Why Methanol is Best for ASA Titration w/ NaOH

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Discussion Overview

The discussion revolves around the suitability of methanol as a solvent for acetyl salicylic acid (ASA) during titration with sodium hydroxide (NaOH). Participants explore the implications of solvent choice on solubility, indicator behavior, and potential sources of error in the titration process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants suggest that methanol is a better solvent for ASA than ether or chloroform due to its polarity, which is necessary for NaOH solubility.
  • There is a question regarding whether ASA refers to acetyl salicylic acid, indicating a need for clarification on terminology.
  • One participant notes that the CTC handbook lists ASA as soluble in alcohols, ethers, and chloroform, suggesting that the solvent may not be the problematic variable in the titration.
  • Phenolphthalein is mentioned as an indicator used to detect the endpoint of the titration, with questions raised about its solubility in different solvents.
  • Some participants argue that the visibility of phenolphthalein in low concentrations means its solubility should not affect the titration results.
  • There is a discussion about the behavior of phenolphthalein in solution, including its transition from colorless to pink as a base is added, referencing Le Chatelier's principle.
  • Concerns are raised about potential errors in the titration process, particularly regarding the possibility of NaOH dissolving in methanol instead of reacting with ASA, which could lead to inaccurate readings.
  • Participants discuss whether similar processes for dissolving phenolphthalein in alcoholic solutions could be applied to chloroform and ether.
  • Questions are posed about the pH at which phenolphthalein changes color and whether this could introduce errors in determining the endpoint of the titration.
  • One participant clarifies that ASA is a weak acid and that the equivalence point for its titration is at a pH of 8.2, which raises further questions about the choice of indicator.

Areas of Agreement / Disagreement

Participants express various viewpoints on the effectiveness of methanol as a solvent and the implications for the titration process. There is no consensus on the best solvent or the potential sources of error, indicating ongoing debate and uncertainty.

Contextual Notes

Participants highlight limitations regarding the solubility of phenolphthalein in different solvents and the conditions under which NaOH may interact with methanol, but these aspects remain unresolved.

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Homework Statement


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.
 
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Do you want us to guess that ASA symbolizes acetyl salicylic acid?
 
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 . . .
 
Phenolphthalein was used to indicate the moment the solution became basic.

Phenolphthalein is soluble in methanol, but is it soluble in chloroform or ether?
 
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).
 
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.
 
rss14 said:
It says it is usually dissolved in alcohol (such as methanol). There has to be a reason for this.

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.
 
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?
 
Borek said:
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.

So this same process can be done by chloroform and ether?
 
  • #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 ?
 
  • #11
rss14 said:
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 ?

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?
 
  • #12
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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