Melting point

Do you have any idea what the melting points and range might be for a 50/50 mixture of:

1. Benzoin (M.P.: 135-136) and succinimide M.P.: 121-123 and
2. Benzoic acid M.P.: 121-122 and succinimide M.P.: 122-124

I'm doing a lab with really laughable melting devices. My unknown is succinimide (visual identification). I know an impure melting point will be beneath the literature value. If it's mixed with the unknown, will the M.P. be below that of the known or of the unknown or both? How far below and what will be the range?

Thanks from Sheryl

If you are sure of the unknown being succinimide, and it is pure, then a mixture of that and a known succinimide (50-50)sample should have the m.p. in the literature with perhaps a few degrees range but very close to the literature. My recommendation is to recrystallize your unknown to make sure it is not impure. In the old days I recrystallized the knowns, too, so I was sure I had a good sample. You can calculate the mp of a 50-50 mixture of the above IF you know the molal freezing point depression constants. I looked in some freshman texts I have and they don't list any of the three. I also checked in the CRC handbook and those aren't listed there, either. I am unable to guesstimate the m.p. of the mixtures since I don't know Kf. Perhaps the idea of the lab is to do mixed m.p.for the practice and not to identify the unknown. If I were identifying an unknown I would make derivatives. However if the idea of the lab is to make 50-50 mixtures of the two above, just make them and do the m.p. I don't know that you get a leg up on the class by trying to calculate a temperature; you already know if will be somewhere between 100 and 135. I don't know what you are calling a laughable melting device but suspect it is something of the order of what I used lo these many years ago. They are laughable, in terms of today's computerized devices, but they were inexpensive and gave great results. I identified 20 out of 20 unknowns correctly. And I was a beginner.

When one of the devices went over 30 degrees past the known M.P., that was my first clue. The next device had so much moisture in the eyepiece that I couldn't see much. It's a small school and it doesn't have much of a budget unfortunately and the equipment is very old. It also takes a very long time for the devices to cool so that you can do the next M.P. Very time consuming with questionable results.

Do you know whether, in the mixed unknown and known solids to be melted, the resulting melting point be below the lowest range of both of them or just below one of them?


Not knowing the Kf, no I don't know. It could be any of those answers. Have you tried the old fashioned method with small melting point tubes, a thermometer, rubber band to hold the tubes to the thermometer, and all immersed in a tube of cotton see oil (or some such) and heated with a bunsen burner? Those work very well. You can even put two or three melting point tubes at the same time and do several simultaneously if all the samples melt at the same temperature OR if they melt at much differenct Ts. One trick I learned, all of my profs frowned on it and all told me it was not a good method; however, I finally stopped using any of those other methods. Just put a speck on the bulb of a thermometer, slowly move the "device" closer to a bunsen flame, and watch the T and the sample. Don't try to move the T up too fast. With practice, and with the right samples, I could find a spot close enough to the flame that I could make the crystal melt, move it away slightly, and it would solidify, Reverse and melt, reverse and solidify. That worked very well for me unless I had one that decomposed. In that case, I had to replenish the crystal on the bulb BUT I had an approximate T then and I could creep up on it next time.

Thanks for the tip. Sounds like fun, anyhow I'm laughing. I'll see if I can get away with it. I'd have to get ahold of a thermometer that goes higher than usual.


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