a pH of 7.3, the half-life becomes 41 minutes while at a pH of 7.0 it becomes 54 minutes. So between a pH of 7.0 and 8.0 the chlorine loss rises by a little more than a factor of 2.

So, in theory, the breakdown of chlorine from sunlight is somewhat faster at higher pH. In practice, there is apparently some chlorine loss from chlorine bound to CYA that is far higher in concentration, but the half-life is much longer -- some number of hours (at least 6). We don't know this half-life and it's hard to figure out from typical pool numbers because of so many other things going on not related to sunlight breakdown. There is a redistribution of chemical species of chlorine bound to CYA at different pH but there is effectively no change in the concentration of the most dominant one, HClCY^- from pH 7 to 8. However, other species do vary including Cl₂CY^-, H₂ClCY, ClCY^2- and Cl₂CY^-, but again, I don't have any rates of breakdown for any of these. At a pH of 7.5 and with an FC/CYA ratio near 0.1, the concentration of HClCY^- is 66 times higher than that of hypochlorous acid (HOCl).

Richard

 

[anonapersona]

Thanks, for the details. You do make my head spin sometimes. So, the short answer is yes but hard to say by how much. Since the pool was typically at pH 7.8 and the big difference was noticed at pH 7.3 (or less) since I was lowering TA at the time and during that next day the pool was probably less than 7.3 for some time then the difference in half life might have been quite large.

Now I wonder if it is worthwhile to try to keep pH lower. I will keep testing and see how it goes.

Would lower pH tend to aggravate decay of the waterfall rocks? Moss rock I think, some sort of sandstone apparently.

 

[chem geek]

You could compensate for the lower pH with a higher Calcium Hardness (CH) level that should protect plaster and limestone surfaces. Not sure what it would do with your specific rock materials. The lower pH will tend to have a faster pH rise.

 

[anonapersona]

Thank goodness for the Pool Calculator. I will look at raising CH to maybe 300. Then maybe try to keep pH closer to 7.5 than 7.8 and see how the chlorine runs. I do think that since the spa is attached to the pool I need to be wary of what happens when we crank that up to 104 degrees and turn the bubbles on for a half an hour or more.

 

[Freelancer]

Richard,

Is not also true that chlorine added to water will produce more hypochlorous acid (the stronger killing chlorine) than hypochlorite ions (the less stronger killing chlorine) at lower PH's as oppossed to higher PH's. It is just something I read somewhere which is why I try to keep my PH's on the lower side. 7.4 - 7.6

 

[JasonLion]

Freelancer, yes and no. When there isn't any CYA in the water, this effect is fairly significant. But as soon as there is some CYA in the water this effect is dramatically reduced and essentially stops making any difference.

 

[Freelancer]

Thanks Jason.. This is why I post what I read elsewhere just so that I can get a balanced picture and more acurate picture.

 

[dschlic1]

A question on this: Does adding MA raise FC?

 

[chem geek]

Richard,

Is not also true that chlorine added to water will produce more hypochlorous acid (the stronger killing chlorine) than hypochlorite ions (the less stronger killing chlorine) at lower PH's as oppossed to higher PH's. It is just something I read somewhere which is why I try to keep my PH's on the lower side. 7.4 - 7.6

You can see the graph of hypochlorous acid vs. pH with no CYA vs. with 30 ppm CYA in the graphs in this post.

 

[chem geek]

A question on this: Does adding MA raise FC?

No it does not. Muriatic Acid contains no chlorine and simply lowers the pH (and increases the amount of chloride in the water -- that's salt, not chlorine which is different at a higher oxidation state).

Free Chlorine is the total measure of all chlorine components in the water that are not tightly bound so that includes hypochlorous acid, hypochlorite ion, and all chlorine attached to CYA (which all gets released quickly in the time of the FC test). The Combined Chlorine (CC) test then measures most of the rest of the chlorine that is more tightly bound such as the chlorine attached to ammonia (i.e. chloramines) and the chlorine attached to organics such as urea (i.e. chloroureas).

I think your question may be asking whether adding Muriatic Acid increases the active chlorine (hypochlorous acid) level and the answer is yes, because the pH is lower. However, as Jason indicated, this effect is muted by having CYA in the water so it's better not to worry about trying to keep the pH so low and instead target a pH that is roughly near 7.5 (the average pH of tears) though it doesn't have to be that precise.