Acid-base equilibria
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I'm doing an investigation on choosing a suitable indicator for a strong acid (hydrochloric acid) and a weak base (ammonia) titration. The options are methyl orange and phenolphthalein. I know that methyl orange is the most suitable indicator. What I don't know is how do I go about making a procedure for this titration? I was told that I had to use a buffer solution and a pH meter. Any suggestions?
020608
Igloo writes ...
You are correct in thinking that methyl orange is the better choice of indicator.
Presumably the buffer solution and pH meter are to help you establish for yourself that the pH at the end point is about 3-5 (the pH range appropriate for methyl orange).
pH meters need to be standardised before use, and it would be a good idea to obtain a buffer solution at a known and accurate pH (say 4.0) and use the pH probe to see if the meter needs adjustment. Once the pH meter has been calibrated in this way you can carry out a titration using the pH probe and using your results to plot pH against volume of acid/alkali added.
If the ammonia solution is being added from the burette to a known volume of hydrochloric acid and a few drops of indicator, the pH should start at a low value (e.g. in the region 0-2) and finish in the region of 10-12. The colour should initially be red, and there should be a large rise in pH in the region of 3-5 near the end point, where the indicator suddenly changes colour to yellow, so this will confirm that methyl orange is appropriate in a subsequent titration without the pH probe.
I presume that this is what you have to do in order to make use of the buffer and the pH meter in order to choose an appropriate indicator.
In reality of course you can obtain very accurate results using the pH probe by itself, and make no use of an indicator whatsoever!
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updated: 03 June 2008
