Recrystallization

Lab # 2

RecrystallizationRecrystallization

-Recrystallization is a laboratory technique used to purify solids based on their different solubilities.

-It works only when the proper solvent is used.

The Steps of The Steps of RecrystallizationRecrystallization

1 -Find a suitable solvent.

2-Dissolve the impure solid in a minimum volume of

hot solvent.

3-Remove any insoluble impurities by hot filtration.

4-Slowly cool the hot solution to crystallize the desired

compound from the solution.

5-Filter the solution to isolate the purified solid

compound.

The Steps of The Steps of RecrystallizationRecrystallization

Choosing a SolventChoosing a Solvent

-The solute must be relatively insoluble in the

solvent at room temperature but much more

soluble in the solvent at higher temperature.

-At the same time, impurities that are present

must be either be soluble in the solvent at

room temperature or at higher temperature.

-N.B. Compound are soluble in solvents with

similar polarity, (like dissolves like).

Recrystallization of Benzoic Recrystallization of Benzoic AcidAcid

-Chemical structure of benzoic acid:

-Suitable solvent: Hot water-Melting point: 121-123 C

Recrystallization of Benzoic Recrystallization of Benzoic AcidAcidProcedure:

1.Place 5 g of impure benzoic acid in a 250 ml beaker.

2 .Add approximately 120 ml of water.

3 .Add the magnetic searing tablet.

4 .Heat a mixture on a hot plate until the solid has gone into solution.

5 .Add additional hot water if necessary until all of the

benzoic acid dissolves

Recrystallization of Benzoic Recrystallization of Benzoic AcidAcid

6 .Once the sample has dissolved it must be filtered hot

7 .After hot filtration, remove the flask from the hot

plate and set aside to cool to room temperature.

8 .Gradual cooling is conductive to the formation of

large, well-defined crystals.

9 .Collecting the purified crystals by vacuum filtration.

10 .Drying the crystals.

11 .Weigh the crystals and take a melting point.

Vacuum filtration:

Vacuum, or suction

filtration is more rapid

than gravity filtration

and is most often used to

collect solid products

resulting from

precipitation or

crystallization.

Melting Point Determination

• Melting point of a solid is the temperature at which the solid and liquid phases are in equilibrium.

• The purity compound is measured by melting point.

• Always record a range of temperature, where it starts melting and where it is completely melted.

• Impurities will change the melting point.

Place a VERY SMALL quantity of the solid of interest on glass capillary, and use a stirring rod to grind the solid to a powder. Use a spatula to gather the powder into a small pile. Stick the open end of a melting point capillary into the pile to a depth of about 1 mm, then invert the capillary and tap the sealed end on the bench to encourage the solid to drop to the bottom. The height of solid in the capillary should be no more than 1-2 mm.

Melting point measuring procedure:

place the capillary into one of the 3 sample wells of the Mel-Temp apparatus . Turn the Mel-Temp power switch on.

While the sample is heating, watch it through the magnifying window, while frequently checking the temperature reading of the thermometer. Observe the sample as the temperature rises .

To measure the melting point of a substance, it is

necessary to gradually heat a small sample of the

substance while monitoring its temperature with a

thermometer.

The temperature at which liquid is first seen is the

lower end of the melting point range. The

temperature at which the last solid disappears is the

upper end of the melting point range.

Melting point measuring procedure:

turn off the Mel-Temp and remove your

capillary. CAREFUL--DO NOT TOUCH THE

END THAT WAS HEATED !

Let the capillary cool, then discard it in the

glass waste .

Melting point measuring procedure:

Melting point: