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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: 777888 on December 16, 2004, 10:51:09 PM

Title: titration and buffer
Post by: 777888 on December 16, 2004, 10:51:09 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!

Title: Re:titration and buffer
Post by: 777888 on December 19, 2004, 02:20:45 AM

Can anyone teach me please? :)

Title: Re:titration and buffer
Post by: kevins on December 21, 2004, 09:52:52 AM

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

Title: Re:titration and buffer
Post by: jdurg on December 21, 2004, 10:27:50 AM

Quote from: kevins on December 21, 2004, 09:52:52 AM

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

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.

Title: Re:titration and buffer
Post by: 777888 on December 21, 2004, 06:14:16 PM

Quote from: jdurg on December 21, 2004, 10:27:50 AM

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

Title: Re:titration and buffer
Post by: 777888 on December 22, 2004, 12:02:22 AM

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!

Title: Re:titration and buffer
Post by: AWK on December 22, 2004, 01:15:38 AM

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

Title: Re:titration and buffer
Post by: 777888 on December 22, 2004, 07:22:34 PM

Quote from: AWK on December 22, 2004, 01:15:38 AM

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"?

Title: Re:titration and buffer
Post by: 777888 on December 22, 2004, 07:31:33 PM

Quote from: kevins on December 21, 2004, 09:52:52 AM

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

so "ALL" indicators work for stonrg acid/strong base titration???
How about weak acid/stron base titration?