Under strength chlorine

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stev32k

stev32k

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LifeTime Supporter
Mar 29, 2009
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Mobile, Alabama
I've been having problems keeping my FC where it should be for the last two weeks. It should be 4.5 - 5.0 but has been 2.0 - 2.5. I have a Stenner pump that has been is service for about 2 months and until the past two weeks it has done a wonderful job. When the FC dropped I put the outlet tube without the check valve in a bottle for an hour and measured the discharge rate. It was right where it should be. Then I removed the check valve attached the discharge line and measured again and again the flow was right where it should be. From those measurements it appears the pump, inlet strainer, and check valve are working like they should. The pool water is still clear as clean glass (I add chlorine manually to keep the levels up) with no hint of algae and there is no combined chlorine in the samples.

The only other thing I can think of that would cause the low levels of FC is the chlorine being under strength. I buy liquid chlorine that is labeled as 10.5% in 2.5 gallon carboys. I've used the same stuff for several years without any noticeable problems, but now I think the pool store has bought some under strength chlorine, or is diluting the material, or maybe it is just old. Whatever the reason I want to test the strength.

The only testing I can do is with the TFP test kit. So is there a procedure I can use to tell me if the liquid is badly under strength. There is a commercial analytical lab it town I could use, but would like to do some preliminary testing before taking samples to them.
 
The other possibility is that as summer progresses you need more chlorine because the UV rays are stronger, thus consuming more of your chlorine. I have to slowly adjust my pump up as the spring turns to summer and down as summer turns to fall. Slowly up, then slowly down.

Are you using bleach, or 10/12.5% chlorinating liquid?
 
Not sure about testing the chlorine itself, but many people are in the habit of looking closely at the bottle for its manufacture date. While you confirmed you have clear water and no CCs, you are still losing FC. Can you confirm a couple things to help us answer better?
1 - What is your current CYA?
2 - At what time of day are you reading your FC levels?
 
As stated above I am using 10.5% chlorinating liquid that comes in 2.5 gal carboys. The carboys are filled from a bulk tank on the pool store site. There is no way to know how long the liquid has been in the tank. My current CYA is 40 and I have been testing FC three times per day, morning, noon, and night. The Stenner pump runs when ever the main pump runs and the FC concentrations stays very constant thought out the day.

I can raise the FC level to 4.5 if I increase the pump output to a rate of about 3.5 gallons per day. I've had the pool for 18 years and have never used anything close to that amount unless I have to shock.
 
Agreed that consuming over 3 gallons per day is excessive. Only 3 things come to my mind in your situation:
1 - Finding-out if there is a way to test that chlorine source; if you cannot, consider turning it off completely for a day or two and rely solely on manual feeding to see if there's a noticeable difference
2 - Perform an OCLT (I didn't see that noted above) to rule-out performing a SLAM
3 - Consider increasing CYA to see if that helps protect your FC
 
I found this on an OLD thread:

If you can measure 100mls, do your dilutions this way:

Measure 100mls of water and place in a cup.
Using your 1ml syringe, remove 1 ml of water
Add 1ml of your "in question" Bleach solution and mix well.
This is your first 1:100 dilution

To another cup, add 100mls of water.
Using your 1ml syringe, remove 1 ml of water
Add 1ml of your 1:100 dilution solution from the above step and mix well.
You now have your 1:10,000 dilution needed for your test

Hope it helps!
 
The procedure in the previous post is better if you have equipment to measure ml accurately (say, pipettes). Otherwise you can do the following.

1/4 teaspoon in 2 gallons will give an FC that is (10,000 gallons)/(128 fl.oz./gallon)/(6 tsp/fl.oz)/(4 1/4tsp/tsp)/(2 gallons) = 1.63 times higher than the Trade % of the chlorinating liquid. So if it's really 10.5%, then you should measure 17.1 ppm FC. Obviously you need to measure this VERY carefully or else you will introduce significant error. The bucket must be clean or else the chlorine will get consumed from substances leftover in the bucket. Repeat the experiment a few times to get a rough idea of your variability from such errors (though that doesn't catch bias errors).
 
I don't have any volumetric glassware or even a bucket that I could halfway measure 2 gal accurately. I do have a pretty good digital scale that will read to 1 mg with a total capacity of 305 g. So maybe I could do the dilutions on a weight basis. My wife gives herself allergy shots with a 0.5ml syringe so I could use one of those to make the serial dilutions. I also wonder if I could use weight alone to get an approximate strength? The specific gravity of 10% hypochlorite is 1.15g/ml and my scale should be fairly accurate at that level if I use a 0.5ml syringe to measure the volume. Would that work?
 

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I had to return some bleach yesterday that was flat. I found the markings and it was almost 1 year old.

It was the Walmart generic bleach...I was VERY surprised to see that it was not fresh. In the middle of a slam
now and had to return 10 bottles...the pain!
 
stev32k said:
I don't have any volumetric glassware or even a bucket that I could halfway measure 2 gal accurately. I do have a pretty good digital scale that will read to 1 mg with a total capacity of 305 g. So maybe I could do the dilutions on a weight basis. My wife gives herself allergy shots with a 0.5ml syringe so I could use one of those to make the serial dilutions. I also wonder if I could use weight alone to get an approximate strength? The specific gravity of 10% hypochlorite is 1.15g/ml and my scale should be fairly accurate at that level if I use a 0.5ml syringe to measure the volume. Would that work?
Click to expand...

I don't think it will be that easy since the degraded chlorine becomes salt and will still be in solution. The specific gravity will remain very close to the same. I would add a measured amount of the chlorine to a known quantity of water and then test the result. For example, a standard 64 oz. juice pitcher could be filled with water to the 64 oz. mark and then add 360 mg (0.36 g) of chlorine and it should test as 20 mg/L.
 
Here is a follow up. I used epro's method of determining chlorine strength and was pleased and surprised at how repeatable and how accurate it seems to be. I bought 4 new carboys of 10.5% chlorine yesterday and tested the strength twice. Both times I got 10%. I was so surprised by the first test reading 10% that I repeated it and got 10% the second time. Next I tested the chlorine in my storage container and got 6%. That's quite a difference so I repeated the test and got 5.5% the second time.

The Stenner pump was started up on May 4 and I added 20 gal of 10.5% chlorine to a plastic storage container. Since then I have added another 10 gal to the tank. So some of the chlorine has been in the tank for 7 weeks. The tank is dark, has a tight fitting lid, and is located in complete shade. The day time outside temperatures have been in the high 80's to low 90's for just about the whole 7 weeks. So does it seem reasonable that chlorine could have degraded to nearly half strength in that period of time under these conditions?
 
YOu said that yoru stenner is at 3.5 gallons per day, and runs when your pump runs. How many hours does yoru pump run per day. For example, if you pump only runs 8 hours per day, then you stenner is at abotu 1 gallon of FC actually added not 3.5 gallons. While 1 gallon is high, it is closer to normal.
 
A more detailed chart I made showing chlorinating liquid and bleach degradation is in this post. You can see that for 12.5% chlorinating liquid at 90ºF average temperature it degrades to 8.36% in one month (4 weeks) and 6.73% over two months (8 weeks). So the degradation rate you are seeing is higher than with high-quality chlorinating liquid. Also, the 90ºF temperature you are reporting is daytime temperature, but at night I expect the temperature to be lower so the degradation rate should be even less.

Even looking at the Odysee Manufacturing link provided in the previous post, 15% chlorinating liquid at 90ºF has a half-life of 48 days so would get to 7.5% in about 6 weeks. 12.5% chlorinating liquid would have a longer half-life since it varies roughly as the square of the concentration.

I suspect that your chlorinating liquid has some metal impurities in it since metal ions act as catalysts to degradation. You might contact the manufacturer and let them know your results.
 
Hmm, after looking at the degradation charts I believe I will cut down on the volume in the storage tank. I've been trying to keep about a 6 week to 2 month supply, but maybe I should not store more than about a 1 month supply. I had no idea the chlorine would degrade that fast.

Edit: It just occurred to me after looking at the degradation charts again that maybe diluting the 10% to 5% would eliminate a lot of the chlorine loss. My tap water seems to be pretty good quality and the test reports the water authority issues never shows any heavy metals. I wonder if that would really reduce the losses?
 

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