Activated charcoal is sometimes used to remove small amounts of colored impurities from solution. Activated charcoal has a high affinity for conjugated compounds, whose flat structures wedge themselves well between the graphene sheets. The quantity used should be limited, as charcoal adsorbs all compounds to some extent and could lead to a lower recovery of the desired compound. Charcoal should of course not be used if the product itself is colored.
Decolorizing charcoal (Norit) is added after a solid has been dissolved in the minimum amount of hot solvent. A small portion should be used at first; just as much that can fit on the tip of a spatula (the size of half a pea, see Figure 3.39b). When adding charcoal, it's recommended to temporarily remove the solution from the heat source or the charcoal's high surface area may cause superheated areas to immediately boil. Superheated areas are regions where the temperature is above the boiling point of the solvent, but lack a nucleation site to create a bubble and initiate the phase change. If charcoal is added directly to a near boiling solution, the solution may boil over.
For fairly colored solutions, the first portion of charcoal is likely all that is necessary, and the resulting solution should be a faint grey (Figure 3.39c). Additional charcoal can be added if the color remains (Figure 3.40c). Charcoal particles are so fine that hot filtration is necessary to remove them before crystallization.
Charcoal does not need to be used with every solution that's colored, even if the desired compound is white. Colored impurities may remain in the mother liquor after filtration. For example, the sequence in Figure 3.41 shows the purification of acetanilide (a white solid) that has been contaminated with several drops of methyl red solution to produce an orange solid (Figure 3.41a). The solid was crystallized from hot water without the use of charcoal, and even though the mother liquor was yellow (Figure 3.41b), the crystallized solid was still a pure white color (Figure 3.41c). The methyl red "impurity" remained in the mother liquor and was not obviously incorporated into the crystal lattice of acetanilide.
As a comparison, the same solid was decolorized with charcoal, filtered and crystallized, as shown in Figure 3.42. The charcoal removed the methyl red color, and the mother liquor was colorless (as indicated with an arrow in Figure 3.42b), but there was no obvious improvement of color in the resulting crystallized solid (Figure 3.42c). Additionally, there was a loss of yield when using charcoal. When charcoal was not used, the recovery was \(63\%\), which was consistent with various other trials of acetanilide. However, when charcoal was used, the recovery was \(53\%\). The loss of yield may be attributed to charcoal's absorption of the target compound along with the colored impurity, as well as loss of compound on the filter paper during the hot filtration. For this reason, charcoal should only be used if specified in a procedure, or if previous crystallization trials failed to remove colored impurities.