Question

Why is charcoal sometimes used in recrystallizations? How does it function? Why should you avoid using large amounts? "Activated carbon" is used in air and water purifiers. How does it work?

Answer 1

Charcoal (aka. Norit, activated carbon) is added during a recrystallization to bind high molecular weight, colored impurites.

Charcoal is added once you have a hot saturated solution of the desired product, and, once added, needs to be removed (along with the adsorbed colored impurities) by hot gravity filtration.

The decolorized filtrate contains the desired product, and the charcoal with the adsorbed colored impurities is retained on the filter paper.

Activated charcoal is charcoal that has been treated with oxygen to open up millions of tiny pores between the carbon atoms.Activated charcoal is good at trapping other carbon-based impurities ("organic" chemicals), as well as things like chlorine. Many other chemicals are not attracted to carbon at all -- sodium, nitrates, etc. -- so they pass right through. This means that an activated charcoal filter will remove certain impurities while ignoring others. It also means that, once all of the bonding sites are filled, an activated charcoal filter stops working. At that point you must replace the filter.