Whenever I freeze precipitate out of naphtha I tend to end up with a bunch of crystals floating around in the solution. When I try to decant the naphtha, a bunch of the crystals end up flowing out too.

I've tried using coffee filters but never had much luck with that.

I also tried filtering it through a cotton ball in a funnel then soaking the cotton in a small amount of naphtha and evaporating it. That seemed to work ok, but I'd like to avoid evaporating solvents at all if possible.

I was considering buying a Buchner funnel and vacuum filtering it, but before I drop the cash on that, does anyone have an effective method using cheaper more readily available materials?

Also, is it possible that I'm just not leaving it in the freezer for long enough? Would a longer freeze make the crystals drop to the bottom of the container and make them easier to separate by decanting? (I think I left the last batch in the freezer for ~10 hrs)

10 hours is a pretty short time to leave it in there. I usually give my trays a full 24 hours.

I've looked at a dish through 8,12,16,20,24 hours of the process and I find that it's not sufficiently crystallized until around the 18-20 hour mark. Leaving it to 24 hours just ensures you've given it plenty of time.

I'll sometimes get crystals floating in my solvent in which case I'll decant everything quickly into a beaker or coffee plunger glass (looks just like a beaker) that has a lip on it. The lip enables you to pour off the solvent much easier without any cotton balls or coffee filters. You'd be surprised how efficiently you can do it. If you keep the beaker/glass on a 45 degree angle, most of the crystals sink as you pour the solvent off slowly.

Then quickly dry those crystals off in the same glass in front of a fan. As long as you get the immediate solvent evaporated out of the glass the crystals will be fine.

I usually leave my plates in the freezer for at least 24 hours, sometimes longer if I don't have a chance to drain the naphtha out. Like Tryptallmine said, you can see a noticeable change over the hours in the size of the crystals. After 24 hours, the crystals are usually stuck to the bottom of the plates and I have no problem with them going back in with the naphtha.

Thanks! I actually thought about trying this, but I think I put it off because I thought I might have a hard time getting the crystals out of the beaker, as they are often quite sticky. This could possibly be due to my insufficient freeze time though. Either way, now that I think of it I do have a suitable beaker and scraping the crystals (or goo) out shouldn't be too difficult with the right tool.

The crystals that free float are generally extremely brittle and flake easily as they didn't form a solid crystalline structure.

Just make sure they are sufficiently dried and you will have no trouble at all scraping them out. Sometimes If I only have a small volume of solvent to decant I'll just use a tiny 50ml beaker which are really no bigger than a shot glass.

Even with a small surface area they will dry crystals fine in front of a fan. You can scrape them out with a spoon handle or butter knife. Easy.

If they are still sticky, give them an hour in front of the fan. It should dry up a bit.

Left a container in the freezer for 24 hrs and still got mostly floating crystals.



Tryptallmine, this got me thinking of the possibility that I may be using too much solvent for my pulls, as I generally am decanting 100-200 mL per pull. Could this be contributing to my problem? I think I recall reading somewhere that higher volumes of solvent can equal less precipitation.

I use syringe. I tilt the dish so it pools in the most vacant corner and suck it up. Then I'll deposit the solvent in a bottle for further refinement at a later time.