Chem 40.1 Lab

8/13/2019 Chem 40.1 Lab

http://slidepdf.com/reader/full/chem-401-lab 1/21

EXERCISE 2:

Recrystallization andMelting Point

Determination

8/13/2019 Chem 40.1 Lab


Recrystallization

A purification technique that take

advantage of:

Effect of temperature on solubility tendency of crystals to form definite lattice and

exclude anything that does not fit

8/13/2019 Chem 40.1 Lab


Impurities

CAN BE ANY OF THE FOLLOWING:

Products of side reactions

Starting materials Solvents

Inorganic materials

8/13/2019 Chem 40.1 Lab


STEPS INVOLVED INRECRYSTALLIZATION

1. Selection of appropriate solvent

CRITERIA:

- Should dissolve the desired substance atelevated temperature but not at room

temperature

- Chemically inert with the compound andimpurities

8/13/2019 Chem 40.1 Lab



- impurities should be totally soluble ortotally insoluble

- boiling point of the solvent should be lessthan the melting point of the desiredsubstance

- solvent must be relatively volatile

- solvent must be safe, nontoxic andinexpensive

8/13/2019 Chem 40.1 Lab



2. Dissolution

- Use only a minimum amount of solvent

- Higher amount of solvent, higher unsaturation, lower

recovery but no effect on purity- Crude samples should be disintegrated or powdered

- Use of activated carbon –adsorbs colored impurities

- amount used should be minimized since it may also

adsorb the desired substance

8/13/2019 Chem 40.1 Lab



3. Hot Gravity Filtration

- Filtration by action of gravity alone

- removes the totally insoluble impurities - used for relatively quick filtration of hot samples

- prevents premature recrystallization

8/13/2019 Chem 40.1 Lab



SETUP

a. Stemless funnel –

prevents rapid cooling of

solutionb. Fluted filter paper –

provides maximum

surface area of contact

for liquid and maximum

contact with the walls of

the funnel

8/13/2019 Chem 40.1 Lab



Recrystallization Proper

- recrystallization can be effected by slow

cooling of the hot mixture- abrupt cooling causes random inclusion of

soluble impurities

8/13/2019 Chem 40.1 Lab



Hot mixture

Slow

cooling

Abrupt

cooing

Crystals withdefinite lattice

Inclusion of

impurities

8/13/2019 Chem 40.1 Lab



Absence of crystals may be due to:

- excess of solvent

8/13/2019 Chem 40.1 Lab



5. Suction Filtration

- fast filtration of mixtures that are not hot

- uses vacuum- optimum removal of large amount of

solvent

8/13/2019 Chem 40.1 Lab



- involves washing of crystals with coldsolvent

- not washed with cold sovent – adsorbed impurities are not removedleading to a high recovery but low purity

- hot solvent is used – crystals will

redissolve leading to a low recovery butno effect in purity

8/13/2019 Chem 40.1 Lab



Setup: Buchner

funnel, suction flask,

filter paper, vacuum

8/13/2019 Chem 40.1 Lab


8/13/2019 Chem 40.1 Lab


Melting Point Determination

Melting point – temperature at which the

liquid and solid phases of a substance

coexist and in a state of equilibrium

8/13/2019 Chem 40.1 Lab


Purpose of Melting PointDetermination

To establish a pure substance’s identity

Pure substances have definite MP value at a

certain pressue

Requires a certain amount of energy (heat) to

melt or to break IMFA

8/13/2019 Chem 40.1 Lab


Purpose of Melting PointDetermination

To serve as an index of purity

Pure substances have sharp MP (falls at a very

narrow range <1°)

Uniformly melt due to uniform, uninterrupted

arrangement of molecules

8/13/2019 Chem 40.1 Lab


NOTE

Pulverization of sample should be done on

a watchglass and not directly on filter paper

Filter paper contains ‘lints’ which may beincorporated as impurities

8/13/2019 Chem 40.1 Lab


Possible Sources of Error in MPDetermination

Rapid heating – higher MP than literature

value

Presence of impurities – widens MP range loose, uneven packing of crystals – widens

MP range

8/13/2019 Chem 40.1 Lab


END

Chem 40.1 Lab

Documents

Transcript of Chem 40.1 Lab