Sciencemadness Discussion Board

Cleaning chemicals via saturated solution wash -> crashed crystals

RogueRose - 7-7-2016 at 09:58

I'm curious whether it is possible to get clean/pure crystals when starting with a "dirty" substance. Let's say some compound that dissolves 200g/L @ 20C but 2300g/L@ 60C. So 2100g crystalizes during the temp change. This question is about anything that can be "extracted" like this - no one specific compound in mind.

Now if the raw/unclean material is dissolved the water is dirty with "god only knows" and probably ranges from compound to compound. The material passes through something like a coffee filter but is very murky brown. If run through a 5 micron filter, the solution just lightly cloudy with a tan tint. 1 micron filter it looks clear but has slight tan tint. Adding charcoal filter gives water clear solution. (these results are basically what I get from a number of different technical or agriculture grade products I have used).

What I want to know is if I dissolve my compound and filter through coffee filter so no large particles or dust/debris is there - then I allow to cool to crystalize (not water is still dirty) and then filter through a pillowcase, catching all small crystals. Wash lightly with ice DH2O while in pillowcase.

My question is, can I do this in the dirty water and get clean/pure crystals? If not, what can get trapped in the crystals, dirt?

Either way, I would do 2 crystallizations so I would suspect the second time there would be little "dirt" in the saturated solution.

I would like to avoid doing any filtering below "coffee filter" level for this but am wondering if there is benefit by going to the 1 or 5 micron or even the activated carbon step.

Texium - 7-7-2016 at 16:44

You'll likely have some inclusions of dirt and silt on the first crystallization, but if you do a few recrystallizations you should be able to get most of the insoluble impurities out by that method. Assuming that the contaminants tend to stay suspended in the supernatant, each time you recrystallize it there will be less and less remaining trapped in the crystals.

RogueRose - 7-7-2016 at 17:14

Quote: Originally posted by zts16  
You'll likely have some inclusions of dirt and silt on the first crystallization, but if you do a few recrystallizations you should be able to get most of the insoluble impurities out by that method. Assuming that the contaminants tend to stay suspended in the supernatant, each time you recrystallize it there will be less and less remaining trapped in the crystals.


Thank you for your reply! It would seem that something like 3-5 crystalizations would be in order to get the purity I am looking for and for what I am doing, that is a lot of steps.

I am trying to determine if it is just not better to dissolve to highest saturation point and then pass through my 2-3 filters (each decreasing in size, ending with Act Carbon). The problem with this is that it is difficult to keep temp at high saturation point while filtering so inevitabley more water is used, which leads to having to evaporate it later down the road. I KNOW I can get super pure crystals on one run with this method but last time I needed almost 3x as much water.


Hmmm, decisions decisions..