Crystallization
Revised 6/4/12
CRYSTALLIZATION
Crystallization is a common technique used to purify solids. Two common methods of
crystallization are “gradual cooling” and “diffusion”.
Gradual Cooling
Gradual cooling involves dissolving the impure solid in a minimum amount of a hot solvent and
allowing the resulting solution to cool slowly to room temperature. During the cooling process,
pure (or almost pure) crystals form and are then collected by vacuum filtration.
The first step in the crystallization process is to find an "ideal” solvent by testing a small amount
of the impure solid. The desired compound should be very soluble in the solvent when it is hot
(near the boiling), but only slightly soluble at room temperature. Sometimes a solvent mixture
(two or more solvents in a specific ratio) is required to achieve this temperature dependent
solubility. At the same time, the dissolved impurities (byproducts and unreacted starting
material) should be highly soluble in the solvent at BOTH high and low solvent temperatures.
The compound must precipitate out of solution at cooler temperatures to allow its separation
from the solvent and the dissolved impurities by filtration.
The next step is to dissolve the solid to be purified in a minimum amount of hot solvent. This
step should be performed quickly, with stirring if necessary. (Note: The solid and solvent
should NOT be combined and then heated – this may cause decomposition of the desired
compound. The solvent should always be heated first, and then added to the solid.) For the
largest and purest possible crystals, the resulting solution should be set in a location where it will
not be agitated.
The solubilities of three compounds in benzene as a function of temperature is shown in Figure
1. The following example illustrates how the relationship between temperature and solubility is
used to isolate a desired compound by crystallization.
A mixture of 5.0 g of A (an impurity), 20.0 g of B (the desired compound), and 3.0 g of C (an
impurity) is combined with 100-mL of boiling benzene (75-80°C). According to the solubility
1
Crystallization
Revised 6/4/12
diagram, all compounds will initially dissolve. Compound B begins to crystallize once the
temperature drops below 67°C. When the solution cools to 15°C, all of A and C remain
dissolved: their solubilities (17.5 g of A and 3.5 g of B per 100 mL benzene at 15°C) are greater
than the amounts of each present in solution (5.0 g of A, 3.0 g of C). However, at 15°C the
solubility of B is 7 g per 100 mL benzene, so only 7 grams of B remain dissolved and 13 g have
crystallized out of solution. The purified yield of B (13 g) will be about 65%. An attempt to
increase the yield of B by lowering the temperature below 15°C would be ineffective because C,
an impurity, would also begin to crystallize, contaminating B again. If more than 3.0 g of C
were present the purified yield of B would be even lower. Why?
Benzene Solubilites of 3 Different Solids
25
50
22.5
45
Solubility (g/100mL C6H6)
Solubility (g/100mL C6H6)
Benzene Solubilites of 3 Different Solids
55
40
35
30
25
20
15
10
20
17.5
15
12.5
10
7.5
5
2.5
5
0
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
0
5
10
15
Temperature (oC)
20
25
30
35
40
45
50
55
60
65
Temperature (oC)
Figure 1. Solubility Curves of A (♦), B (■), and C(▲) in benzene.
Diffusion
Crystallization by diffusion is an alternative to gradual cooling that does not use heated solvents.
This crystallization process is preferable if the desired compound degrades at the elevated
temperatures of solvent boiling points. The impure solid is dissolved in a minimum amount of a
solvent in which it is highly soluble (the “good” solvent). This solution is placed in a test tube
anchored at a 45° angle by a clamp (or leaned at a similar angle in a beaker). The next step is to
carefully “layer” a solvent in which the desired compound is insoluble (the “bad” solvent) on the
top of the first solution. The bad solvent is slowly delivered by disposable pipet that is in contact
with the lowest edge of the test tube opening (Figure 2). Slow diffusion of the “bad” solvent into
the “good” solvent will cause crystals to slowly grow at the interface between the two solvents.
For diffusion to work, the impurities must be soluble in both solvents.
2
70
75
Crystallization
Revised 6/4/12
"Bad" Solvent
"Good" Solvent
with dissolved
Impure Product
Figure 2. Layering of Solvents for Diffusion Crystallization
Review Questions:
•
In the slow cooling crystallization process, is it possible to recover 100% of the desired
product that is dissolved in a minimum amount of boiling benzene? (No)
•
Assume 15 g of substance B is mixed with 5g of substance C in 100-mL of benzene (Fig 1).
a) At what minimum temperature are BOTH substances completely dissolved?
b) At what temperature range will B crystallize but all of C remains in solution?
c) What is the mass and percent yield of B that is recovered as a crystallized solid if the
benzene solution is heated to 65°C and then cooled to 40°C?
( (a) about 54°C
(b) 35 to 54°C
(c) about 3 g or 20% )
3