Chlorine/pH/CYA Relationships

[Jakebear]

I’m full of questions tonight. What is the consequence of running the pH a bit low to try to make up for the “Chlorine/CYA lock?

I know the “ideal” is 7.5 pH & 40 ppm CYA but--------

I’ve seen charts that put chlorine effectiveness at around 50% at 7.5 pH, but up around 75% at 7.0 pH. Then the next Chart says Chlorine effectiveness is reduced by high CYA. (>50ppm).

What I am getting at is can the two conditions (low pH & high CYA) off-set the effects of each other or doesn’t it work that way??

27000 gallon In ground Vinyl DE filter

[chem geek]

I’m full of questions tonight. What is the consequence of running the pH a bit low to try to make up for the “Chlorine/CYA lock?

I know the “ideal” is 7.5 pH & 40 ppm CYA but--------

I’ve seen charts that put chlorine effectiveness at around 50% at 7.5 pH, but up around 75% at 7.0 pH. Then the next Chart says Chlorine effectiveness is reduced by high CYA. (>50ppm).

What I am getting at is can the two conditions (low pH & high CYA) off-set the effects of each other or doesn’t it work that way??

27000 gallon In ground Vinyl DE filter

Jakebear,

These are good questions and it turns out that because CYA acts like a chlorine buffer (similar to the way that alkalinity acts like a pH buffer), the presence of CYA reduces the pH effect on chlorine's effectiveness. Of course, the presence of CYA itself significantly reduces the absolute amount of disinfecting chlorine, but once that is done it acts to somewhat keep that amount more constant when pH swings compared to not having any CYA at all. Of these two effects that CYA has -- 1) reducing chlorine's effectiveness and 2) buffering chlorine against pH changes, it turns out that the reduction of chlorine's effectiveness is huge. The bottom line is that you have absolutely no hope of restoring significant amounts of chlorine effectiveness when CYA is present by simply changing the pH.

I have to leave right now, but will add another post with some numerical examples to give you a better feel for this. It turns out that this question comes up quite a bit as people think they should run at lower pH to get greater chlorine effectiveness and don't realize that when CYA is present this inhibits their ability to do this (i.e. to lower pH to increase chlorine's effectiveness).

Richard

[Simmons99]

Also - why risk damaging your liner by running a PH below 7.2 consistantly?

[chem geek]

Simmons99 makes an excellent point. I don't know what the recommended pH range is for vinyl liners, but as the numbers show below it doesn't make much sense to try and vary pH to improve chlorine effectiveness rather than simply adding more total chlorine.

The following shows the amounts of disinfecting chlorine (HOCl) and non-disinfecting chlorine (OCl-) when the total chlorine is 3 ppm at various levels of pH with different CYA amounts for each table. The "missing" chlorine (to add up to 3) when CYA is present is bound to CYA, better protected from sunlight (UV) destruction, but not immediately available to do work (disinfect or oxidize). It's like being in reserve and can be called up rather quickly (in seconds).


for Total Free Chlorine = 3 ppm

. . . . . . pH
0 ppm CYA 7.0 7.5 8.0
ppm HOCl 2.243 1.451 0.685
ppm OCl- 0.742 1.520 2.271

. . . . . . pH
15 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.142 0.107 0.087
ppm OCl- 0.047 0.112 0.289

. . . . . . pH
30 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.060 0.045 0.038
ppm OCl- 0.020 0.047 0.126

. . . . . . pH
70 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.023 0.017 0.015
ppm OCl- 0.008 0.018 0.050


You can see that though the ratio of HOCl to OCl- is only a function of pH and is independent of CYA, the presence of CYA cuts the total amount of both HOCl and OCl- way, way down and also makes the disinfecting form of chlorine, HOCl, less variable with pH changes while the non-disinfecting form, OCl-, becomes more variable.

Specifically, without CYA, changing the pH from 7.5 to 7.0 increases chlorine effectiveness by 55% while with CYA present (in any amount much larger than the total amount of chlorine), this same pH change only increases chlorine effectiveness by 33%.

If you were to increase the total chlorine from 3 ppm to 4 ppm, you would get the following.


Total Free Chlorine = 4 ppm

. . . . . . pH
0 ppm CYA 7.0 7.5 8.0[/B]
ppm HOCl[/B] 2.991 1.934 0.913
ppm OCl-[/B] 0.990 2.027 3.028

. . . . . . pH
15 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.219 0.167 0.134
ppm OCl- 0.072 0.175 0.442

. . . . . . pH
30 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.086 0.064 0.055
ppm OCl- 0.028 0.067 0.181

. . . . . . pH
70 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.032 0.024 0.021
ppm OCl- 0.011 0.025 0.068


Notice how without CYA you get the expected 33% increase in chlorine effectiveness (4/3 = 33%), but with a CYA of 30 ppm you get a larger 42% increase in chlorine effectiveness when you increase total chlorine from 3 to 4. This is because having the total free chlorine level closer to the CYA level makes the CYA's buffering of chlorine less effective.

Yes, that's a lot of information, but the bottom line is that when you have CYA in your pool it is much easier to increase chlorine's effectiveness by adding more chlorine than it is by lowering the pH (which, as pointed out earlier, has other side effects).

Richard

[PoolDoc]

It turns out that this questions come up quite a bit as people think they should run at lower pH to get greater chlorine effectiveness and don't realize that when CYA is present this inhibits their ability to do this (i.e. to lower pH to increase chlorine's effectiveness).

The reason that people think that is that every singple pool publication I've seen fails to consider the effect of CYA on the HOCl / OCl levels in pool water, and pretends that the simple, no-CYA, curves still apply. Of course, they don't.

I've known this for a long time, but until Richard got his calculations to the point where they are today, I had no way to actually quantify the effects.

-- Richard, it would be great if we could come up with a set of graphs, to publish here, alongside of the simple Cl / HOCl / OCl 3-hump graphs that are present in all the pool manuals. A visual representaion of the *real* situation would be a good first step toward combating this almost universal error.

Anyhow, you have my gratitude for all the work you've done. I think it has the potential to help a lot of people. --

Ben

[Jakebear]

Thanks to All

This has really helped me to understand what is really going on with these relationships. The graph would be wonderful --- a picture is worth a million words when it comes to the technical stuff. We always try to out think the numbers when we don't fully understand.

Thanks again ---- it's 7.5pH and Ben's chart for me

[chem geek]

I'd be happy to create a graph, but please excuse my ignorance since I'm not exactly sure what the graph should show. I have seen the graph of HOCl vs. OCl percentage (a single S-shaped curve) as a function of pH and that graph won't change due to CYA since the HOCl/OCl ratio is independent of CYA.

You mention a 3-hump graph, but I haven't seen that (or don't recognize it by how you are describing it). Can you point me to a Internet website or other link where I can see what you are talking about?

If you wanted to see how HOCl levels change vs. pH with different levels of CYA, then perhaps the thing to do is to only show HOCl (forget OCl-) absolute levels at a certain Total Free Chlorine (FC) level and show multiple lines on the graph representing different levels of CYA. This would show how the S-shaped curve changes in angle (flattens out) at higher CYA which means that you can't change HOCl as readily by changing pH when CYA is present. The main problem with such a graph is that the scale when CYA is 0 is hugely different than the scale when there is CYA present. I could show the HOCl level (the Y-axis) on a logarithmic scale which would readily accommodate the 0 CYA and high CYA cases, but I don't think this is the "familiar" scale used on the graphs you are talking about.

[EDIT] Or I can just show two graphs, one without CYA and one with CYA (say, with a level of 30 ppm), and obviously the scales could be different for the two graphs and yet each have normal linear scales that people are familiar with. This sounds more like what you were asking for. [END-EDIT]

Of course, I can just come up with something and you can give me feedback -- that's what I've done with some of the other graphs in The China Shop.

Thanks,
Richard

[chem geek]

OK everyone. Here are some graphs I put together so now you can give me feedback as to whether this is what you are looking for. First, I show the traditional HOCl/OCl- relationship on the left including a total line (for HOCl + OCl-) that is always at 100%. It should be noted that the chart on the left is valid for any Total Free Chlorine (FC) level.

The chart on the right shows the same situation when there is 30 ppm CYA and in this case the Total Free Chlorine (FC) level matters and is 3 ppm for this chart though when CYA >> FC it is roughly the ratio of CYA to FC that determines HOCl and OCl- levels. Also note that the percentage of disinfecting chlorine (HOCl) at a pH of 7.5 fell from about 50% on the chart on the left to around 1.5% on the chart on the right. Note that the total HOCl+OCl level is not 100% when CYA is present. The difference from 100% (the Cl-CYA curve) is the amount of chlorine "bound" to CYA and though it is better protected from degradation from sunlight, it is also not immediately available for disinfection or oxidation (but is available "in reserve" as HOCl gets used up). Finally, notice how much "flatter" the HOCl curve is in the graph on the right indicating that the presence of CYA has made the amount of HOCl less sensitive to changes in pH (though we really need to look at a log scale for relative changes -- more on that next).



If we want to see changes in disinfecting chlorine (HOCl) in percentage terms, then a logarithmic scale is more appropriate so that equal distances on the chart represent the same relative amount of change. That is, it answers the question of how much improvement there is in the relative amount of chlorine when you lower pH. It is not quite as obvious in this graph, but the HOCl curve is a bit flatter on the right with CYA present, though at lower pH at around 7.0 the pH sensitivity of HOCl is about the same at 30 ppm CYA as it is with no CYA (and below 7.0 the pH sensitivity of HOCl is actually greater with 30 ppm CYA than with no CYA, but this is mostly due to the fact that with no CYA and at low pH most of the chlorine is already HOCl so there's no room for relative "growth"). Also note that at higher pH above 7.5 that the presence of CYA allows one to operate at higher pH without losing that much chlorine effectiveness (without CYA the effectiveness of chlorine drops rapidly above pH 7.5). The graph on the left cannot show the 0% flat line for Cl-CYA since it is off the chart (the 0% is at negative infinity since this is a logarithmic scale).



How's that?

P.S.
It is interesting to note that the traditional HOCl/OCl- graph with no CYA showed the large variation in HOCl percentage vs. pH, but that this was rather pointless (for pools; not for drinking water disinfection) because the absolute concentration of HOCl was typically so large that it didn't really matter if only 10% of the total was HOCl. The minimum HOCl concentration for preventing algae is on the order of 0.05 ppm (disinfection minimum is around 0.01 ppm) whereas even a pool with no CYA and a pH of 8.4 (which is only 10% HOCl) with even a low total FC of 1 ppm still gives 0.10 ppm HOCl which is double where we normally run our pools today when we use CYA!

Richard

[chem geek]

Bump, bump, bump...still looking for feedback. Are these the graphs y'all were looking for or do you want something else?

Richard

[waterbear]

still trying to digest them. I have to applaud you for all the work you are doing here!