AvBaeyer
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Cleaning Up Hexane with Rooto Sulfuric Acid
I typically purchase tech grade hexanes in bulk and then clean it up and distill as required. The first step in the clean-up is to wash the hexane
with concentrated sulfuric acid (ca 5 vols hexanes:1 vol acid.) I use high quality sulfuric acid (Duda) which turns black after the wash and separates
cleanly. Today I decided to use some Rooto acid that had been around for a while which turned out to be a mistake. The acid turned black as expected
but a quite serious emulsion formed along with significant "rags" after it cleared. To make a frustrating story short, it appears that the Rooto acid
does contain some kind of emulsifying agent which has been discussed here in the past. Just a heads up if you do use Rooto acid in chemistry which may
be affected by an emulsifying agent. I hope you find this observation useful.
AvB
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draculic acid69
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Just out of curiosity what causes the h2so4 to turn black from the hexanes?
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Bonee
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the black stuff is carbon from dehydrated -OH containing stuff most likely.
maybe you could try putting a speck of aluminium foil in the emulsion to make some aluminium sulfate which may flocculate the emulsifying stuff ? just
excise care for the hydrogen bubbling.
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draculic acid69
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What type of OH species exist in hexanes?
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Tsjerk
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Sulfur species are what form black grut with dirty toluene.
[Edited on 27-7-2022 by Tsjerk]
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PirateDocBrown
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Yes, washing and distilling off of sulfuric (even Rooto) is a good way to get rid of a lot of thiophenes.
If you're going to distill after the wash anyway, just leave the sulfuric in (even the emulsions) and let the distillation do the separation. Then,
I'd distill a second time.
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Tsjerk
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Quote: Originally posted by PirateDocBrown  |
Yes, washing and distilling off of sulfuric (even Rooto) is a good way to get rid of a lot of thiophenes.
If you're going to distill after the wash anyway, just leave the sulfuric in (even the emulsions) and let the distillation do the separation. Then,
I'd distill a second time. |
If you are going for a second distillation, just as well add some sulfuric acid to that distillation. If the acid was very dark after the first wash,
you will see it also colors during the second wash.
[Edited on 27-7-2022 by Tsjerk]
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AvBaeyer
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Perhaps I can expand a bit on the process to address some of the foregoing comments. Technical hexanes contain some olefinic compounds based on
qualitative chemical tests. The sulfuric acid wash removes these compounds (there is no thiophene or hydroxylic compounds in the hexanes.) After the
sulfuric acid wash the hexane is washed with water, dilute NaOH, then brine. After drying over sodium sulfate, >90% of the hexane distills at
56-57C. The remainder distills from 58-70C to dryness leaving behind a trace of tarry material. None of the distillate tests positive for olefins
(KMnO4 or bromine water).
The main point of my post is that Rooto acid which is quite easily available may cause problems when it is used in reactions due to additives which
are likely present. Just a note of caution.
AvB
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PirateDocBrown
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| Quote: Originally posted by AvBaeyer | Perhaps I can expand a bit on the process to address some of the foregoing comments. Technical hexanes contain some olefinic compounds based on
qualitative chemical tests. The sulfuric acid wash removes these compounds (there is no thiophene or hydroxylic compounds in the hexanes.) After the
sulfuric acid wash the hexane is washed with water, dilute NaOH, then brine. After drying over sodium sulfate, >90% of the hexane distills at
56-57C. The remainder distills from 58-70C to dryness leaving behind a trace of tarry material. None of the distillate tests positive for olefins
(KMnO4 or bromine water).
The main point of my post is that Rooto acid which is quite easily available may cause problems when it is used in reactions due to additives which
are likely present. Just a note of caution.
AvB |
We had slightly changed the topic, to using sulfuric to clean toluene, which can indeed have thiophene. Just to clarify.
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AvBaeyer
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I erred in the boiling point data re hexanes. It distills at 66-67C with the remainder 68-70C to dryness. Just want to make sure things are correct.
AvB
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draculic acid69
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Thanks for the clarity about the impurities.
That makes more sense to me than thiophene and alcohols.
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Amos
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In general technical grade reagents(at least in the US) usually contain some kind of contaminant as a result of them being recycled or cheaply
processed after manufacture. My technical grade methanol contains 1-2% xylenes.
Given that the emulsifiers present in the drain cleaner are most likely organic, I wonder if drain cleaner sulfuric acid could be pre-treated by a
combination of intense heating and addition of hydrogen peroxide. This way it would behave more like laboratory-grade stuff without needing
distillation.
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clearly_not_atara
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H2O2 might work, but I would give Bonee's aluminium sulfate idea a shot first. Using H2O2 to oxidize sulfurous contaminants could
release significant SO2, but I don't think that the Al + H2SO4 reaction should produce any SO2. Do vent the hydrogen, of course.
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Texium
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| Quote: Originally posted by Amos | | In general technical grade reagents(at least in the US) usually contain some kind of contaminant as a result of them being recycled or cheaply
processed after manufacture. My technical grade methanol contains 1-2% xylenes. |
Worth noting that there are
still quite a few tech grade solvents that are sufficiently pure off the shelf (see the NMR analysis I did a few years ago: https://www.sciencemadness.org/whisper/viewthread.php?tid=10... )… though it’s probably still best practice to test for impurities or purify
solvents before using them if you are uncertain.
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