Hi,

I added citric acid to remove some rust stains and it worked great. I had brough my chlorine level up to 25 before hand knowing that it was going to drop and I did not want it going to zero like the last time I did this. Its been about 4 days now since I added it, the pH is back up to normal (I added some baking soda yesterday to get it back in range), but my chlorine is being consumed quite rapidly.

I shocked over the weekend to be sure. Sunday night I passed the overnight FC test. FC was 33.5 at night and 33.0 in the morning. That afternoon it was down to 25. Of course, I checked CYA and it is at 70 which I expected because I had just brought it up to there from 50 a week before.

So at this point I still going to assume the worst and keep shocking, but just curious how quickly will the citric acid deplete the chlorine? I would think as soon as it got mixed into the water, but ?????

At high FC levels you lose FC very fast due to the sun. Order of magnitudes faster than at normal levels. If you passed the OCLT that says that it's not the citric acid doing it. I'd allow the FC to come back to normal levels and see what happens. I bet your loss returns to normal.

Good point. Glad I asked the question here because I forgot about that minor detail. I'll let it come down to 8 and see where it goes from there.

As described in this paper, chlorine oxidizes citric acid to produce significant amounts of chloroform (a trihalomethane, THM), especially at lower pH, so it is better to use ascorbic acid instead even though it is more expensive. Another alternative is oxalic acid, but one has to be careful to not overdose or use it in high CH conditions due to the precipitation of calcium oxalate (i.e. it's OK as a local spot treatment, but not so good for a broad amount -- probably no more than 1 ounce per 10,000 gallons). If you are only using the citric acid for a single treatment and keep your water exposed to air, then that's probably OK, but I would not use citric acid for repeated multiple treatments.