Post on 04-Jun-2018
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
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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
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Impurities
CAN BE ANY OF THE FOLLOWING:
Products of side reactions
Starting materials Solvents
Inorganic materials
8/13/2019 Chem 40.1 Lab
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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
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STEPS INVOLVED INRECRYSTALLIZATION
- 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
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STEPS INVOLVED INRECRYSTALLIZATION
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
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STEPS INVOLVED INRECRYSTALLIZATION
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
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STEPS INVOLVED INRECRYSTALLIZATION
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
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STEPS INVOLVED INRECRYSTALLIZATION
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
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STEPS INVOLVED INRECRYSTALLIZATION
Hot mixture
Slow
cooling
Abrupt
cooing
Crystals withdefinite lattice
Inclusion of
impurities
8/13/2019 Chem 40.1 Lab
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STEPS INVOLVED INRECRYSTALLIZATION
Absence of crystals may be due to:
- excess of solvent
8/13/2019 Chem 40.1 Lab
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STEPS INVOLVED INRECRYSTALLIZATION
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
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STEPS INVOLVED INRECRYSTALLIZATION
- 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
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STEPS INVOLVED INRECRYSTALLIZATION
Setup: Buchner
funnel, suction flask,
filter paper, vacuum
8/13/2019 Chem 40.1 Lab
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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
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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
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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
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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
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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