3.3B: General Procedures for Removing Impurities

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Crystallization works well as a purification technique if impurities are present in very small quantities (less than \(5 \: \text{mol}\%\) of the solid), or if the impurities have a very different solubility profile from the desired compound. Impurities can be easily removed if they are either much more soluble or much less soluble in the solvent than the compound of interest.

Figure 3.9 shows the procedural sequence used to remove a "soluble impurity" from an impure soled, meaning an impurity that is embedded in the crystalline matrix, but would be completely soluble in the crystallization solvent. The impure solid is first fully dissolved in the minimum amount of hot solvent to liberate impurities trapped in the solid's interior. Upon cooling, a completely soluble impurity will remain dissolved in the mother liquor while the desired compound crystallizes. The crystallized solid can then be collected by suction filtration.

Figure 3.9: Crystallization sequence in order to remove a soluble impurity, using black circles and white squares to represent individual particles. As in reality individual particles are too small to see, the liquids in each of these scenarios would appear completely transparent.

Figure 3.10 shows the procedural sequence used to remove an "insoluble impurity" from an impure solid, meaning an impurity that is embedded in the crystalline matrix, but would be insoluble in the crystallization solvent. The impure solid is heated in the minimum amount of hot solvent needed to dissolve the desired compound. The insoluble material is then filtered while the solution is kept hot (called "hot filtration"), and then the desired compound is crystallized and collected by suction filtration.

Figure 3.10: Crystallization sequence to remove an insoluble impurity. In the second flask, the white circles aggregate as an insoluble material, while the black circles are dispersed and therefore dissolved and invisible.