Lab 03: Purification of a Solid by Recrystallization

Chem 221
Orange Coast College
Lab 03: Purification of a Solid by Recrystallization
Pavia et al., Introduction to Organic Laboratory Techniques: A Microscale Approach: Technique 5, Sections 5.0–
5.3 (pp. 558–566), 5.5–5.8 (pp. 569–574), 4.3 (pp. 551–553), and 4.5 (pp. 553–555).
Pre-Lab Planning
Prepare planning section in the lab notebook prior to working on the procedure. Present the lab book with the
completed planning section to the instructor to receive an unknown for the lab.
The planning portion of the notebook should have the following sections: Date, Title, Introduction, Haz-Mat (see
below), Procedure.
The Haz-Mat portion should be completed for the following compounds: isopropanol (isopropyl alcohol) and
triphenylmethanol. It may be necessary to find MSDS information online for one or more of these compounds.
In this exercise we will purify a measured mass of an impure compound using the technique of
recrystallization. Our goal will be to attain the highest possible purity while maximizing the percent recovered.
This exercise is designed to illustrate the principles of crystallization in particular; however, it also illustrates an
unhappy fact of life: every purification technique (and almost every transfer technique) results in the loss of
desired substance. While it is relatively easy to obtain low yields of product in a purified state or high yields of
product that is still impure, it requires skill and careful planning to obtain high yields and high purity of the
desired substance. Consider the effect of each procedural step on the yield of desired product while conducting
the experiment.
Obtain a numbered test tube from the instructor that contains a mass of impure triphenylmethanol
(Ph3COH) which has been prepared by students from previous semesters. Record the test tube number in your
notebook and then determine the mass of the sample. Examine the appearance of this solid (and all solids
obtained in the present and all future labs) with the aid of a magnifying glass or the dissecting microscope
(located in the instrument room). Note the color and type of crystals (cubes, platelets, needles, powder, etc.).
Prepare two small melting point samples of the impure compound in melting point capillaries. The
melting point of the sample before purification should be determined using the melting point apparatus (do not
use Thiele tubes or open flames during this lab). However, if there is a long wait for the melting point
apparatus, save the samples in a labeled test tube and continue with the experiment. The melting point may
be determined later. Look up the melting point value for triphenylmethanol in the Aldrich Catalog and in the
CRC handbook. Compare these numbers to the value given in the lab text (p. 298).
Recrystallize the triphenylmethanol from isopropanol according to the general procedure given in the text
(Technique 5, Section 5.3) and this handout. Obtain 10 mL of isopropanol in a 25 or 50 mL Erlenmeyer flask,
and cork the flask until ready to use. Never use a rubber stopper to contain non-polar solvents (such as
hydrocarbons, alkyl halides, alcohols, aldehydes, ketones, and esters), since such solvents will dissolve the rubber
and contaminate the solvent. Contrary to the instructions in the lab book, do not heat the pure solvent; on
our small scale this is unnecessary, and it wastes solvent through evaporation.
Transfer all of your triphenylmethanol to a second 25 mL flask, add a carborundum boiling chip, and then
proceed with dissolving the solid as described in Technique 5, Section 5.3. Use a hot plate (set to 2 or 3) to boil
the solution (no open flames). Use a Pasteur pipet to add the solvent to the solid and keep track of approximately
how much solvent you transferred to just dissolve the solid. Remember, sometimes solids may be contaminated
with insoluble impurities, so try to distinguish between the solid and impurities.
When all (or as much as possible) of the solid has dissolved, cool the hot solution slowly as described in
the technique. The slower the rate of crystal formation, the purer the recovered crystals. We want to maximize
the %-recovered so follow the appropriate suggestions made in the technique. We should end up with well-
Chem 221
Lab 03: Purification of a Solid by Recrystallization
Page 2
formed crystals ALONG WITH some small amount of liquid. If there is no visible liquid, repeat the
crystallization and add a slightly larger amount of solvent.
After cooling in a ice bath, filter this mixture using a Hirsch funnel setup similar to Figure 4.5, page 552.
CAUTION! Be sure that all glassware under a vacuum is securely clamped.
Wash the crystals with ice cold solvent. Air dry by keeping the crystals in the Hirsh funnel for 5
additional minutes with the vacuum source still attached. Transfer the crystals to a watch glass and continue to air
dry. When the solid is dry (if dry, it will not clump or stick to your stirring rod) re-weigh, re-examine the crystals,
and re-determine the melting point range.
Some additional crystals may start to form from the filtrate inside the filtering flask. These crystals did
not go through the Hirsch funnel but, rather, were formed from the filtrate solution as the volume of filtrate
decreased along with the temperature. These generally contain large amounts of impurities, and can be discarded.
However, if the compound were valuable, these would be saved and repurified.
Transfer all of the purified product to a previously tared 75 mm sample tube. Cork the tube and label it.
Save the sample in the lab drawer until the instructor requests that it be submitted.
In the conclusion, summarize the experimental findings including %-recovered of purified
triphenylmethanol, and note all indications of improvement in purity of triphenylmethanol . Also report the
melting points of the impure and purified product, and discuss the significance of any difference between the two.
Postlab Questions:
Answer the following questions after your conclusions in a section titled “Post-Lab Questions”.
Answer questios 1, 2, 7, and 8 (pages 575, 576).