Ascorbic Acid and Chlorine

Homebrewale

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Apr 21, 2020
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Holly Springs, NC
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I tried to find in old threads the amount of bleach needed to neutralize an amount of ascorbic acid. For example, how many gallons of 10% bleach are needed to use up 1 pound of ascorbic acid?

I tried the ascorbic acid method for getting rid of rust stains. I lowered the FC to zero and pH to 7.2 before I started. I added 2 lbs of Vitamin C powder. After I left all that circulate in the pool for a day, it was time to start increasing the FC and pH. For the FC, I added a gallon of 10% Pool Essentials bleach on Sunday afternoon. I tested Sunday evening and the FC was still 0.0. I expected that result. I added a second gallon Sunday evening. On Monday evening after work, I tested for FC. Again it was zero. I added a third gallon of bleach. On Tuesday morning before work, I tested the FC and it was 2.0 ppm. I thought maybe I finally neutralized the ascorbic acid. I added my 4th gallon of bleach to add another 5 ppm to the water to get to 7 ppm. After work on Tuesday, I tested FC and got 1.5 ppm. I guess I wasn't done yet. I added my 5th gallon now. This morning, the FC was 7.5 ppm. It looks like the process is finished.
 

JoyfulNoise

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May 23, 2015
21,405
Tucson, AZ
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Sounds about right ....

2 lbs of ascorbic acid in 20,000 gallons of water is about 12ppm. It takes 3.7 parts of AA to neutralize 1 part chlorine so your first gallon of 10% should have handled it. However, your liquid chlorine may not be totally fresh and, if the AA process resulted in some "biological growth", then the FC you add is fighting both ascorbic acid AND anything that might be growing in the water. So, in reality, it can take a lot more chlorine to establish a residual than just what would be predicted from the standard neutralization reaction.
 

Homebrewale

Silver Supporter
Apr 21, 2020
891
Holly Springs, NC
Pool Size
22000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair iChlor 30
I'm not sure if there was any biological growth since the water temperature was 63F and I added Polyquat 60 before I started the process. What I didn't say in my first post is that I added some citric acid to the water before I started. The FC was 10 ppm at the time. I had two packets to citric acid lying around. I don't know how much was in each one since I didn't weigh them. I would guess 3 to 4 ounces each. Since I needed the FC to be zero and citric acid also removes rust, I decided to use one packet to get rid of the FC in the water. When it dropped the FC but not to zero, I added the second packet. I didn't think there was much residual citric acid left in the pool but there may have been more than I expected.

As for the chlorine, it wasn't the freshest but at the same time not real old. It was bottled on 12/06/2021 - about 4 months old.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
21,405
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
I’d have to look it up but I’m pretty sure citric acid requires more chlorine to neutralize it than ascorbic. 4 month old 10%, even when stored correctly, is probably closer to 8.5%. Polyquat-60 also causes a small FC demand too because chlorine will oxidize it.

All in all, I think you used as much chlorine as can be expected. The downside of the AA treatment is that it zeros out the chlorine and takes time to slowly reestablish the residual. It’s that time period of low FC that can lead to algae outbreaks and then completely reverse all the hard work that was done removing stains.
 
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Homebrewale

Silver Supporter
Apr 21, 2020
891
Holly Springs, NC
Pool Size
22000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair iChlor 30
Let me just say it was a waste of time. The ascorbic acid didn't touch the rust stains. Even though I didn't do the Vitamin C tablet test, I know the stains were rust stains. They were caused by a sloppy roofer who put a few nails in the deep end of the pool. That was 3 years ago and I ignored the stains. I thought I would try since I'm selling the house in July. Maybe these stains are like grease stains in a shirt. If you wash the shirt right away, you can get the grease stain out. If you leave it for awhile, it gets embedded in the fabric. Maybe the rust stains are just now part of the vinyl and won't come out.
 
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