Acid & base titration

  • Thread starter Chocolaty
  • Start date
  • #1
Chocolaty
48
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Why is it better to use phenolphthalein than bromothymol blue to determine the equivalence point which is equal to 7?

bromothymol blue: 6.0 - 7.6 yellow @ blue
phenolphthalein: 8.3 - 10.0 colorless @ pink

:grumpy:
 

Answers and Replies

  • #2
zwtipp05
107
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My guess would be that since 7 falls inside the range for the blue you would have to stop the titration while the color is still mixed. With phenolphthalein you stop when it completely changes color which makes it easier to obtain consistent results rather then trying to interpret the mix of colors.
 
  • #3
christinono
211
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zwtipp05 said:
My guess would be that since 7 falls inside the range for the blue you would have to stop the titration while the color is still mixed. With phenolphthalein you stop when it completely changes color which makes it easier to obtain consistent results rather then trying to interpret the mix of colors.
Actually, that is not quite right. The best indicator to use for a titration is one which the equivalence point (pH) falls into the range - preferably the middle - of the indicator. In this case, the bromothymol blue would be ideal, because it would change colour when the pH equals about 7. The phenolphthalein won't even start to change colours.
 
  • #4
siddharth
Homework Helper
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Assuming this is a strong acid vs strong base titration, what solution is in the burette?

From what Zwtipp05 posted, I think he means that the acid is in the burette and the base is in the conical flask along with the indicator.

And according to christinono, the base is in the burette and the acid is in the conical flask along with the indicator.
 
  • #5
The Bob
1,119
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As siddharth said, let's assume it is a strong acid/base titration.

Either way (acid or base in burette), only one of the indicators is the best to use but can Chocolaty tell me which and his reasons why?

However, even though two ways have been suggesed, only one is correct. The acid should go in the burette.

The Bob (2004 ©)
 

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