Chromatography Forum

Hi guys,

I want to switch solvent of polychlorinated n-alkanes (containing 5 individual compounds) from cyclohexane to methanol. I dont want to evaporate to dryness and then switch to methanol.

I will be grateful for your advice

This is an interesting challenge. If a methanol-cyclohexane azeotrope has lower boiling point than methanol then you may be able to switch solvents by adding methanol and distilling off the azeotrope.

I was also thinking there may be a way to add a higher boiling component such as water, boil off the cyclohexane, add methanol and then dry with mole sieve.

Of course, your chlorinated compounds would have to have a higher boiling point than your distillation temperatures.

In simple terms; add methanol, evaporate under a stream of nitrogen, and keep topping up with methanol until all the cyclohexane is gone.

Are you sure that the chlorinated alkanes are soluble in methanol ?

Peter

Thanks peter and chromatographer for your replies.

In fact, I need to prepare loading solutions of chloroalkanes in methanol. Yes, they are quite soluble in methanol (thu not as much as cyclohexane). If I keep topping up with methanol, do you think i will get rid completely of cyclohexane keeping in view boiling point of cyclohexane (80.7 oC) versus methanol (65 oC)?

You will never get rid of the very last traces of cyclohexane of course, but you should be able to get it down to a few percent I would guess. Although you will always be evaporating more methanol than cyclohexane because of the higher boiling point of cyclohexane, you replace only the methanol, so the cyclohexane must get less and less.

Peter

But how volatile are your analytes?

How much of them will you lose?

That is the issue, is it not?

best wishes,

Rod

from the patent: Recovery of components of a binary azeotrope

3. Process of claim 2 wherein the azeotrope is composed of about 37% methanol and 63% cyclohexane, on weight basis, and boils at about 54.degree. C. at atmospheric pressure.

so you should be able to boil off the cyclohexane with excess methanol. I would recommend using a kuderna danish concentrator to do this. You could easily monitor the progress with GC.

Thanks everybody for the input.

@Chromatographer: Thanks for sharing this idea. Seems promising one

@ Rod: Vapor pressure of my compounds are from < 3.01 mmHg at 25°C. I have not done recoveries stuff so far. In fact, my compounds are quite expensive, so don't want to loose them. Just wanted to get insight before going to lab .

Chroma11,

I would vote for Chromatographer's solution, the KD. One minor twist I would add, I would use a micro-kd unless you are dealing with really big solvent volumes. They are available from glass houses and some GC suppliers like Supelco.

Best regards,

AICMM

Thanks AICMM