Superchlorination & CYA

[sbcpool]

After discovering that I had a mustard algae problem ("hey, why does this algae keep showing up only in the shady areas even after I brush it off and keep FC good?"), I started the recommended SLAM procedure using levels for mustard algae. When I started the SLAM my CYA was 40, and it has been 40 since I got my house. After 24 hours of SLAM levels my overnight FC loss was less than 1 mg/L and the pool was clear, but I thought I'd run it another 24 to be safe. The next day (the 2nd SLAM day), I noticed that I'd lost about 8 mg/L of FC and it was only noonish. I added some more to bring it back up. Same thing the next night - FC loss under 1 mg/L, but when I tested in the afternoon I had lost over 10 mg/L FC.

Now I'm scratching my head and thinking, "why isn't the CYA protecting the chlorine?" So I do a CYA test. ZERO! Bubkiss, notta, zilch. I run another one. Same result. I fill the tube to the top and I can still make out the black dot. It's very faint, but I can make it out.

So, assuming that I didn't accidentally discover that you can destroy CYA with 20+ mg/L FC, what could be going on? Do high levels of HClO affect the CYA test?

 

[chem geek]

It takes a combination of very high FC with high pH (approaching 9) for chlorine to oxidize CYA so it's possible but we usually don't see it happen. Did you lower the pH before doing the SLAM? As described in Degradation of Cyanuric Acid (CYA), It takes 2.5 times as much FC to oxidize an amount of CYA. So it should take 100 ppm FC to get rid of 40 ppm CYA and does not seem that you used anywhere near that amount of chlorine.

So instead I think your other explanation is the correct one which is that high levels of chlorine do in fact affect the CYA test. The chlorine bound to CYA doesn't precipitate with melamine. If you add sodium thiosulfate (R-0007, but not sure of how much to add) to your water sample before adding the melamine reagent then you should be able to measure the true CYA level.

Also, the high loss rate during the day is also explained because the high hypochlorite concentration breaks down in sunlight more quickly. So the higher FC and especially the higher pH has a much higher hypochlorite ion concentration that is most sensitive to sunlight with a half-life of only 20 minutes (in depth of a pool it may be more like 40 minutes). With the high SLAM level more of the chlorine will be unbound.

With a very high SLAM level as with yellow/mustard algae, it is important to lower the pH a lot, down to at least 7.2. That reduces how high the pH will get and will reduce the rate of loss from sunlight and also has more hypochlorous acid to kill the algae. Be sure to read the Pool School article on yellow/mustard algae that has you get behind light niches, etc.

 

[sbcpool]

It takes a combination of very high FC with high pH (approaching 9) for chlorine to oxidize CYA so it's possible but we usually don't see it happen. Did you lower the pH before doing the SLAM?

I did, but I've been having a lot of trouble keeping my pH down. I've added about 8 gallons of MA in the last month and it still keeps rebounding to a range above what the test kit can measure. I had initially thought it was a TA issue, but my TA has remained constant.

So instead I think your other explanation is the correct one which is that high levels of chlorine do in fact affect the CYA test. The chlorine bound to CYA doesn't precipitate with melamine. If you add sodium thiosulfate (R-0007, but not sure of how much to add) to your water sample before adding the melamine reagent then you should be able to measure the true CYA level.

I was wondering about doing that. I was just going to use H2O2 to destroy the FC in the sample prior to testing.

 

[chem geek]

Using hydrogen peroxide to get rid of the FC should be fine. Test to see that the FC is indeed 0 after using the hydrogen peroxide. I don't know how long that takes so that would be good to know (I think it's pretty fast).

As for your use of lots of acid but not having the TA drop over time, that means the source of pH rise is not from carbon dioxide outgassing, but something else like curing (or degrading) plaster. If you have a lot of evaporation and refill with water high in TA, then I suppose the TA could remain stable in spite of carbon dioxide outgassing and acid addition, but this seems less likely.

 

[sbcpool]

Using hydrogen peroxide to get rid of the FC should be fine. Test to see that the FC is indeed 0 after using the hydrogen peroxide. I don't know how long that takes so that would be good to know (I think it's pretty fast).

Above pH 7.0 it's just about instantaneous. I use this trick to chlorinate filtered water when hiking and then remove the chlorine taste before drinking. The filter removes the really hard to kill bugs and bacteria and viruses are wiped out by a couple mg/L of chlorine. After a few minutes I add a couple of drops per gallon of H2O2 and the chlorine taste disappears.

As for your use of lots of acid but not having the TA drop over time, that means the source of pH rise is not from carbon dioxide outgassing, but something else like curing (or degrading) plaster. If you have a lot of evaporation and refill with water high in TA, then I suppose the TA could remain stable in spite of carbon dioxide outgassing and acid addition, but this seems less likely.

Given the condition of the pool I'd say it's the plaster degrading. There has to be a better way than adding two gallons MA per week.

 

[chem geek]

Unfortunately, if the pH rise is from calcium hydroxide or calcium carbonate getting into the pool from degrading plaster, there isn't anything you can do to compensate for that chemically other than adding acid. Have you noticed the CH climbing over time (more than would be accounted for by the increase from evaporation and refill with CH from fill water less the decrease from water dilution)?

 

[sbcpool]

Have you noticed the CH climbing over time (more than would be accounted for by the increase from evaporation and refill with CH from fill water less the decrease from water dilution)?

That's the one test I can't perform myself because of abnormal color vision. Getting someone to assist with that test has been problematic.

I've been wondering if I could add some food coloring to the test without affecting it too much. Hopefully I could find a combination that would make the color change more apparent.

 

[chem geek]

Yeah, that red to blue change is subtle and hard even for those with good color vision because it's really more of a purple than a clear red. It's not nearly as obvious as the green to red transition of the TA test.

I wouldn't add food coloring, but what you can try doing is looking at the sample against a strong yellow background. When the color is a red/magenta/purple, you should still be able to see the red show through (as a dark red that is no longer purple), but when it switches to blue then it should look almost black. So hopefully that lightness difference will be enough to be obvious to you.

 

[sbcpool]

That gives me an idea. I have these shooting glasses that are really strongly tinted yellow. I will try running the test with these glasses.

 

[chem geek]

Let us know how that works out.

 

[sbcpool]

Well, I couldn't find the yellow glasses but I did find some deeply red tinted glasses that came with my laser level. The calcium test appears clear when it's red and turns dark when it turns blue. The problem is that there isn't a dramatic change in color. It's more of a fade from red to purple (according to the wife) to blue (and that was with adding 8 drops of R-0012 first - without that I just get purple-pink blobs), and each step is small so it becomes difficult to tell where the exact cutoff is. Even with the glasses the change in color was just a tiny bit with each drop to the point that I couldn't tell if I was just imagining the slight shift. There has to be a better way to test for calcium.

 

[chem geek]

That sounds like a fading endpoint though your adding the R-0012 drops first is supposed to help with that. Are you using a 25 ml sample size or a 10 ml sample size? The transition will probably be easier to see with the 10 ml sample size though your resolution will be 25 ppm instead of 10 ppm, but that should be good enough.

 

[sbcpool]

I am using 10 mL. I wonder if there is any relatively easy way to precipitate the Mg out of solution while preserving Ca, but since they're both alkaline earth metals I'm not sure how that would work.

 

[chem geek]

The Calcium Hardness (CH) test from Taylor already does exactly what you propose. The R-0010 Calcium Buffer is sodium hydroxide that precipitates magnesium as magnesium hydroxide. This is how the test is able to measure Calcium Hardness. Without the R-0010, you would be measuring Total Hardness instead.