Crystallization and recrystallization

Crystallization can be used both for production and purification of compounds. A simple crystallization aims at the isolation of different compounds, while the so-called recrystallization is used for the purification of the isolated crystalline compounds. For crystallization, a supersaturated solution of the crystallized material should be prepared, e.g. with cooling of a hot saturated solution, concentration of an unsaturated solution, or changing the composition of the solvent of the nearly saturated solution. Crystal nuclei start to develop in the supersaturated solution, on which the dissolved particles can precipitate, and the impurities remain in the solvent, or do not dissolve at all.

The size of the crystals depends on the relation between the development of crystal nuclei and speed of crystal growth. The faster the rate of cooling, the smaller the crystals form; in this case numerous small crystals form. The slower the rate of cooling, the larger the speed of the crystal formation is; in this case fewer big crystals form.

If the crystallization is difficult to start (the nuclei do not start to develop), the process can be successfully promoted by rubbing the inner surface of the container, or putting a few grains of the seed crystal into the supersaturated solution.

If crystallization aims at purification, the process is called recrystallization: the substances are dissolved, and crystallized again. Recrystallization can be used for

Identification number:

TÁMOP-4.1.2.A/1-11/1-2011-0016 69

purification of such compounds, whose solubility changes with temperature more than those of the impurities. The solubility of the majority of water-soluble substances increases with temperature; therefore a hot saturated solution should be prepared for their recrystallization. To improve the effectiveness of the purification, a so-called activated carbon with high surface area is added to the hot saturated solution. This activated carbon is able to adsorb impurities on its surface (usually with the help of van der Waals forces) (flocculation). If the filtered hot solution is cooled, it becomes supersaturated, and crystals of the substance start to form. The remaining cool solution is called the mother liquor; it is saturated with the crystallized substance and keeps the soluble impurities of the crystallized material in solution.

Some practical aspects of recrystallization:

The temperature of the hot saturated solution should be at least with 10-20 ºC 1.

below the boiling point of the solvent.

The hot saturated solution should be filtered through pleated filter paper with the 2.

help of a preheated funnel.

The crystals can be filtered at atmospheric pressure using conventional filter paper, 3.

or under vacuum using a Büchner funnel or a sintered glass funnel.

The mother liquor should be washed from the surface of the filtered crystals with a 4.

small amount of cold solvent.

The impurities adsorbed at the surface of the crystallized substances can be 5.

removed with clarifying agents or recrystallization.

Calculation examples for recrystallization III.5.3.1

Prepare a saturated solution of KNO3 at 75 ºC by dissolving 90.0 g of KNO3 in 1.

water, and cool the solution to 20 ºC. Calculate the mass (in g) of the deposited KNO3. What is the yield of recrystallization? Concentration of the saturated solution of KNO3 is 150.0 g KNO3/100 g water at 75 ºC, and 32.0 g KNO3/100 g water at 20 ºC.

at 75 ºC 150.0 g KNO₃ in 100.0 g water

90.0 g KNO3 in x g water

x = 90.0 ⋅ 100.0

150.0 = 60.0 g

at 20 ºC in 100.0 g water 32.0 g KNO₃

in 60.0 g water x g KNO3

x = 60.0 ⋅ 32.0

100.0 = 19.2 g

The mass of KNO₃ deposited on cooling is 90.0 - 19.2 = 70.8 g.

The yield:

70.8 ⋅ 100.0

90.0 =78.7%

Thus, 70.8 g of KNO₃ will be deposited. The yield of recrystallization is 78.7 %.

70 The project is supported by the European Union and co-financed by the European Social Fund What a mass (in g) of CuSO4 ⋅ 5 H2O will be deposited if 200.0 g of 40 m/m%

2.

CuSO₄ solution is cooled to 30 ºC. Concentration of the saturated solution at 30 ºC is 20 m/m%. (M(CuSO4) = 159.6 g/mol; M(CuSO4 ·5 H2O) = 249.6 g/mol)

Mass of the dissolved material in 200 g solution = ^{200.0 ⋅ 40}

100

=

^{80 g}

Mass of the deposited CuSO4 ⋅ 5 H2O = x g Mass of CuSO4 in x g CuSO4 ⋅ 5 H2O = ^{x ⋅ 159.6}

249.6

=

0.639 ⋅ x g Mass of CuSO4 remaining inthe solution = (80.0 – 0.639 x) g Mass of the solution = (200.0 – x) g

(200.0 – x) ⋅ 0.2 = 80.0 – 0.639 x x = 91.11

Thus, 91.1 g of CuSO₄ ⋅ 5 H2O will be deposited.

Experimental task: Purification of benzoic acid with alkaline III.5.3.2

precipitation and recrystallization

Measure 1.0 g of benzoic acid (technical grade) on an analytical scale into a 50 cm³ Erlenmeyer flask. Add 5 cm³ of sodium hydroxide solution and shake until it will dissolve. If it does not dissolve completely at room temperature, you can mildly warm it through asbestos wire gauze.

Filter the cool (or cooled) sodium benzoate solution, which still can contain insoluble impurities, into a 10 cm³ beaker. Add a pinch of activated carbon to the filtrate and mix it well (see flocculation). Filter the suspension again into a 10 cm³ beaker.

While vigorously stirring the mixture with a glass rod, gently pour 1 cm³ of concentrated hydrochloric acid to the filtrate.

Filter the white curdy precipitate through Büchner funnel under vacuum. Tamp down well the precipitate with a flat-ended glass rod and wash it with 5 cm³ of ice-cold water with which the beaker contained the precipitated benzoic acid has been washed.

Transfer the filtered material into a 50 cm³ Erlenmeyer flask with the help of a spoon and a glass rod. Wash the adhered benzoic acid from the funnel, the spoon, and the glass rod into the Erlenmeyer flask with about 15-20 cm³ distilled water.

Warm the solution through asbestos wire gauze. When the solid completely dissolved, filter the hot solution into another 50 cm³ Erlenmeyer flask. Allow the solution to cool to room temperature until crystals start to form. Then put the flask into an ice-cold water bath. Filter the crystalline benzoic acid by means of a water aspirator and wash it with a small amount of ice-cold distilled water. Continue aspiration until water no longer drips from the funnel.

Spread out the substance on a filter paper and allow to dry. When it is completely dry, weigh it together with the filter paper. Put the pure substance into a watchglass, and measure the weight of the filter paper. Calculate the loss of recrystallization process.

Identification number:

TÁMOP-4.1.2.A/1-11/1-2011-0016 71

Purification of contaminated alum with recrystallization III.5.3.3

Contaminated crystalline materials can be purified by recrystallization if solubility properties of the substance and its impurities make it possible.

If impurities are present in only a few percent, and solubility of the compounds is similar, the procedure as follows: Prepare a saturated solution of the substance at an elevated temperature. Then cool down the solution, in order to get a supersaturated solution. Wait for precipitation of the crystals, then filter and dry them. In the following experiment crystalline alum (KAl(SO4)2 ⋅

12 H2O) contaminated with 5 m/m% copper sulphate (CuSO4 ⋅ 5 H2O) and 1 m/m% insoluble substance (quartz) will be purified by recrystallization.

Measure 30 g of contaminated alum on analytical scale. Suspend it with 30 cm³ of distilled water and warm up to boil to dissolve alum and copper(II) sulphate. Filter the hot solution through pleated filter paper using a preheated funnel. Cool the Erlenmeyer flask (containing the filtrate) with tap water and filter the precipitated crystals with a conventional filter paper. Wash the filtered crystals with 2-3 cm³ of cold distilled water to remove the adsorbed copper(II) sulphate. Transfer the crystals into a 50 cm³ Erlenmeyer flask and dissolve the crystals in a minimum amount of boiling water.

Allow the solution to cool to room temperature until crystals start to form. Then put the flask into an ice-cold water bath. Filter the crystalline alum by means of a water aspirator and wash it with a small amount of ice-cold distilled water. Continue aspiration until water no longer drips from the funnel.

Spread out the substance on a filter paper and allow to dry. When it is completely dry, weigh it together with the filter paper. Put the pure substance into a watchglass, and measure the weight of the filter paper. Calculate the yield of recrystallization process.

In document Chemistry – Laboratory (Pldal 68-71)