Question

1. Describe the process of selecting a suitable recrystallization solvent. (4 pts)

Answer 1

If the solvent is not specified, you will need to test a variety of solvents to determine what will work best for the solute you are trying to recrystallize.

This testing can be accomplished by putting a small amount of your solute (about the size of a pea) into three small test tubes.

In each test tube, place 0.5 mL of each potential solvent. Use a stirring rod to agitate the solute or "flick" the bottom of the test tube with one finger while holding the top with the other hand. If the solute dissolves at room temperature with stirring, the solvent should be discarded as a potential recrystallization solvent. If the sample does not dissolve at room temperature, place the test tube in a hot water bath and stir the contents. If the solute partially dissolves add more solvent and continue stirring.

If the solute dissolves completely, remove it from heat and place it in an ice-water bath.

If crystals do not form, try to scratch the inside of the test tube with a stirring rod. If crystals form, you have found an appropriate recrystallization solvent; if no crystals form, keep looking for the right match between solvent and solute.

If no solvent can be found, perhaps you will need to use a mixed solvent. If two solvents in which the solute has different solubility characteristics are mixed, sometimes an appropriate solvent can be made.

Commonly Used Solvents

Solvent B.P. Dielectric Constant* Safety Concerns

Water 100 78.4 None

Ethanol 78 24.5 Flammable; irritating to the eyes, respiratory system, and skin

Acetone 56 20.6 Flammable; Irritating to the eyes; Vapors may cause drowsiness or dizziness

Tetrahydrofuran 66 7.58 Flammable; Harmful if swallowed; Irritating to the eyes, skin, and respiratory system

Ethyl acetate 78 6.02 Flammable; Irritating to the eyes; Vapors may cause drowsiness or dizziness

Cyclohexane 81 1.89 Flammable; May damage lungs if swallowed; Vapors may cause drowsiness or dizziness

*The dielectric constant is a measure of the solvent's ability to separate ions. In general, ionic compounds are more soluble in solvents with high dielectric constants.