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Name: James 
Status: educator 
Grade: 9-12 
Location: Outside U.S. 
Country: USA 
Date: Summer 2010


Question:
After applying citric acid to pool water, 50lbs, to remove stains, and returning the pH to a range of 7.2-7.8 with sodium bicarbonate 36, lbs and sodium tetraborate, 8 lbs, can the citric acid still neutralize the chlorine? The alkalinity was reading at 400+. We have added 25+ lbs of hypochlorite three times, within 24hrs all chlorine is gone. I plan to use this expensive experiment as a lesson in my middle school science class. Your suggestions and help are appreciated.



Replies:
Acid, any acid, neutralizes hypochlorite by shifting its equilibrium-with-H2O towards being elemental chlorine Cl2, which evaporates out of the pool right away and is faster to react with any remaining organics in the water.

Pool Chlorine is : x H2O + Na(+)aq + OCl(-)aq
OCl(-) + H(+)  <-->  HOCL 
OCl(-) + 2 H(+) + Cl(-)   <-->  H2O + Cl2(gas) 

Citrate ion neutralizes pool chlorine in a different way, simply by using it up. Citric acid is an organic molecule, a fuel, a reducing agent. Sugar would be too, and Tartaric acid, and even oxalic acid. Citric acid or citrate both add to the chemical oxygen demand level of the water. ("COD" as opposed to BOD, biological oxygen demand)

Chloride or sulfate ions are not easily oxidized, which is probably why Muriatic acid and Sulfuric acid have been the traditional pool acids.

I found some advice on the web ( www.pool-guy.com/pool_spa_faq_tips.htm ) which indicated that, yes, a citric acid pool treatment will clean well but then it absorbs a lot of chlorine. How much one must add to overcome this was not said. I do not know my organic chemistry well enough to tell you the first reaction when hypochlorite attacks citrate. I also do not know whether a citrate ion consumes one hypochlorite ion and then slows down, or consumes several until completely oxidized to CO2 and water, or something in between.

The advice also said that citric acid treatment should only be done in winter. It seems plausible to me that the reaction rate of citrate with hypochlorite decreases dramatically as the temperature declines. In any case, your pool will survive the low chlorine levels much longer in the winter. In summer the algae would take advantage too fast.

From the amount of citrate you used, I guess you are treating a junior-Olympic size pool. If smaller, perhaps you overdid the citrate, thereby making the price you must pay to consume all the citrate and re-establish normal chlorination levels higher than expected.

I also wonder if the chlorine stabilizer in your water helps or hurts while you are trying to do this titration. Imagine that a stabilizer molecule traps an elemental chlorine atom, but preserves its oxidizing potential. Then an equilibrium ensues, with a small percentage of the chlorine "free". If something consumes the free chlorine, dissociation of more chlorine from the stabilizer molecule may be kind of slow. It is possible that citrate consumes the chlorine faster than the stabilizer releases it. The free chlorine levels would be low during that time. Then when the citrate is done being consumed, if you had lots of chlorine stored up in the stabilizer, the free chlorine level might rebound to a slightly high level and require largish quantities of dechlorination or time to come down.

Or conversely, you had 100ppm of chlorination saved up, hidden in the stabilized form, and all that got borrowed to use up the citrate, and now you have to pay it back. It might be an amount equivalent to several normal chlorinations.

I have not figured out why citric acid is supposedly a good thing in pools, other than it may be a mild chelating agent to help pick up some metals such as iron, which would make it good for the acid-cleanup phase of treatment. But then it seems to me it would make more sense if one intended to throw out the iron-contaminated citrated water and replace it with new water, which I realize is expensive. So I am not sure whether everybody who uses citrate has given in and made the necessary huge additions of chlorine, or what.

cordially,

Jim Swenson



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