Chlorine/pH/CYA Relationships

[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!

[PoolDoc]

Very interesting, very informative, and *definitely* something that I'll need to spend some time thinking about . . . once I have some time.

But, it still all goes into the China Shop*. Richard, please remember (or you may not know) that probably fewer than 10 regular users on the Forum are able to BOTH understand the data you've presented AND work out its practical implications. My guess is that less than 5% of the people who've read your post so far noticed the log scaling you used on some of the data, and less than 1% can comfortably and easily interpret data presented on a log scale.

Because of the complexity of the data, the tendency will be for some to fixate on some data point or other, make an application from it, and then that will become their 'pool Gospel'. This is an outcome I fervently hope to be avoid!. The last thing I want to do is complicate people pool lives, or to create yet another 'must adjust' parameter, like has been the case with TDS.

Even for me -- and as you know, I was already generally familiar with the data behind your graphs, even before you developed your spreadsheet -- it will take some time for me to digest all data so neatly contained in your graphs, and to think through what the useful and helpful implications of that data are.

Beyond that, we all need to think carefully about how this data can be presented so that it's not only accurate, but so that it will usually be interpreted and applied correctly and helpfully by folks who found algebra a trial!

Meanwhile, it needs to rest in the China Shop, where all us 'pool geeks' can digest and apply it. It's something worth doing! The Cl2/HOCl/-OCl chart has done a world of damage, and it would be a huge shame if careless presentation of this data prevented it from helping to correct that damage!

Sincerely,

Ben

* I need to rename the China Shop, or maybe make a parallel section named something like "For Pool Geeks Only", to distinguish arguments from technical discussions.

[milynor]

WOW! Taking me back to chem101! Still don't understand it all, but I'm working on it. Is the chlorine "used up" faster with lower stabilizer levels with normal PH (7.5)?