CaptainPike
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Trouble Cleaning DCM
I bought a can of dichloromethane on eBay, a half-gallon, or something – in a metal can just like the big sizes in the hardware store of acetone,
toluene etc.
I have been using this off and on since 2013, no reason to suspect anything. But I noticed that the can was rusting on the top (outside). DCM
typically has 1.5% water azeotropiclly. I decided to distill what was left (about 1 L) because it looked a little orange, I figured from rust. When I
distilled the DCM, my thermometer read not much more than 35°C, and the product is still yellowish. Could it be I have appreciable methanol dissolved
in this "DCM"? What would come over so that the distallate was yellowish? The still pot contained a small puddle of deeply rust colored gunk as I
expected.
Any ideas what might be going on here?
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MrHomeScientist
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Rust does sound likely. Try a second distillation at lower temperature, to avoid entrainment of whatever the contaminant is?
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CaptainPike
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+Mr.HomeScientist – good idea. I looked up the word, "entrainment", Dictionary.com says
to carry along (a dissimilar substance, as drops of liquid) during a given process, as evaporation or distillation.
Chemistry has such cool words. You know, I believe I have learned more great and useful words in my study of chemistry than I have, the chemistry
itself! Like supernatant (pronounced the best by UC235). Anyway, I had initially tried filtering the methylene chloride, thinking that the rust
particles would be caught in some good VWR filter paper I have.. Indeed, some large pieces were stopped but the filtration was essentially useless.
A funny thing happened: the weting of filter paper by the DCM did nothing to help stick it down on the porous plate, the way a little water will.
But I think you're right. I decided to go ahead and use the tainted solvent for an extraction I wanted to finish. And wouldn't you know it – in
the recovery of the solvent, it came out crystal clear (with little gobs of water floating on top). But here, I was moving much more slowly, not
wanting to oxidize my product. And my still pot was much smaller. I can't help feeling there's another factor at work here, however.
And one more thing (sounds like Columbo) here is where a little anhydrous calcium chloride really does the trick. I had expected that the DCM
purchased by a supply, (even on eBay) would be dry. But I'll bet it came with the nasty little azeotrope comfortably in place.
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MrHomeScientist
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Glad to help!
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