Recrystallization Lab

Percent Recovery: Increase Purity: Decrease

If not all the solid impure benzoin was dissolved in the ethanol, how would this effect the purity and percent recovery?

Benzoin

In this experiment, what is being recrystallized from ethanol?

Yes, if the compound is impure.

Is it possible to have percent yield greater than 100%?

Decolorizing charcoal will not be used. Hot filtration will not be needed.

Is the starting material in this lab colored? What is the result of this?

1) Choose a solvent. 2) Add hot solvent just until all of the compound you are trying to purify dissolves. 3) If colored impurities are present, add decolorizing charcoal. 4) If charcoal was added, filter the resulting solution while it is still hot. 5) Cool the solution. 6) Separate the solid compound from the solvent. 7) Dry the isolated crystals.

What are the 7 steps of recrystallization?

All slightly toxic (ethanol also extremely flammable).

What are the hazards associated with the compounds in this experiment?

This solvent must not have a high solubility of the compound when cold, but does have a high solubility when hot. (This allows the solid to first be dissolved in hot liquid, then upon cooling to precipitate the solid. These solvent properties make the recrystallization process possible.) The solvent also cannot chemically react with the material to be purified. Lastly, the solvent should have a reasonably low boiling point (<120 degree C) to ensure the solvent can be evaporated or otherwise easily removed from the isolated product.

What are the properties of a good recrystallization solvent?

If you use too much solvent, less of the compound you're trying to purify recrystallizes (more remains in solution), and you'll get a low percent recovery. This does not impact the purity of the recovered material. If you use too little solvent, not all of your crystals will dissolve in the hot solvent, and they will retain some impurities. The purity will decrease and the percent yield will increase slightly. Slower cooling tends to give larger more pure crystals. Faster cooling results in smaller crystals of lower purity. Plunging hot liquid directly in ice water forces very rapid cooling and traps impurities within the crystal lattice.

What errors effect only purity or percent recovery or both? (Review notes, data sheet and quiz)

If you use too much solvent, less of the compound you're trying to purify recrystallizes (more remains in solution), and you'll get a low percent recovery (no impact on purity).

What happens if additional solvent is added?

If you use too little solvent, not all of your crystals will dissolve in the hot solvent, and they will retain some impurities. The purity will decrease and the percent yield will increase slightly.

What happens if not enough solvent was added?

The amount of recovered material will be assessed by calculating a percent recovery. Percent Recovery = (𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑜𝑢𝑛𝑑 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑 / 𝑠𝑡𝑎𝑟𝑡𝑖𝑛𝑔 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑) 𝑥 100%... Since the starting benzoin contains impurities and the purpose of the recrystallization is to remove impurities, a percent recovery of 100% would signal a failed recrystallization.

What is percent recovery and how is it calculated?

This step separates two kinds of impurities from the substance you want. The charcoal binds with the colored molecules. When filtering the charcoal coated with impurities is removed along with particles of impurities that didn't dissolve in the hot solvent to begin with.

What is the importance of filtering the solution while it is still hot, if charcoal was added?

The purpose is to separate impurities from the desired solid substance thereby purifying the target compound.

What is the purpose of recrystallization?

Smaller crystals of low purity form.

What is the result of faster cooling?

This solvent must not have a high solubility of the compound when cold, but does have a high solubility when hot. (This allows the solid to first be dissolved in hot liquid, then upon cooling to precipitate the solid. The solvent also cannot chemically react with the material to be purified. Lastly, the solvent should have a reasonably low boiling point (<120oC) to ensure the solvent can be evaporated or otherwise easily removed from the isolated product.

What must be considered when choosing a solvent?

We want it all to dissolve because if it is not all dissolved, the purity will be much lower and the mass recovered will be higher.

When recrystallization why should solvent be added until the entire solid dissolves and no more?

This removes the remainder of the recrystallization solvent, which is an impurity.

Why do we dry the isolated crystals?

Allowing the product to crystallize slowly tends to give larger, more pure crystals.

Why should the product be allowed to crystallize slowly and not plunge directly into ice water?