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INTRODUCTION

•  The most common method of purifyingsolid organic compounds is byrecrystallization. In this technique an

impure solid compound is dissol!ed in asol!ent and then allo"ed to slo"lycrystallize out as the solution cools. #s thecompound crystallizes from the solution

the molecules of the other compoundsdissol!ed in solution are e$cluded from thegro"ing crystal lattice gi!ing a pure solid.

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%urpose of process

• &ainly for the puri'cation of solidcrystalline compounds.

• &elting point is one of the "ays ofdetermining the purity of thecompound.

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%RINCI%()* O+R)CR,*T#(((I-#TION

•ased on the di/erence bet"een the solubilities ofthe pure substance and the impurities in the sol!entor mi$ture of sol!ents

•0 di/erent situations could arise

•)ither the impurities ha!e greater solubility in thesol!ent than the pure substance being recrystallized

•In "hich case the impurities "ill be retained in themother liquor and se!eral recrystallizations "illyield the pure substance

•Or the impurities "ill ha!e less solubility in thesol!ent than the substance being recrystallized in"hich case the impurities "ill not dissol!e and morecrystals "ill be obtained

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%RINCI%()* O+R)C,*T#((I-#TION

• #t normal laboratory temperaturesthe substance being recrystallized isnot soluble. 1o"e!er increasing the

temperature results in increasedsolubility of the pure substance

• #t the boiling point of the pure

substancema$imum solubility isobtained.

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%RIN I%() +R)CR,*T#((I-#TION

• #fter reaching the boilingpointreduction of temperatureresults in precipitation of crystalnuclei

• %resence of crystal nuclei promotescrystal gro"th "hich continues untilthe temperature is stabilized at the

normal laboratory temperature

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%RINCI%()* O+R)CR,*T#((I-#TION

• #ssumptions made are2

•  The impurities are present incomparati!ely small quantities ie lessthan 34

•  The e/ect of either component 5iethe impurity and the pure substance6on the solubility of the other isneglected

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%R)C#UTION*

•  The mi$ture is "armed slo"ly tofacilitate the crystallization process.

•  The solution is allo"ed to cool naturally

to room temperature to aid theformation of 'ne crystals.

•  The 'lter paper should adhere o!er the

perforated plate the suction 'lter.•  The 'lter paper must be moistened

before 'ltration "ith the chosen sol!ent.

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%R)C#UTION*5cont6

• *uction 'ltration should be donegently.

• )$cess addition of sol!ent should bea!oided.

• 7hen t"o sol!ents # and are usedfor a &ulti8sol!ent recrystallizationprocess sol!ents # and must bemiscible.

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C1OIC) O+ *O(9)NT

• *ubstance should be insoluble insol!ent at room temperature butsoluble at ele!ated temperatures.

• Impurities should be either soluble orcompletely insoluble in sol!ent.

• *hould yield "ell de'ned crystals ofrequired compound.

• *hould possess lo" boiling point.

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C1OIC) O+ *O(9)NT5cont6

• It should not react "ith thecompound of interests.

• *hould ha!e similar chemicalproperties as the compound to berecrystallized.

• Other factors to be consideredinclude ease of manipulation:ammability and cost.