Chlorine Loss?

[H2O_Keeper]

Shortly after closing my pool for the winter I got a gallon sample of pool water so I could play with my testkit which I was to get soon. Didn't order right away and I just got my TF100 and was starting to play and the first thing I did was the OTO for FC. It read zero.

Where I am confused is that I had a cheap pentair OTO and I was over 3 (strong yellow) when I tested when I got the sample. I did not have any algae or apparent CC. The jug sat in the shade in my poolhouse probably averaging 50 degrees over the last 7 weeks.

If I missed this in PS or another thread please advise. I thought FC would only break down by sunlight, bather load, or CC. Any idea how the FC got consumed in my jug (I rinsed with just water pretty good a used apple juice jug)? Noticed the PH is lower than I left it as well. Reading around 7.3 (strong 7.2) was at 7.8 with my pentair PH test when I did the sample back on 11/1/08.

Any idea>?

 

[chem geek]

If you had 4 ppm FC and 80 ppm TA, then dropping to 0 ppm FC would have the pH drop from 7.8 to 7.6. If the FC was higher or the TA lower, then that could explain at least part of the pH drop since the consumption of chlorine is acidic.

That is unusual for the chlorine to get used up quickly when the water is cold, but 7 weeks is not a short period of time. My FC in the pool which has an opaque pool cover will drop over time, but usually not more than 0.5 ppm FC per week at 50F. Since it was 7 weeks in your case, it's possible that the chlorine was slowly breaking down something in the water. The bucket would have to be exceptionally clean to not have anything to react with the chlorine -- the standard rinsing for glassware in experiments is usually at least 3-4 times (some say 7 times). The chlorine can also slowly outgas if the bucket isn't covered, but that wouldn't have the pH drop.

My best guess is that there were some minor contaminants in the pool water and possibly also a very small amount of apple juice left. 1 ppm in 1 gallon is about one-tenth of a drop.

If you repeat the experiment, use two similar containers rinsed out thoroughly and cover one of them. That will then distinguish between outgassing vs. breaking down from the contents of the water. You could also try a third container adding chlorine to distilled/filtered water instead of pool water to isolate that factor.

Richard

 

[chem geek]

I forgot to mention that chlorine also slowly oxidizes Cyanuric Acid (described in this post), but that should be pretty slow at cold water temps. For every 1 ppm CYA that is oxidized, this consumes about 2.5 ppm FC.

 

[H2O_Keeper]

Very interesting. Thanks for the tip. I may do a mini-study of a control group, covered, and uncovered to see if CYA was the reason for the breakdown. Thinking about this after your post the water may of been warmer most of the last 7 weeks than 50 degrees. In the midwest we have seen swings from 1 degree to 70 degrees (F) in this period. My poolhouse has no HVAC system so I dont know for sure what temp its been staying at.

My pool is at a CYA of 50 according to my new TF100, any reccomended starting point for the FC level to start at to observe FC breakdowns?

 

[chem geek]

If you go with 4 ppm FC with your 50 ppm CYA and you use a 25-ml sample size in the FAS-DPD chlorine test so you can measure within 0.2 ppm, then that should give you the ability to distinguish between different rates of chlorine usage.

I can't really tell you, however, when you'll see FC usage associated with CYA. Wojtowicz has some models and measurements, but they don't tie to what we see in this forum nor in my own pool. Some pools seems to have a greater chlorine usage than others, but it's hard to know if it has anything to do with CYA or something else.

 

[H2O_Keeper]

Have considered doing a rough experiment of tracking FC loss in a CYA sample and water sample per this post (no light). Ideally I would just put some CYA in distilled water but I used the last of my on hand supply a few months ago.

I tried to find on the net and TFP data on what happens to CYA at water boiling temps and was unsucessful. I was considering boiling my sample of pool water before adding FC to try and get rid of any contaniments. Anyone know if my CYA would change properties if I exposed it to 100 degrees celsius?

 

[chem geek]

I don't think that boiling is going to get rid of chlorine demand. If there are chemicals in the water that chlorine can react with, such as certain organic compounds, then that will just happen and boiling the water won't get rid of most of those organic compounds unless they're volatile. To have water without any chlorine demand, you'd have to start out with at least tap water if not distilled/filtered water and then add CYA and chlorine to it. Since you don't have the CYA, I wouldn't worry about it. Eventually we'll get enough data to sort this out, but it's not a huge issue.

 

[H2O_Keeper]

Well I just found some great articles about CYA and FC loss in the deep end....

I did not yet thoroughly digest it all but it appeared the threads I found all had sunlight involved (which is real world). I also do not have a SWG involved like some of the threads. Can you stir me in a direction if any studies have been done in the absence of light (pool water)?

I realize this is not real world but am curious what that rate is. THanks and Happy New Year!!!!!!!!

 

[chem geek]

You can contact the Journal of the Swimming Pool and Spa Industry where you can order the book "The Chemistry and Treatment of Swimming Pool and Spa Water" by John A. Wojtowicz. The address, phone number and FAX number may be found at the bottom of this link. I believe the book costs $40 (it may have gone up with current shipping costs). The specific paper is "Oxidation of Cyanuric Acid with Hypochlorite" described here (the book is a collection of the various Wojtowicz papers).

The summary of the data in his paper that I wrote in this thread would indicate a faster chlorine loss than is seen in many pools. My best guess is that such loss by oxidation of CYA is very dependent on other subtle factors in the water that may act as catalysts for this reaction. See this thread for one such discussion about iron though the levels in the pool are typically far smaller than referred to in the linked article.

Richard

 

[dschlic1]

I have been told that plastic bottles, even if extremely clean will have a small chlorine demand. It appears that the chlorine reacts slightly with plastic. I found this out while investigating sources of chlorine standard solutions.

 

[budster]

Why would one try to test anything but "fresh" pool water, and not use as pristine as possible container (like glass)? I'm really impressed with the back-and-forth, and with the great (seriously) information, but one of the basics is to test fresh, frequently, and be sure your sample is not contaminated with container residue.

 

[Guest]

Why would one try to test anything but "fresh" pool water, and not use as pristine as possible container (like glass)? I'm really impressed with the back-and-forth, and with the great (seriously) information, but one of the basics is to test fresh, frequently, and be sure your sample is not contaminated with container residue.

I have to second this! A lot of people seem to forget we are talking about swimming pools and not science experiments. Some people get so caught up with the results of their tests and forget to consider the precision of the test kits we use also.
If you are testing your pool (or spa) on a regular basis and getting constant, repeatable results then you are doing fine!
I guess the test kits satisfy a need to play with a chemistry set like when we were kids! :-D