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Author Topic: titration and buffer  (Read 11310 times)

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777888

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titration and buffer
« on: December 16, 2004, 04:51:09 PM »

1) For strong acid(HCl)/strong base(NaOH) titration. Would phenolphthalein be a bad choice for an indicator? (Because phenolphthalein changes to violet at pH=10.0, the pH range of phenolphthalein is 8.0-10.0) Are there any other indicator that can be used?

2) For weak acid/strong base titration, why is there a buffer region?
Is there an equilvalent region on the strong acid/strong base titration curve? (The strong acid/strong base titration curve also increases very slowly)

3) What indicator is the best for weak acid(HC2H3O2)/strong base(NaOH) titration? (phenolphthalein??) Are there any other indicator that can be used?

Thanks for teaching me!
« Last Edit: December 21, 2004, 12:12:07 PM by 777888 »
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777888

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Re:titration and buffer
« Reply #1 on: December 18, 2004, 08:20:45 PM »

Can anyone teach me please? :)
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kevins

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Re:titration and buffer
« Reply #2 on: December 21, 2004, 03:52:52 AM »

For strong acid and strong base titration, any indicator can be used. It is because the sharp pH change at the equivalent point.
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jdurg

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Re:titration and buffer
« Reply #3 on: December 21, 2004, 04:27:50 AM »

For strong acid and strong base titration, any indicator can be used. It is because the sharp pH change at the equivalent point.

Absolutely correct.  With a strong acid and strong base titration, the end point is where the pH = 7.00 (Neutral).  As soon as you add one more drop of NaOH, the pH will shoot up into the basic range since NaOH is a strong base.  (And the addition of one drop to a neutral solution will cause a huge increase in the OH- concentration).  So while you're titrating, the solution will be clear until it suddenly becomes basic due to that addition of one extra drop of NaOH.

For a weak acid/strong base titration, a buffer range exists because as you are adding the base to the acid, you are forming water and the conjugate base of that acid.  Any combination of a weak acid and its conjugate base can be considered a buffer.  However, once you've added enough base to the solution, there won't be enough of the weak acid floating around to provide a buffer.
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777888

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Re:titration and buffer
« Reply #4 on: December 21, 2004, 12:14:16 PM »

Absolutely correct.  With a strong acid and strong base titration, the end point is where the pH = 7.00 (Neutral).  As soon as you add one more drop of NaOH, the pH will shoot up into the basic range since NaOH is a strong base.  (And the addition of one drop to a neutral solution will cause a huge increase in the OH- concentration).  So while you're titrating, the solution will be clear until it suddenly becomes basic due to that addition of one extra drop of NaOH.

For a weak acid/strong base titration, a buffer range exists because as you are adding the base to the acid, you are forming water and the conjugate base of that acid.  Any combination of a weak acid and its conjugate base can be considered a buffer.  However, once you've added enough base to the solution, there won't be enough of the weak acid floating around to provide a buffer.
WHY? Say
methyl orange      red          3.1-4.4    yellow
This won't work becuase the indicator has completely changed to yellow at a pH of 4.4, the solution is not yet neutral....


So why there isn't a buffer region in strong acid/strong base titration curve?

Thanks
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777888

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Re:titration and buffer
« Reply #5 on: December 21, 2004, 06:02:22 PM »

My text book says:
BUFFER is a mixture of a conjugate acid-base pair that maintains a nearly constant pH when diluted or when a strong acid or base is added; an equal mixture of a weak acid and its conjugate base

I am confused if the strong acid/strong base titration curve have a buffer region because at first, the strong acid/strong base titration curve increases VERY slowly every though large amounts of NaOH is added...

Please help me! I want to understand this concepts! :) Thank you!
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777888

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Re:titration and buffer
« Reply #7 on: December 22, 2004, 01:22:34 PM »

http://employees.oneonta.edu/schaumjc/chem361/titration.doc
http://chemistry.beloit.edu/Rain/pages/titr.html
http://dwb.unl.edu/calculators/activities/titrate.html
http://www.sparknotes.com/chemistry/acidsbases/titrations/section1.html
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
Thanks!
I read them and concluded there is NO buffer region in strong acid/strong base titration cruve (but I am still somehow confused...the strong acid/strong base curve also has a region that is relatively horizontal(constant) despite the amount of strong base added, why isn't it considered buffer?)
(See http://img.sparknotes.com/chemistry/acidsbases/titrations/gifs/sasb.gif for the graph)

There is a buffer region in weak acid/strong base titration curve. Is there an equilvalent region on the strong acid/strong base titration curve?
Would the answer be "no"?
« Last Edit: December 22, 2004, 01:29:20 PM by 777888 »
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777888

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Re:titration and buffer
« Reply #8 on: December 22, 2004, 01:31:33 PM »

For strong acid and strong base titration, any indicator can be used. It is because the sharp pH change at the equivalent point.
so "ALL" indicators work for stonrg acid/strong base titration???
How about weak acid/stron base titration?
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