Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Extra love on January 28, 2008, 10:01:04 PM
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I did a titration experiment which i used NaOH to titrate vinegar. I also put indicator to it to see the colour change when it reached an endpoint but why the solution start change colour at a pH=8.3 instead of at a PH=7?
Please *delete me* Thank you!
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You've left some information out I think.
What is the indicator -- does a quick Google confirm that that particular indicator changes color at 8.3? It is not a given that the indicator will change color at pH 7.0
How do you know the pH, did you have a pH meter in your titration as well?
Or are you asking why your experiment chose an 8.3 indicator for a pH titration?
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Could be as well that Extra love was told that endpoint pH of acetic acid is 8.3. That's my bet. It holds only for 0.01M acetic acid and even than it is not exactly true, but close enough to reality.
Extra love - what ions do you have in solution at the endpoint? Have you heard about hydrolysis?
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You've left some information out I think.
What is the indicator -- does a quick Google confirm that that particular indicator changes color at 8.3? It is not a given that the indicator will change color at pH 7.0
How do you know the pH, did you have a pH meter in your titration as well?
Or are you asking why your experiment chose an 8.3 indicator for a pH titration?
Indicator is Phenolphthalein. Actually, I don't know. This is the question from the lab manual.
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Could be as well that Extra love was told that endpoint pH of acetic acid is 8.3. That's my bet. It holds only for 0.01M acetic acid and even than it is not exactly true, but close enough to reality.
Extra love - what ions do you have in solution at the endpoint? Have you heard about hydrolysis?
This is the titration reaction: CH3COOH + NaOH ---> CH3COONa + H2O And Phenolphthalein is the indicator.
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Indicator is Phenolphthalein. Actually, I don't know. This is the question from the lab manual.
OK. So back to the original question, as posted:
... but why the solution start change colour at a pH=8.3 instead of at a PH=7?
Because that is the pH at which phenopthalein changes color. Now, You're probably wondering, how a titration can be done when the visual change occurs at a pH greater than 7. But if you think about it, you'll see why it is possible.
Hint: You've neutralized all the acid. The pH is 7.0. You won't see a color. You add one drop of NaOH to the neutral solution of sodium acetate. What pH are you likely to get?
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Hint: You've neutralized all the acid. The pH is 7.0.
Shouldn't the solution be basic? (when acetate reacts with water)
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Hint: You've neutralized all the acid. The pH is 7.0.
Shouldn't the solution be basic? (when acetate reacts with water)
True enough. A weak acid neutralized with a strong base will make a weakly basic salt solution when all the acid is neutralized.
Haven't you answered your own question?
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... but why the solution start change colour at a pH=8.3 instead of at a PH=7?
Because that is the pH at which phenopthalein changes color.
To be more precise - that is the pH at which there are chances that you will be able to see color changes. pKa for phenolphtalein is above 9 and it is listed as changing color in the 8.2-9.8 range (or something like that). For 0.01M solution of acetic acid pH=8.3 is almost exact pH of endpoint (for the reasons that Brett pointed out).
Haven't you answered your own question?
Brett ≠ Extra love :)
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Haven't you answered your own question?
Brett ≠ Extra love :)
Uh, oops. Heh. Srry. ::)