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Topic: Choosing the indicator  (Read 15936 times)

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Offline shakajjh

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Choosing the indicator
« on: February 10, 2008, 12:00:11 AM »
Ok so based on what I know about PH and indicators I'm thinking that the answer for this would be Methyl red.  am I on the right track? 

My professor said that on a test he could just give us a reaction between a strong acid and weak base or weak acid and strong base and based on that we have to pick an indicator?  How is that related to this problem?

Thanks for helping me

Suppose that the equivalence point pH in the titration of an acid with a base is 5.1. To obtain the most accurate results, which of the following indicators would you select for the titration? (Choose from A - E)

 Indicator  pKIn 
A methyl orange 3.7
B methyl red 5.2
C alizarin yellow 12.0
D bromthymol blue 6.8
E thymol blue 8.9
« Last Edit: February 10, 2008, 12:21:55 AM by shakajjh »

Offline Yggdrasil

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Re: Choosing the indicator
« Reply #1 on: February 10, 2008, 01:00:25 AM »
You are correct.  You want to choose the indicator whose pKa is closest to the equivalence point of the titration.

Offline shakajjh

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Re: Choosing the indicator
« Reply #2 on: February 10, 2008, 01:28:36 AM »
thank u so much have a snack on me :)

Offline Borek

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Re: Choosing the indicator
« Reply #3 on: February 10, 2008, 04:50:08 AM »
Ygg didn't mention WHY you want it this way - and that's the most important point. If you will use some other indicator - say thymol blue - it will change color at at pH around 9. It means AFTER reaching equivalence point you have to add more titrant (base in your case) to push pH from 5.1 to 9 - that will give additional error in your titration. It may also work other way around, if the equivalenc point is at 9, and you use methyl red, you see end point earlier than you should.
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Offline shakajjh

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Re: Choosing the indicator
« Reply #4 on: February 10, 2008, 03:47:13 PM »
awesome! now I understand it a heck of a lot better.  Now to memorize the Strong acid + bases and basing indicator off their reactions is a whole new story

:(

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