Sciencemadness Discussion Board

identifying phthalic acid

jamit - 13-11-2015 at 00:24

today i acquired a melting point apparatus and so i decide to test it out using phthalic acid (MP ~207C) which I extracted from vinyl gloves several years ago. What I thought was "phthalic acid" would not melt even at 340C, so I realize that it must be massively contaminated or it may not even be phthalic acid at all.

My question is, how can I find out if my sample is phthalic acid? Is there a chemical test for phthalic acid? I googled it but the results came out negative. If someone could steer me in the right direction, I would appreciate it. thanks.

Volanschemia - 13-11-2015 at 00:31

ortho-Phthalic Acid (what most people refer to when they say Phthalic Acid) will melt at 207C and will then decompose to Phthalic Anhydride and sublimate. To test if it is Phthalic Acid, heat some up in a beaker and when it melts, put a round bottomed flask filled with cold water on top. If it is o-Phthalic Acid, you will get cotton candy like crystals of Phthalic Anhydride condense on the bottom of the RBF.

If it doesn't melt and sublimate then it is not o-Phthalic Acid, but it could quite possibly be another isomer (iso- or tere-) or not even Phthalic Acid at all.

Another way you could test it (which I have never done, but I think it would work), is to place a small amount in a flask with some Phenol and a couple of drops of conc. H2SO4. If you get the red colour of Phenolphthalein form, then it should be Phthalic Acid.
Someone please correct me on that if it is plain wrong.

UC235 - 13-11-2015 at 06:34

Is it possible that you also did a hydrolysis of PET plastic and may have your jars mixed up? Terephthalic acid sublimes at 400C.

CharlieA - 13-11-2015 at 08:26

Quote: Originally posted by TheAustralianScientist  

Another way you could test it (which I have never done, but I think it would work), is to place a small amount in a flask with some Phenol and a couple of drops of conc. H2SO4. If you get the red colour of Phenolphthalein form, then it should be Phthalic Acid.
Someone please correct me on that if it is plain wrong.


Wouldn't you have to basify the phenolphthalien to observe the red/pink color? I believe that phenolphthalien is colorless in acidic solution.

I also wonder if this might be a way to synthesize phenolphthalein relatively cheaply. With my wife's appetite for Mountain Dew, I have an endless supply of PETE starting material!:D

gdflp - 13-11-2015 at 09:01

PETE is polyethylene terephthalate, or polyethylene para-phthalate so that won't work. Phenolphthalein is prepared by the condensation of phenol with ortho-phthalic acid.

CharlieA - 13-11-2015 at 13:31

Of course! Duh!!!!!! I guess my head us up my....My only defense is not enough coffee at the time of the post...or too much wine?

Volanschemia - 13-11-2015 at 14:07

Quote: Originally posted by CharlieA  

Wouldn't you have to basify the phenolphthalien to observe the red/pink color? I believe that phenolphthalien is colorless in acidic solution.


I think it would have to be in aqueous solution to go colourless, and as there is extremely little water in the reaction, most of the Phenolphthalein formed is not dissolved.
I'm not sure if that's correct, but I know that the Phenolphthalein is dark red when it forms in the reaction between Phenol and Phthalic Acid.

jamit - 13-11-2015 at 15:56

Thanks guys. I'll give it a try this weekend.