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Strong Base Weak Acid Titration and WeakBase Strong Acid Titration
1) Calculate the volume of the endpoint when 0.1 mol dm-3 HCl solution reacts with 25 ml
of 0.2 Na2CO3 solution if the phenolphthalein is used as the indicator.
2) Calculate the volume of the endpoint when 0.1 mol dm-3 HCl solution reacts with 25 ml
of 0.2 Na2CO3 solution if the methyl orange is used as the indicator.
Theory
A standard Choosing indicators for titrations
Remember that the equivalence point of a titration is where you have mixed the two substances in exactly equation
proportions. You obviously need to choose an indicator which changes colour as close as possible to that equivalence
point. That varies from titration to titration.
Strong acid vs. strong base
The next diagram shows the pH curve for adding a strong acid to a strong base. Superimposed on it are the pH ranges for
methyl orange and phenolphthalein.
You can see that neither indicator changes colour at the equivalence point. However, the graph is so steep at that point
that there will be virtually no difference in the volume of acid added whichever indicator you choose. However, it would
make sense to titrate to the best possible colour with each indicator.
If you use phenolphthalein, you would titrate until it just becomes colorless (at pH 8.3) because that is as close as you
can get to the equivalence point. On the other hand, using methyl orange, you would titrate until there is the very first
trace of orange in the solution. If the solution becomes red, you are getting further from the equivalence point.
Strong acid vs. weak base
, This time it is obvious that phenolphthalein would be completely useless. However, methyl orange starts to change from
yellow towards orange very close to the equivalence point. You have to choose an indicator which changes colour on the
steep bit of the curve.
Weak acid vs. strong base
This time, the methyl orange is hopeless! However, the phenolphthalein changes colour exactly where you want it to.
Weak acid v weak base
The curve is for a case where the acid and base are both equally weak - for example, ethanoic acid and ammonia
solution. In other cases, the equivalence point will be at some other pH.
You can see that neither indicator is of any use. Phenolphthalein will have finished changing well before the equivalence
point, and methyl orange falls off the graph altogether. It may be possible to find an indicator that starts to change or
finishes changing at the equivalence point, but because the pH of the equivalence point will be different from case to
case, you can't generalize. On the whole, you would never titrate a weak acid and a weak base in the presence of an
indicator.
Sodium carbonate solution and dilute hydrochloric acid.
This is an interesting special case. If you use phenolphthalein or methyl orange, both will give a valid titration result - but
the value with phenolphthalein will be exactly half the methyl orange one.
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