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Homework Help: Help with Calculating PPM of Chlorine

  1. May 20, 2015 #1
    We are currently undertaking an investigation into chlorinated pool water and how various factors can affect the quality of it. We are attempting to determine the amount of "free chlorine" (HOCl/OCl-) in a given sample of pool water.
    This pool water is labelled as 200 ppm, and is "stabilized" with an unknown amount of cyanuric acid. We do know that the water is chlorinated with sodium hypochlorite.

    We have tried to determine the amount of free chlorine in solution by using methyl orange. Acidified methyl orange (red) reacts 1:1 with HOCl/OCl-, and turns clear. The experiment we conducted is as follows:
    2.00mL of methyl orange (0.5g/L or 0.00153mol/L) was put in a reaction vessel
    HCl was added until the solution turned red
    Pool water was titrated into the solution until it turned clear

    The experiment was repeated 3 times, and on average 0.14mL of pool water turned the solution clear.
    However, when we try to use these results and find out what the ppm of pool water is, we keep getting around 775ppm, which seems way too high. We calculated it as followed (this method was given to us in an experimental booklet):
    ((concentration of methyl orange)*volume of methyl orange/volume of pool water)*35.5*1000

    Is this sort of discrepancy normal? If so, why? If not, what and where could we have gone wrong and why?
  2. jcsd
  3. May 20, 2015 #2


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    Staff: Mentor

    Doesn't look normal, but the general idea and your calculations look reasonably correct.

    Perhaps 200 ppm is a concentration of "free" chlorine, one that is not bind with cyanuric acid? But that's just a random guess, I know nothing about the pool chemistry.
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