My name is Heather Cowdry and i am doing an A level project about testing samples of Aspirin for purity. To do this i am using chromatography, colorimetry, melting point analysis and acid-base titrations.

In my initial plan for the titration i was going to use phenolphthalein indicator and titrate the aspirin solution using NaOH. It has since been suggested that i use phenol red indicator (i think that is the correct name) instead of the phenolphthalein. I do not understand why this is so. What are the differences between the two indicators and will the experiment work regardless of which of the two i use?
I would really appreciate some help in this matter as the other internet sources i have used have been quite confusing!

Thank you

For carboxylic acids such as aspirin, use an indicator in the pH 3 to 4 area, such as phenol red or methyl orange or methyl orange-xylene cyanole. Remember all such titrations are best done potentiometrically first, then an indicator is chosen which delivers its color change at the potentiometric equivalence point. Phenolphthalein changes color at about 8 or 9.

It would be well if the rest of you didn't give this one away............

There are some important lessons to be learned.
1. be wary of anonymous advice on the internet.
2.The internet is not the best place to find what you are looking for. What you are asking was studied and described 50 years before the internet was invented.
3.I had to think a minute why phenol red is a better choice, but it probably is if you can see the change as well as phenolphthalein.

Find a copy of Quantitative Chemical Analysis by Kolthoff, et al. This is a must book for any general analytical chemist to know about. Study chapter 34.In particular consider Fig 34-3 with respect to the chemical properites of what you are titrating. Do a titration with each indicator and pay particular attention to what happens at the end point. If after a good faith effort of studying the chemistry involved you haven't discovered the answer to your question, email me and we will discuss it. This is a good learning problem; my compliments to the teacher that came up with it. What school are you at?

Thanks Bill for that reference! I'll try and find it and then if im still unsure i'll come back to discuss it further. Thank you!

Heather, If you can't find that Kolthoff reference, any old, like around '40 or 50's, book on titrations will have a chapter on acid base titrations and indicators. What you are looking for is what kind of indicator is best for weak acids. But, in addition, you need to consider the functionality of aspirine. What groups does it have and what might happen to them during titration? How does the range for phenol red differ from phenophthalein and why might that be better in this case?

Heather,

A 30 second search got this site discussing everything every one else did.

http://www.carlton.paschools.pa.sk.ca/ chemical/ equilibrium/ abindicators.htm

Reading it would have raised your question if you still had one to the A level. What several people are telling you is the answer for a student is not what is important. Learning to use your god-given intelligence to solve problems is important. Use your own resources before asking someone else. You will benefit yourself and others around you instead of becoming a liability.

Get on the internet and find the acid disassociation constant of asprin. Find an acid base indicator with the same value.
Jim

Hi Heather, it's Kate. I notice you got a few less rude responses than I did! I have since had another that said information on aspirin should be easy to find. I never knew just how many horrible people there were in the world who are not willing to share their useful knowledge with people who require it. I think the subject of titration obviously brings in more friendly people than chromatography. See you at school and if any of these lovely people give you any good info on chromatography, let me know and I'll do the same for you.

Image that! Heather and Kate (Aspirin discussion) know each other... neither did their homework before asking this forum for help. All of you who are "willing to share your usefull knowledge" beware. I side with Jim Gorum...do some research BEFORE you beg for answers.

How about adding the following to Policies:
Before asking a specific "how to" please check google, alltheweb..... + the links given in an earlier chain.

From some of the responses it appears that chromatographers are a bit rusty on their titration theory.

To approach this problem theoretically one calculates the pH where 99.9 or 99.99% of the asprin is neutralized. Then one picks an indicator that changes in this region. This pH will be somewhere more basic than 7. It is most easily visualized by plotting the acid and salt forms of the indicator and acid and salt forms of the analyte vs pH. Most ancient titration books will have such plots as examples. A good rule of thumb is you won't see a color change until 90% of the indicator is neutralized.

having done this calculation, one will conclude that phenolphthalein would be a good choice. However, that phenol ester in asprin is quite labile and it will begin to hydrolyze at the pH where phenolphthalein turns. I haven't done it, but I will bet that the end point will fade with this indicator as aspirin hydrolyzes. So, perhaps phenol red, which turns around pH 7, a pH of minimum hydrolysis rate, could be a better choice. Of course the best choice is potentiometric with a titrator.

Do you suppose by now Kate has discovered that TLC is a poor way of finding low level impurities, which may be why no one on this forum had much experience with her problem?

Heather, I'm in the soap industry in U.S., and the standard AOCS titration test for fatty acids (carboxylic acids) even in the presence of triglycerides (potentially saponifiable) is titration with standard NaOH in hot neutral ethanol to the phenolphthalein end point. AOCS is American Oil Chemists Society.

Dear all, aspirin should be assayed by adding an excess of NaOH+heating and titrated using HCl to pH 7.5 (phenolphtalein color dissapear). It is not possible to get a good end point due to the labile phenolic ester as pointed by Bill.
Triglecerides are much more stable.
Regards

I completely agree with heather, SOmetimes it is very difficult to find information on the web.