Hello,

after recently performing my first extraction followed by freeze precipitation, I had *a lot* of trouble with floaters in the solvent: free crystals floating instead of sticking to the sides of the containers. In some jars/pulls, almost all crystals were this way.

I spent quite a while searching the nexus for easy solutions to this, finding a lot of topics on them but with conflicting information: sometimes coffee filters were recommended, other times there were suggested as a terrible idea (with the crystals dissolving in the filter itself). As an aside, the Nexus FAQ item on this subject could be improved.

Anyway, thought I'd post the way I personally ended up dealing with it, and it worked great: just remove the solvent very slowly from the container after removing from the freezer (and of course quickly after doing so); and rotate it while pouring as well. The idea is to spread the crystals as much as possible along the container's surface, while still dripping the solvent slowly into another container. If all goes well, most (>90% - that's conservative - in truth I managed by slowly pouring + rotating to save almost all xtals) of the floaters should stick to the container's walls as you slowly poor/as the solvent moves out. You can ignore the ~10% or less that don't stick and end up being poured - let it dissolve in the other container's warming solvent and use it for future pulls to avoid loss. Of course after doing that, put the freeze precip container against a fan to evap the remaining solvent as usual.

This method worked perfectly; and it will work best with a large container, where you can have a large surface area that is *not* covered in sticking crystals to make the floaters stick as you rotate (also, I mention rotating because I used round jam style jars - not sure how you'd solve the problem with say, a flat dish).

This also brings me to a question: did anyone attempt some experiments to pinpoint the main root cause of floaters ? is it linked to the speed at which temperature of the NPS drops ? to the max temperature it reaches in the freezer ? possibly due to the NPS itself, or the container shape/surface ? More generally, I would specifically like to know what helps the crystals to stick to the walls to try and avoid this in the future. There too, searching the nexus forums has failed to produce a definitive answer.

Thanks in advance !

It seems likely that floating nucleation points will lead to floating crystals. If dust drifts onto the surface of your precipitation, or was in the container to start with it could give rise to this phenomenon.

(Top-of-the-head answer...)

That's interesting. I know little about crystallization in general, but it intuitively seems like this could indeed be a reason (and in fact I imagine any solid anchor - be they dust or something else - in the NPS would help to induce this). Especially likely since I pre-evaped a lot of the solvent against a fan before freeze precipitation, so the presence of dust is definitely possible. Thanks !

I'd still be curious to know if anyone actually experimented with say, speed of temperature drop and container material/form factor. Do those have an impact ?

Yeah, I'm guessing the cooling process is too rapid.

Are you perhaps agitating the solvent during the cooling/crystallization process?

I usually tilt the dish carefully until the floaters stick to a wall.

In case of too many crystals floating I vacuum filtration - Buchner funnel with standard qualitative filter paper (wetted with fresh cold naphtha before use).

I would avoid coffee filters as they suck up naphtha leading to a great loss of solvent.

I also rinse the Pyrex dish with distilled water and dry it thoroughly in the oven before use. Maybe this helps at least I have better feeling that no water is present.

I did during my first freeze precip (wanted to see the snow :-)) but not on later ones and the issue still occured. Still, as I said I have a handle on how to remove almost all of them now. Thanks !



Yeah, didn't even try the coffee filter. Your oven drying idea is a good one, will try it for further precipitations (still got 150g MHRB), thanks ! though I will say I did not notice any water crystals at all, and was very careful to avoid water contamination. I think downwardsfromzero got it right with his dust hypothesis.

You can pour the solvent in a new clean dish/jar let it sit in room temperature till you don't see floating crystals anymore. Then mix by shaking/tilting de dish/jar and then put it in the fridge for a few hours before putting it back in de freezer. For me thats the trick that works perfect