Chlorine Demand from Ascorbic Acid Treatment

NHPool

Member
Jun 22, 2010
5
New Hampshire
Ascorbic Acid treatment was successful.

All numbers look good (except CYA high at 80), but I can't hold FC -- it drops about 2 ppm per hour.

I put in 2 pounds ascorbic acid, and 10 pounds citric acid, into my 30,000 pool.

How much chlorine will it take to burn out any remaining ascorbic/citric acid? I read that it takes 2.5 pounds of ascorbic acid to de-chlorinate 1 pound of chlorine, which would mean I'd only need 5 pounds of chlorine...but I've added about 14 pounds of chlorine so far.

Does the answer depend on my 80 CYA level? I understand that higher CYA levels can make chlorine less effective, but I am not sure whether CYA level is relevant in this case. If the high CYA level is the problem here, then could I need 50 pounds of chlorine (10x the scenario above)?

Does it matter whether how I add the chlorine - a little every day vs. a big shock - in terms of wiping out any remaining ascorbic/citric acid?

Thanks!
 

JasonLion

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May 7, 2007
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There are several different things consuming chlorine: ascorbic acid, citric acid, sunlight, and organics, so it is impossible to put a specific number of the total amount of chlorine you will need. Your calculations are also off because you didn't take the citric acid into account. Citric acid will also consume chlorine.

You want to add chlorine slowly to prevent the stains from coming back. If it wasn't for that, you could add the chlorine much more quickly. Too much chlorine added at one time will cause a short term increase in the FC level which could cause the stains to come back, even if the chlorine does all get consumed in under an hour.
 

chem geek

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Mar 28, 2007
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The CYA level doesn't matter in figuring total quantities. The CYA just slows reactions down but the chlorine reaction with ascorbic and citric acids is fairly fast in spite of CYA in the water. 2 pounds of ascorbic acid in 30,000 gallons would require around 3.2 ppm FC to get quickly consumed. The 10 pounds in 30,000 gallons of citric acid can also be a reducing agent reacting with chlorine and would require around 14.6 ppm FC assuming you used the anhydrous form of citric acid. So that's a total of around 18 ppm FC. That's almost 9 gallons of 6% bleach or 7 pounds of 65% Cal-Hypo or 5 pounds of Trichlor or 8 pounds of Dichlor.

So I don't know why it's taking more chlorine in your situation. Make sure you aren't bleaching out your chlorine test. Is there a reason why you are using pounds of chlorine which I presume is something like Cal-Hypo instead of using chlorinating liquid or bleach?

(Jason, the molecular weight of citric acid is only a little higher than that of ascorbic acid and each is equivalent in terms of consuming the same amount of chlorine on a molar basis. The strength of the reducing agent should not affect its quantity; it would only affect its ability to react with certain chemicals)
 

NHPool

Member
Jun 22, 2010
5
New Hampshire
Jason, I tried to take both the 2 lbs ascorbic and 10 lbs citric into account. 12 pounds divided by 2.5 = 5... in any case, this all assumes that the 2.5 ratio is right....

But Richard seems to have the calculation to estimate 18 ppm of FC to knock out the 12 pounds of acid.

I've put 12 bottles of 12% liquid chlorine into the pool over the past 5 days...which is a lot more than 18 ppm.

This suggests something else may be going on, correct? I also used 3 pounds of chlorine neutralizer (more than I needed), which would add to chlorine demand...perhaps 1.5 pounds wasn't necessary.

I also put a quart of Metal Magic in and I read that can have its own demand.

I am using the drop test from TF-100 so the chlorine test isn't bleaching out.

Finally, the pH was riding a bit low for the days after the AA treatment...which could have made the chlorine less effective?

In any case, let me know what you think about this plan...

1. 1 gallon 12% liquid every morning and every evening until FC starts to hold
2. after 9 more days (which will take me to 2 weeks), bring the pool to FC shock levels (per the CYA table) and keep adding chlorine to maintain shock levels

This is a ton more chlorine, but not sure what else to do.

Alternatively, over the next 9 days I could just add 1 gallon per day and put in some algaecide to protect me from FC=0...

Thoughts?

Brad
 

JasonLion

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May 7, 2007
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Ideally, you should add chlorine more than twice a day, and use less each time. If you are consistent at that, I doubt there will be any need to shock. Algaecide is a good idea in this situation, especially if you haven't already put some in.
 

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