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Experiment 1: Recrystallization

Read pp 221-231 and 234-235, Chapter 15, in LTOC. The theory and technique

of the purification of solid compounds by recrystallization is fully explained in these

pages. Read also pp 73-74 (6.1) to learn about boiling stones and sticks, and page 75 to

review the use of hot plates for heating flat-bottomed containers.

Part 1. Recrystallization of Pure Phthalic Acid

In the first of today's procedures, you will

recrystallize phthalic acid, a compound that is used to make

synthetic polymers such as Dacron. Phthalic acid is very

soluble in boiling water, 18 g/100 mL, and is much less

soluble in chilled (14°C) water, 0.54 g/100 mL. Thus, water

is a very good recrystallization solvent for phthalic acid.

Outline the steps of the following procedure. As part

of your pre-lab assignment, calculate the volume of boiling

Phthalic acid

water required to dissolve 1 g of phthalic acid (use the data

given above).

Set up a hot plate in your fume hood. You do not need a temperature probe since

you will be heating water to the boiling point. When you turn on the hot plate, be sure

the letter B is displayed on the left-hand side (no temperature probe required). If it is not,

turn off the power switch, then press down and hold the Temp knob while turning on the

power switch. There are three heating modes (A, B and C). Repeat this procedure until

the letter B is displayed. Turn the Temp knob until the “set temp” on the bottom of the

display reads 110°.

Place 1.0 g of phthalic acid in a 25 or 50 mL Erlenmeyer flask. Place 10 mL of

water in a graduated cylinder. Add 4-5 mL of the water to the solid in the flask along

with a wooden boiling stick, then set the flask on the hot plate. As soon as the water

begins to boil, continue to add water dropwise from the graduated cylinder until the solid

completely dissolves. If 10 mL of water has been added and the solid still has not

completely dissolved, refill the graduated cylinder, and continue to add water, but at a

faster rate. Record the total volume of water required to dissolve the crystals.

Take the flask off the hot plate and remove the boiling stick. Allow the solution

to cool undisturbed to room temperature. During this slow cooling, crystals of the solid

will form. When the flask has reached room temperature, place it in an ice bath for at

least 10 minutes.

Set up a vacuum filtration apparatus using the small Buchner funnel and 125 mL

side-arm flask from your drawer (see p. 230 in LTOC and the

Figure Page, Expt.

1).

Place a disk of filter paper in the funnel and collect the crystals by vacuum filtration.

You may use a spatula to assist in transferring the crystals to the funnel, then wash the

phthalic acid in the funnel with small amounts of ice cold water. Transfer the crystals

from the funnel to a watchglass, place another piece of filter paper on top of the crystals

and press firmly to remove some of the water. Leave them in your drawer to dry until

your next lab period. You will obtain the weight of your recovered product next week.

Two days after your next lab period, you will be required to hand in the Results to your

TA. The filtrate in the side-arm flask may be poured into the sink.

Recrystallization

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