Sublimation of Benzoic Acid
September 25, 2006
This procedure follows closely a similar procedure by Dr. Mark Hemric, and
utilizes principles from Technique 9 of Microscale Organic Laboratory Fourth Edition
by Mayo, Pike and Trumper. As you follow the directions, refer to your lab textbook
to clarify any difficulties with the procedure.
- Transfer the contents of your bicarbonate extractions from the previous
week into a 30 mL beaker. Dissolve 200 mg NaCl in the solution, heating
- Add 6 M hydrochloric acid to the beaker, gently swirling the beaker to
mix the acid and the solution. As you add the acid to the solution, a white
precipitate should form, and the solution should bubble or foam. What is
precipitating out of solution? Why is the solution bubbling? Continue to
add acid until the bubbling stops or until no more precipitate forms on
addition of acid.
- Filter your precipitate using your Hirsch funnel and vacuum ﬁltration ﬂask
as described in your textbook under ﬁltration techniques. Continue the
vacuum on the ﬂask until it is reasonably dry (covering with Saran Wrap
- Scrape the precipitate onto a watchglass and set the watchglass on the
warm sandbath. Continue heating the solid at about 100C until it is
completely dry. Determine the mass of the solid. What is the percent yield
of the benzoic acid from the extraction?
- Reserve a small amount of the crude benzoic acid for use in a melting point
determination. If the recovered amount of benzoic acid is more than 100
mg, place 100 mg of your benzoic acid into a round-bottom ﬂask (either
size will do). If you have recovered less than 100 mg but more than 50 mg,
place all of your crude benzoic acid (less your melting point sample) into
a round-bottom ﬂask. If your benzoic acid weighs less than 50 mg, place
the solid into a 5 mL reaction vial.
- Attach your sublimator to the top of the vial or ﬂask, using vacuum grease
to seal the joint. Attach the aspirator to the other end of the sublimator.
Submerge the vial or ﬂask into the sandbath. Fill the tube in the top of the
sublimator (the cold ﬁnger) with ice water. Start water running through
the aspirator in order to create a vacuum in the vial or ﬂask.
- Slowly increase the temperature of the sandbath, stirring the sand to
distribute heat. Frequently exchange the water in the cold ﬁnger for cooler
water, taking care not to spill any water onto the joint between the
sublimator and the ﬂask.
- When the temperature of the ﬂask reaches a certain point, the sample in
the bottom of the ﬂask will sublime from the ﬂask and deposit on the
cold ﬁnger. When this process begins, turn off the heat to the sandbath
and continue the deposition for another 10 minutes, making sure not to
disturb the sublimator and knock your crystals from the cold ﬁnger.
- Remove the ﬂask and sublimator from the sandbath. Carefully disconnect
the aspirator from the sublimator and turn off the water. Carefully remove
the sublimator from the ﬂask and scrape the crystals from the cold ﬁnger
into a tared beaker. Determine the mass of the sublimed benzoic acid, and
calculate a percent yield for the puriﬁcation.
- Measure a melting point for the crude benzoic acid, the sublimed benzoic
acid, and the residue left in the sublimation ﬂask. Are the melting points
consistent with a puriﬁcation of benzoic acid?