Isolation and Purification of an Organic Compound
EXPERIMENTALPROCEDURE
A. Choosing a Suitable Solvent
The choice of solvent is crucial in a separation-and-purification . One essential
characteristic of a suitable solvent is that the desired compound must be
considerably more soluble in hot solvent than in cool solvent. A second desirable
characteristic is that impurities should be either insoluble in the solvent at all
temperatures or very soluble in the cool solvent. soluble in hot solvent than in
cool solvent. A second desirable characteristic is that impurities should be either
insoluble in the solvent at all temperatures or very soluble in the cool solvent. In
Part C of this experiment, acetylsalicylic acid (we will call it aspirin from that point
on) will be separated from the impurities sugar and sand. The sand is insoluble in
all solvents that might be used to purify aspirin by recrystallization, so we will
focus our attention on the solubility characteristics of acetylsalicylic acid (aspirin)
and sugar in various solvents in this part of the experiment.
Procedure
1. Label four clean, dry 10-cm test tubes with the numbers 1, 2, 3, and 4.
2. Put a 0.10-g sample of acetylsalicylic acid into each test tube. It is important that
the samples be weighed carefully even though the sample masses are not
recorded in any data table. Use plastic weighing dishes and either a centigram or
electronic balance to weigh each sample to .01 g.
3. Add 2.0 mL of ethanol to test tube 1 (measure all solvent volumes with a 10-mL
graduated cylinder). Rinse the graduated cylinder with water and dry it with a
paper or cloth towel. Add 2.0 mL of 2-propanol (isopropyl alcohol) to tube 2.
Rinse and dry the cylinder and add 2.0 mL of distilled water to tube 3. Rinse and
dry the cylinder and add 2.0 mL of toluene to tube 4.
4. Grasp each test tube between your thumb and forefinger and agitate each tube
vigorously for 2 minutes or until the solid dissolves, whichever comes first.
Observe and record in Table 17.1 the results of this solubility test. Use the
symbol to represent those solvents in which the solute totally dissolved and to
represent those solvents in which some solid remained undissolved after the
agitation was completed.
5. Solid should have remained in two of the solvents used in Step 4. Put only the
two test tubes containing those solvents and undissolved solid into a boilingwater bath. The bath is made by half-filling a 250-mL beaker with water and
adding one or two boiling chips .Allow each of the two test tubes to remain in the
bath for 5 minutes. Agitate each tube several times while the heating proceeds.
After 5minutes of heating, observe the test tubes and use the (+) and (–) symbols
to represent the solubility behavior.
6. On the basis of the results obtained so far, you should be able to identify one or
two solvents that have suitable solubility behavior toward the acetylsalicylic acid
that you want to separate and purify. Identify any suitable solvents by putting an
x in the blanks under their names.
7. Determine the solubility behavior of sugar in cool samples of the solvent or
solvents identified in Step 6 as suitable. Do this by putting 0.10 g samples of
sugar into clean, dry 10 cm test tubes and adding 2.0 mL of the appropriate
solvent (remember, do not heat the solvents). Agitate each test tube as you did in
Step 4. Record the behavior in , using the + and - symbols as before in the
blanks under the recorded names.
Based on your observations, identify a single solvent that has solubility
characteristics toward sugar that make it suitable for use in the separation
and purification of acetylsalicylic acid . Remember, a suitable solvent will
be one in which the sugar will dissolve at a low temperature.
B. The Effect of the Amount of Solvent Used:
After selecting a good solvent for a recrystallization, care must be taken to use the
proper amount of solvent. Enough solvent must be used so that the sample dissolves
when heated, but if too much solvent is used the solution does not become saturated on
cooling and nothing is recovered. The most common error in recrystallizations is using
too much solvent. To demonstrate this principle, you will observe the effect of water
quantity on the recrystallization of acetylsalicylic acid.
Procedure
1. Label three 10-cm test tubes with the numbers 1, 2, and 3.
2. Put 0.10-g samples of acetylsalicylic acid into each test tube.
3. Add 2.0 mL of distilled water to test tube 1, 4.0 mL of distilled water to test tube 2,
and 6.0 mL of distilled water to test tube 3.
4. Put all three test tubes into a boiling-water bath and allow them to heat until all the
acetylsalicylic acid has dissolved in each test tube. Agitate each test tube occasionally
during the heating.
5. After the acetylsalicylic acid has completely dissolved in each test tube, remove all
test tubes fromthe bath and allowthemto cool for 10minutes. After 10 minutes, agitate
each tube vigorously, allow the solid to settle and record in Table 17.2 the amount of
solid that has come back out of solution. Use relative words like none, some, and a lot
to describe the amounts.