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Author: Subject: unknown DCM azeotrope
Ubya
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[*] posted on 14-3-2019 at 05:57
unknown DCM azeotrope


I've been trying to purify some DCM out of paint stripper.
the formula for the can i found is not the viscous methanol-DCM i always see in youtube videos, mine was supposedly just toluene and dichloromethane. i didn' find any azeotropes for this mixture so i was quite happy to get toluene and dcm at the same time. i started a fractional distillation with a 30cm vigreaux column, i used boiling stones and a water bath set to 50 degrees thinking i could get clean separation (bp dcm 39°C, bp toluene 110°C), but something was off, nothing came over. i raised the water bath temperature to about 70 degrees, i looked at the vapour front rising slowly, but as it hit the thermometer probe i can seeon the display the temperature raising immediatly over 40 degrees and oscillating between 50 and 55. i tried insulating the column, lowering the bath temperature (but nothing would come over). i stopped the distillation, i let everything cool and then started again, but samething as before, first vapour front at around 50 degrees. i gave up to try to get a lower boiling point mixture, so i just distilled everything until the temperature rose to 60+ something (i don't have my lab book with me right now, i'll post more detailed data in a few hours). from the smell i could imagine it contained methanol or acetone, so i did some water washes of the distillate, the volume decreased significally but the even stranger thing is that the dcm layer was on top:o i tested every solution with the beilstein test to locate where the halogenated solvent (DCM) was, for sure the distillate had DCM in it, but now was mixed with some other apolar solvent (it burns with a sooty flame so it could be aromatic or very long chain aliphatic). i can't find any information on DCM azeotropes with aromatic or aliphatic compounds, any help is appreciated





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DavidJR
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[*] posted on 14-3-2019 at 07:28


I think you might be better off to do a simple distillation first to get rid of the gunk, then fractionate to separate the DCM/toluene.
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Ubya
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[*] posted on 14-3-2019 at 08:15


the lable of the can is 100% wrong. if i had only DCM and toluene i shouldn't have a water miscible phase. anyway, tomorrow is friday, my girlfriend goes back to her mom for the weekend so i have some time to work on this




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Ubya
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[*] posted on 16-3-2019 at 17:36


i looked at my other can of solvent, and looking by its composition and a few azeotrope tables i think i solved part of the mistery(the labeling of the solvent i distilled is 100% wrong....)
the first fraction had not a stable temperature and it slowly raised to 56°C, probably due to the azeotrope methyl acetate/methanol, methyl acetate and then maybe acetone(not mentioned even in the other can). then it got to 62-65°C, toluene/methanol and methanol have their bp in this range. i collected everything until the temperature was 70°C and then i washed the distillate with water( methanol, acetone and methyl acetate are now in the water phase), i should now have only dcm and toluene mixed together. but still, how isit possible that i couldn't get a separation from the dcm/methanol azeotrope (bp 37°C) and (supposing the composition to be the same as the other can i have) the next azeotrope being methyl acetate/methanol (bp 54 °C)?





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Ubya
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[*] posted on 30-3-2019 at 16:54


Alright, distilled everything until I got to 80°C, washed the distillate with water to remove methanol and methyl acetate, distilled the clean DC-toluene to get the pure dcm, aaaaand the first fraction comes at 55°C.
Wtf
*me searching again every fking azeotrope I can think of*
EUREKAAAAAA
DCM-Water boiling point 55°C
The solvent was literally watered, the probably used wet methanol *facepalm*
Gosh all this time and I simply had wet dcm, all I have to do is drying the distillate after the washings and distill again

Finally cracked the mistery





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