Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: MrPopo on March 25, 2010, 06:26:56 PM
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I need two different reasons why there are so many types of acid and base indicators.
Thanks in advance,
MrPopo
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Well, the reason lies in the need to identify the equivalence point pH for lots of different reactions, thus having lots of different indicators (each one having its own pKi) so that you can have one indicator for every acid base reaction that you need to study.
For instance, if you need to do a HCl titration with NaOH, you may just pick phenolphthalein to indicate the end of the titration, although it's pKi is about 8.2 (which lets you see the colour change, from colourless to pink, at pH about 9.2, which doesn't matter because once you've got to pH=7 one single droplet of base makes the pH go up quickly, think of the titration curve).
But if the titration is of a weak acid, the equivalence point will be far higher than pH=7, then phenolphtalein will be useless and you'll need another indicator with a bigger pKi.
Hence, ideally the amount of acid-base indicatiors should cover the complete range of pH from 0 to 14 so that you can pick the one that's suitable for your acid-base titration.
Hope this helps.
Cheers.
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I think you have addressed question "what for do we use so many", not "why there are so many".
Not sure which question OP really asked, I thought about the second, but you may be right answering the first.
More detailed discussion of the problem of correct indicator selection:
http://www.titrations.info/acid-base-titration-end-point-detection
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I'm sorry, I don't quite follow. Isn't "why there are so many" a consequence of "what for do we use so many"? I mean, the more that you need, the more that have to be synthesized. The more that are syntesized, the more that exist. I'm afraid, I just don't see the difference.
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You are taking fact that we can synthesize indicator changing color around any pH for granted. This is the case, but it doesn't have to be obvious for everyone.
What if I will answer the original question "there are so many types of indicators because small changes to parts of the molecule surrounding chromophore change hue and pH at which change occurs"? Do you see that I am answering different question, or is my English failing me again?
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Well, I see that your interpretation was far different than mine, what I did take for granted was that the OP was interested in "why there are so many indicators" in a high school student way, I mean, he was interested in why, when he goes to the lab, finds so many indicators when he really uses two or three for the common titrations.
Your answer may be right, of course, I just wouldn't have thought of that answer to this particular quiestion in this context though.
Now I see the difference, thanks for making it clear to me.
Let's give the OP the chance to make himself clear as what exactly he wants to know.
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Its like saying why do we have so many cars?
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Sorry about being unclear, but Francopper's answer
Well, the reason lies in the need to identify the equivalence point pH for lots of different reactions, thus having lots of different indicators (each one having its own pKi) so that you can have one indicator for every acid base reaction that you need to study.
is what I was really looking for. Thanks for everyones help.