Distillation techniques used in pharmacy

1.  Distillationisthe processbywhichcomponentsof aliquidare separatedbyusing
vapourizationandsubsequentcondensation.
Feedliquidcalleddistilland.
Condensedliquidcalleddistillateorcondensate.
 Raoult`slaw:Partial vapourpressure of eachvolatile constituentisequal tovapourpressure
of pure constituentmultipliedbyitsmole fractionata giventemperature.
Partial V.P.=V.P.of pure liquid*mole fractionof liquid
 Ideal solution:whenthere isnochange inthe propertiesofcomponentsotherthandilution.
Example:Methanol andwater
 Real solution:
Postive deviation:Vapourpressureisgreaterthanthe sumof the partial pressure of
individualcomponents. Thisbehaviouroccurwhencomponentdiffersfrom polarity,length
of hydrocarbon chain or degree of association.
Example:carbontetrachloride andcyclohexane,benzene andethanol.
 Negative deviation:V.P.islowerthansumof partial pressure of individual component.This
deviationoccurswhen hydrogenbonding,saltformation andhydration occurs between
componennts.
Example:choloroformandacetone,pyridine andaceticacid,waterandnitricacid.
 Volatility:
volatility=partialvapourpressure of A/mole fractionof A insolution
 Simple distillation:Itisasimple processof convertingsingle constituentfroma liquidinto
itsvapour andthenconvertingthisvapourintoliquidatanotherplace.Alsocalled
differential distillation(becausebasedonrelative volatilityof constituents).
 Purifiedwater andwater for injectionare made by thismethod.
 Fewofficial compoundslike spiritof nitrousetherandaromaticspiritof ammoniaare
prepaedbyit.
 Azeotropicdistillation:itisadistillationmethodinwhich azeotropicmixture broken by
additionof thirdsubstance,whichforms anew azeotrope withone of the component.E.g.
azeotrope of waterandethyl alcohol isbrokenbybenzene.Itisusedfordeterminationof
watercontentinsubstance using toluene accordingtoIP,1996.
 Extractive distillation:Inextractive distillationthirdsubstanceaddedtoazeotropicmixtureis
relativelynonvolatile comparedtosubstancestobe seprated.E.g.1) Phenol isusedfor
seprationof azeotropicmixture of tolueneandiso-octane.2)Furfural isaddedforthe
seprationof butadiene from itsmixture containingbutane andbutane.
 Steam distillation:
1)It isusedfor separationof highboilingsubstancesfromnonvolatile impurities.
2)Steamdistillationisusedforseparationof immiscible liquid.
3)Mainlyusedforseparation of volatile oils.
Steamdistillationisthe commonexample of differential distillation.Itisbase onthe principle
that whenwaterismixedwithotherimmiscible highboilingpointliquid,thentheirmixture

2. starts boilingwhentheirmixturevapourpressure becomes equaltoatmosphericpressure
and thistemperature islessthanboilingpointof pure liquid.
 Flash distillation:Inthisprocessthe entire liquidissuddenlyvapourizedbypassingthrough
highpressure tolowpressure zone.
Alsocalled EQUILIBRIUMDISTILLATION.
Thismethodmainlyusedforcomponentswhichboilatwidelydifferenttemperature.Itis
widelyusedin petroleumindustryfor refiningcrude oil.
 Fractional distillatinisusedforseprationof miscibleliquidsuchasacetone and
water,chloroformandbenzene.Thismethodisalsocalledrectification. Butitcan not be used
for miscible liquidwhichformmiscible liquid