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Titration of ASA

  1. May 30, 2009 #1
    1. The problem statement, all variables and given/known data
    Why would methanol be a better solvent for ASA compared to ether or chloroform, when NaOH is used for titration?

    I think I have an idea why:

    NaOH is only soluble in polar solvents. Methanol is polar, while ether and chloroform are not. . . .

    Does this have anything to do with it.
    Last edited: May 30, 2009
  2. jcsd
  3. May 30, 2009 #2


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    Do you want us to guess that ASA symbolizes acetyl salicylic acid?
  4. May 30, 2009 #3
    Yeah, I know it's an acid. . .

    "The CTC handbook lists ASA to be soluble with alcohols, ethers and chlorofoam"

    If the solvent is not a problematic variable, then it has to be the titration substance (NaOH)

    For this particular lab, NaOH is the reason ethers and chlorofoams are not acceptable as solvents . . .
  5. May 30, 2009 #4
    Phenolphthalein was used to indicate the moment the solution became basic.

    Phenolphthalein is soluble in methanol, but is it soluble in chloroform or ether?
  6. May 30, 2009 #5


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    Phenolphthalein is pretty well visible in very low concentrations, so its solubility is not a factor here (it is not a factor even during titration in water).
  7. May 30, 2009 #6
    From wiki

    "Phenolphthalein is insoluble in water, and is usually dissolved in alcohols for use in experiments. It is itself a weak acid, which can lose H+ ions in solution. The phenolphthalein molecule is colorless. However, the phenolphthalein ion is pink. When a base is added to the phenolphthalein, the molecule ⇌ ions equilibrium shifts to the right, leading to more ionization as H+ ions are removed. This is predicted by Le Chatelier's principle."

    It says it is usually dissolved in alcohol (such as methanol). There has to be a reason for this.
  8. May 30, 2009 #7


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    An obvious one - it has to be added to water solution, but it is insoluble in water. Hence it is dissolved in methanol (or ethanol) and you add it to water in form of alcoholic solution. This way after dilution of alcohol you end with water pheolpthalein solution, could be also with a very fine supsension. That's enough.
  9. May 30, 2009 #8
    And another question. Can the methanol provide a source of error? Can a certain volume of NaOH dissolve into methanol instead of react with ASA to neutralize it? Will this lead to a higher reading of NaOH used to neutralize ASA?
  10. May 30, 2009 #9
    So this same process can be done by chloroform and ether?
  11. May 30, 2009 #10
    Another question:
    Phenolphthalein stays purple when the pH is between 8.2 and 12. When NaOH completely neutralizes ASA, the pH will be 7. This means an excess of NaOH (the base) had to be added until the solid purple appears.

    This is a source of error correct? A universal indicator should have been used ?
  12. May 30, 2009 #11
    Nvm, ASA is a weak acid, and the equivalence point is at a pH of 8.2.

    Can someone please answer my other question though?
  13. May 31, 2009 #12


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    Staff: Mentor

    What do you mean by "NaOH will dissolve into methanol". Try to precisely describe the titration set up and where this methanol is.
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