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I've realised that the majority of the solvent that we use in my lab are contaminated with grease (peaks at 1.26 and 0.86 ppm in $^1H$ NMR in $CDCl_3$), since they come in plastic containers (I guess that's the reason).

One solution would be to buy better grade solvents, but we have a large amount of these solvents in the plastic containers. I was thinking about distilling them to purify them and keeping them in glass bottles, but I thought of some issues that could appear.

Some organic solvent present stabilizers. For example, dichloromethane is often added amylene (like in my case), so I'm afraid of losing the stabilizer when distilling. Amylene has a boiling point of 38.5 ºC, so it should distill with the DCM. However, I was thinking of using the rotavap for practical reasons, but I think that maybe the amylene would not condense at the conditions used for the DCM. Also, I don't know if more stabilizers are added, since sometimes I get peaks that resemble BHT but I don't use any solvent that has BHT in it.

Could I use the rotavap for distilling the DCM or should I perform a simple distillation? Would the simple distillation work for keeping the stabilizer and preserving the DCM for long periods? Also, in the bottles of ethyl acetate and n-hexanes it doesn't say anything about stabilizers, is it alright to assume that they don't need any and I can distill them without problem?

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  • $\begingroup$ If your solvents come from the producer in plastic bottles, the impurity is probably not grease. It is probably some impurity coming from the plastic itself. You can distill DCM in a Rotavap : the amylene will distill together with the dichloromethane. I have never heard that ethyl acetate and hexane contain stabilizers. $\endgroup$
    – Maurice
    Jun 15, 2023 at 15:48
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    $\begingroup$ I'd argue that, if a supplier has used plastic bottles, they are not the most likely source of contamination. They will be designed to transport pure solvents. More likely, other lab practices are creating impurities. We used to use cheap PVC piping for inert gas transport (when doing air sensitive chemistry). The plasticisers (often alkyl phthalates) and preservatives in those pipes were a big source of contaminants. $\endgroup$
    – matt_black
    Jun 16, 2023 at 15:37
  • $\begingroup$ Can you filter thru activated charcoal? Or thru a silica column? $\endgroup$ Jul 16, 2023 at 13:25

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Purifying solvents can be difficult. I sort of solved it by encountering a local glassblower and devising a minimal holdup system with a Vigreux column. I used a dedicated distilling flask for each solvent. Collection was into a glass bottle saved from purchases of HPLC grade solvent. I tested by evaporating about 10 mL in a CLEAN [washed, Oven dried, pyrolyzed, at high T], Erlenmeyer flask and letting the last bit dry on a 4-bounce atr cell No detectable residue. This worked for methanol, methylene chloride, n-pentane, chloroform, cyclohexane, benzene, DMF and heptafloro-2-propanol[used to extract fluorinated oils]. Ethanol and iso-propanol were distilled from activated magnesium to remove water. The alcohols and DMF were checked by infrared or the nmr OH peak.

The removal of stabilizer is not a problem if purified solvents are used deliberately and not stockpiled.

A story about stabilizers: A colleague synthesized a rather large molecule with an ethyl group. He had an nmr with a weak quartet and came to us to corroborate his assessment. the chemical shift looked suspicious. It turned out that the deuterochloroform was stabilized with regular ethanol; we never heard back.

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Using the rotovap to distill the DCM proved useful, as it left a yellow oil as a residue. After that, the solvent was clean by NMR, only with some peaks that seem to be from amylene (the stabilizer present in DCM).

As far as I know, n-hexane and ethyl acetate don't need stabilizers. The ones we have in our lab are clean, although they also come in a plastic container.

I don't know if the contaminated DCM that we have is due to a bad batch, the plastic being dissolved or the actual quality of the DCM. It could be easily tested, but I haven't done it.

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