Why recrystallize

Therefore, the solvent is heated to its boiling point remember to use boiling stones! If too much solvent is added, the solution will not be saturated upon cooling and no crystals will form. Dissolving the solute generally involves adding a small volume of hot solvent, swirling the flask or stirring the solution , and watching to see if the solute dissolves.

Decolorize the solution. If the solute is supposed to be white in its pure solid state most organic solids are and the solution is colored after dissolving all the solute, it will be necessary to add decolorizing carbon to the solution. This will cause the colored molecules to adsorb onto the surface of the decolorizing carbon, thereby ridding the solution of these impurities.

Should these impurities remain in solution, they may become trapped in the developing crystal during cooling. Review the material about decolorizing carbon. Filter any solids from the hot solution. If decolorizing carbon was used as in step 3 or undissolved impurities remain in the hot solution, it is necessary to gravity filter the solution while it is still hot.

Review the information about hot gravity filtration and decolorizing carbon. Under no circumstances should the hot solution be vacuum filtered with a Buchner funnel. This leads to premature crystal development as the solution passes through the vacuum filter. The vacuum reduces the pressure, but also the temperature. Impurities will be trapped in the crystal lattice and steps 1 through 3 will need repeated!

Crystallize the solute. This involves allowing the hot solution with the solute dissolved to return to room temperature slowly. The slower the cooling process, the less chance of trapping impurities in the developing crystal lattice. Allow the solution to reach room temperature. When the solution is later cooled, after filtering out insoluble impurities, the amount of solute that remains dissolved drops precipitously.

At the cooler temperature, the solution is saturated at a much lower concentration of solute. The solute that can no longer be held in solution forms purified crystals of solute, which can later be collected. Recrystallization works only when the proper solvent is used. The solute must be relatively insoluble in the solvent at room temperature but much more soluble in the solvent at higher temperature.

At the same time, impurities that are present must either be soluble in the solvent at room temperature or insoluble in the solvent at a high temperature. For example, if you wanted to purify a sample of Compound X which is contaminated by a small amount of Compound Y, an appropriate solvent would be one in which all of Compound Y dissolved at room temperature because the impurities will stay in solution and pass through filter paper, leaving only pure crystals behind.

Also appropriate would be a solvent in which the impurities are insoluble at a high temperature because they will remain solid in the boiling solvent and can then be filtered out.

When dealing with unknowns, you will need to test which solvent will work best for you. According to the adage "Like dissolves like," a solvent that has a similar polarity to the solute being dissolved will usually dissolve the substance very well.

In general, a very polar solute will easily be dissolved in a polar solvent and will be fairly insoluble in a non-polar solvent. Frequently, having a solvent with slightly different polarity characteristics than the solute is best because if the polarity of the two is too closely matched, the solute will likely be at least partially dissolved at room temperature. There are five major steps in the recrystallization process: dissolving the solute in the solvent, performing a gravity filtration, if necessary, obtaining crystals of the solute, collecting the solute crystals by vacuum filtration, and, finally, drying the resulting crystals.

Figure 1. Hot gravity filtration. Keeping the set up hot prevents crystals from forming prematurely. If crystals don't form upon slow cooling of the solution to room temperature there are a variety of procedures you can perform to stimulate their growth. Disturbing the solution can break up any seed crystals 6 that have started growing. Such crystals are often referred to as single crystals, and not only must they be completely pure, but also the crystal lattice and growth must be highly ordered.

This task can be monumental, as very small variables can be detrimental to the growth of a single crystals. During grad-school, a post-doc told me to leave the crystals growing in a room where nobody ever went, as even the vibrations of footsteps or laboratory equipment could be detrimental. Please also note that some compounds simpl crystallize more easily than others.

More rigid molecules are, as a rule, easier to crystalize. Let us go through a recrystallization process, focusing on technical aspects and trouble shooting. Typical problems: Adding too much solvent so that the product does not crystallize later.

Typical problems: Crystals do not form at all too much solvent , precipitate forms instead of crystals temperature has dropped too quickly, or an oil forms.

Typical problems: Crystallization can be a slow process, and impatience can lead to low recovery. How can we tell if a recrystallization has been a success? Sim- ple visual inspection is a good start: The crystals should have shiny surfaces and catch the light.

They should appear uniform, and you should have crystals of similar structure and size.