Chlorine Locked?

[chem geek]

It's too bad we don't yet know if sodium percarbonate will work at this late stage. Otherwise I'd suggest it. Does the pool store near you carry any small containers of ProTeam® System Support? Or can you order a small jar from The Chemistry Store. You could then try a small amount in a 2-gallon bucket of your pool water -- about 1/8th of a teaspoon -- to see if it can oxidize the CC.

Another alternative would be your own combination of sodium carbonate with hydrogen peroxide since sodium percarbonate is technically one part sodium carbonate with 1.5 parts hydrogen peroxide. By weight, 1 pound of sodium percarbonate is 0.675 pounds sodium carbonate and 0.325 pounds hydrogen peroxide. So you can use pH Up product or Arm & Hammer Super Washing Soda for the sodium carbonate. This would then be closer to 1/16th of a teaspoon of pH Up / Washing Soda. As for the hydrogen peroxide, you can easily get 3% solution from a drug store (though you probably have more concentrated as oxidizer for your pool, we don't know its concentration for sure). The amount needed is 1 teaspoon.

If you have FC from chlorine, then the hydrogen peroxide will neutralize that so we'll need to increase the dose accordingly. The rough rule-of-thumb is that a volume of 3% hydrogen peroxide neutralizes the same volume of 6% bleach. So for the 2 gallon bucket, 10 ppm FC gets neutralized by 1/4 teaspoon of hydrogen peroxide.

So let's make this simple and have you add pool water to a clean 2-gallon bucket. Wait one hour and then measure FC and CC. Add 1/8th of a teaspoon of pH Up / Washing Soda, mix, then add 2-1/4 teaspoons of 3% hydrogen peroxide and mix. Then wait for 1 hour. Then measure the FC and CC. Wait for 8 hours and measure again. Wait for 24 hours from your initial addition and measure again. This dosage is double the amount recommended by Orenda, but should tell us whether or not sodium percarbonate will work. You should definitely see the FC going to 0, but we'll see if the CC drops as well. If it does drop enough, then we'd add chlorine back into the bucket using 6% bleach, 1/4 teaspoon at a time, until FC registers again and make sure that CC doesn't return.