Purification of Solid Organic Compounds

Tishk International UniversityFaculty of ScienceMedical Analysis Department

Practical Organic Chemistry

For

First grade Students

Lab. 04

Aim of the Experiment

Separation of an organic compound from impurities based on their solubility differences.

Organic compounds prepared in the laboratory or isolated from natural sources are usually found as mixtures. Some of the common methods used for their purification are:

• Sublimation

• Recrystallization

• Distillation

• Chromatography

Recrystallization

Recrystallization is a technique used to purify solids. The process relies

on the fact that solubility increases with increasing temperature. As a

hot saturated solution cools, it becomes supersaturated and the solute

precipitates (crystals) out. In a recrystallization procedure, an impure

(crude) solid is dissolved in a hot solvent. As this solution is cooled, the

pure product crystals out and the impurities stay dissolved.

Recrystallization Process

Recrystallization :is a very important purification technique, purifying substancesby removing unwanted by-products. It is also used tomanufacture the correct crystal size and shape of a material.

What are the principles behind recrystallization?

The process depends on two principles; the fact thatsubstances tend to be more soluble in a hot solvent than incold solvent, and that each solute tends to behave as though itwere alone in the solvent.(solubility)

Recrystallization Steps

• Choose a suitable solvent• Dissolve the impure solid in the minimum volume of the hot solvent• Remove insoluble impurities by hot filtration.• Slowly cool the hot solution to recrystallize the desired compound from the

solution.• Filter the solution to isolate the purified solid compound.• Drying the purified crystals.

Impure benzoic acid

Recrystallization

after recrystallization

Pure benzoic acid

Recrystallization Solvent

The choice of solvent is perhaps the most critical step in theprocess of recrystallization.

• The desired compound should be reasonably soluble in the hotsolvent, about 5 g/100 mL being satisfactory, and insoluble ornearly insoluble in the cold solvent. The impurities should beinsoluble or slightly soluble in the cold solvent.

• The solvent must have moderate boiling point, generally below themelting point of the desired compound.

• The solvent should be inert (chemically unreactive) towards thecompounds in the mixture.

• Inexpensive, non-toxic and nonflammable.

Some common recrystallization solvents

Procedure

1. Weigh (0.2 g) of the impure sample and place it in a conical flask.

2. Add (15 mL) of solvent (start by adding 1 mL portions and note the solubility).

3. Heat until the desired compound is dissolved completely (at or near the b.p. of the solvent).

4. Filter the hot solution and let the filtrate cool down slowly to allow crystallisation of the pure compound.

5. Separate the pure crystalline solid by filtration.

6. Calculate the percentage of the pure product.

Drying Agents

Drying agents (also called Desiccants)

Desiccant is a hygroscopic substance that induces or sustains a state of

dryness in its vicinity.

Chemically stable and inert.

Calcium chloride (CaCl2), sodium sulfate (Na2SO4) calcium sulfate (CaSO4)

and magnesium sulfate (MgSO4), all in their anhydrous form.

If the compound is pink, the water can be removed by heating the compound to 210 oC for an hour.

Dry (blue) Wet (pink)

Example: Calcium sulfate

Desiccator