<a href="/assets/img/recrystallization/scales.JPG"><img src="/assets/img/recrystallization/scales_s.JPG" width="250"></a> <br>Click on the image for the large version.
The Wikipedia article on recrystallization pretty much explains it all. Recrystallization works because crystals don’t like impurities in the crystal structure. Impurities are left in solution as the crystals form. The solution can then be poured off and you are left with a more pure compound than you started with. Recrystallization can be repeated as many times as you want. A little of the crystal is lost in the solution each time though. The solubility and temperature determines how much is lost. Recrystallization works best when solubility increases quickly with temperature.
How To Do It
I will be purifying potassium chlorate from a chlorate cell. The cell also produces some insoluble compounds so I will add a filtering step. Hot filtering works best with large quantities and if you are going to recrystallize in the same step.
Dissolve your chemical in a minimum amount of solvent. I usually add an extra 10% of solvent if you are doing a hot filtration. Check out the solubility chart to determine where that is for your chemical.
I had some small particles and some large particles so I used two filters: a paper towel first and then a coffee filter. The hot solution is poured through both. You will most likely have some crystals form on the edges of your filter. Just pour some hot water down to dissolve them. Be careful not to add to much. The more solvent present, the more compound that gets lost.
Now that your solution is filtered it is time to cool. The solubility of potassium chlorate decreases rapidly with temperature. The crystals will precipitate out of the solution and sink to the bottom. Cool your solution as much as possible. I put my container in a freezer until a small amount of ice formed.
Removing the Waste
Toss out your filters with the waste. The first picture shows the insoluble impurities in my chlorate. Get a new filter and pour out your precipitate. It might help to squeeze the filter to remove more water or pour ice cold water over it. Toss the liquid remaining.
All that is left is to dry your product. Depending on your chemical you might be able to just leave it out to dry. The final crystals are now very pure (>99%). Mine look like fish scales. Use them with care!