Chemistry 29a M. Walker

Recrystallization (referred to as crystallization in the book), is a very useful method of purification for solid substances. In essence, the process involves taking a solid sample, dissolving it in a hot solvent, then slowly cooling the solution to allow crystals to form (most substances are less soluble in the cold). In an ideal situation, the substance is totally insoluble cold, but soluble hot, and any impurities trapped the starting sample end up dissolved in the cold liquid (removed by filtration) at the end. Thus the sample is purified.

Solubility depends on several things, but of great importance is the match between the polarities of the material being dissolved (called the solute) and the solvent, based on the rule "like dissolves like". Thus a nonpolar solute (e.g. an alkane) will tend to dissolve in a nonpolar solvent such as ligroin, but be insoluble in a polar solvent such as water. Note, however, that for recrystallization we want our solid to be insoluble in the cold, so a good solvent for dissolving will make a poor recrystallization solvent.

In many cases, the "seven steps of crystallization" (see book) are unnecessary, and when we recrystallize phthalic acid, we will only be using steps 2, 5, 6 and 7.

We will be performing experiment 1 (p57) (solubility tests), and the recrystallization of phthalic acid from water from experiment 2 (p59). I suggest doing the phthalic acid experiment first.

We will modify the procedure slightly: we will not be doing the solvent pair solubilities with methanol, ethanol, acetone and acetic acid combined with water. We will instead do single solvent tests using water, ethanol, toluene and ligroin, using each of the solvents shown, i.e. 16 tests in all. Make up a table to record results for each test, for cold, hot, and further comments (e.g. crystal form). If your sample is soluble in the cold solvent, you need not test hot (most materials are more soluble hot). You should try to discuss in your reports why you observed the results you did- look at the chemical structures of the substances used.

Make sure that you use a reaction tube from your kit, and weigh it empty; do not use a disposable tube, or an accident is likely later. Filter as shown on p50, ask your TA to demonstrate. We will dry the sample (without removing it) in the tube using vacuum, subtract the tube’s mass to find % recovery.

Assigned problems from chapter 3: 2, 3, 5, 9.

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