Nernst-Distribution-Law (2).pdf

Nernst Distribution Law

1. DISTRIBUTION LAW
(LIMITATIONS AND APPLICATIONS)

3. PROBLEM 1.
AsolidXisaddedtoamixtureofbenzeneandwater.
Aftershakingwellandallowingtostand,10mlofthe
benzenelayerwasfoundtocontain0.13gofXand100
mlofwaterlayercontained0.22gofX.Calculatethe
valueofdistributioncoefficient.

4. SOLUTION 1 -
Concentration of X in benzene (Cb) = 0.13 / 10 = 0.013g ml–1
Concentration of X in water (Cw) = 0.22 / 100 = 0.0022g ml–1
According to Distribution law :
Cb/ Cw= 0.013 / 0.0022 = =5.9

5. SOLUBILITIES AND DISTRIBUTION LAW
Whenasoluteisshakenwithtwonon-misciblesolvents,atequilibriumboththe
solventsaresaturatedwiththesolute.Sincethesolubilityalsorepresentsconcentration,
wecanwritethedistributionlawas
C1/C2 = S1/S2 = KD
where S1 and S2 are the solubilities of the solute in the two solvents.
HenceknowingthevalueoftheDistributioncoefficient(KD)andthesolubilityof
soluteinoneofthesolvents,thesolubilityofsoluteinthesecondsolventcanbe
calculated.

8. APPLICATIONOFDISTRIBUTIONLAW
There are numerous applications of distribution law in the
laboratory as well as in industry
(1)SolventExtraction-
Thisistheprocessusedfortheseparationoforganic
substancesfromaqueoussolutions.Theaqueoussolutionis
shakenwithanimmiscibleorganicsolventsuchasether(or
benzene)inaseparatoryfunnel.Thedistributionratiobeingin
favourofether,mostoftheorganicsubstancepassesintothe
ethereallayer.Theethereallayerisseparatedandetherdistilled
off.Organicsubstanceisleftbehind.

9. 2) Partition Chromatography
Apasteofthemixtureisappliedatthetopofacolumnof
silicasoakedinwater.Anotherimmisciblesolvent(hexane)
isallowedtoflowdownthecolumn.Eachcomponentofthe
mixtureispartitionedbetweenthestationaryliquidphase
(water)andthemobileliquidphase(hexane).Thevarious
componentsofthemixtureareextractedbyhexaneinorder
oftheirdistributioncoefficients.

10. (3) Desilverizationof Lead (Parke’sProcess)
Whenmoltenzincisaddedtomoltenleadcontainingsilver
(argentiferouslead),zincandleadformimmisciblelayers
andsilverisdistributedbetweenthem.Sincethedistribution
ratioisabout300infavourofzincat800ºC,mostofsilver
passesintothezinclayer.Oncoolingthezinclayer,analloy
ofsilverandzincseparates.TheAg-Znalloyisdistilledina
retortwhenzincpassesoverleavingsilverbehind.Thelead
layerstillcontainsunextractedsilver.Thisistreatedwithfresh
quantitiesofmoltenzinctorecovermostofthesilver

11. (4) Confirmatory Test for Bromide and Iodide
Thesaltsolutionistreatedwithchlorinewater.Smallquantity
ofbromineoriodineisthusliberated.Thesolutionisthen
shakenwithchloroform.Onstandingchloroformformsthe
lowerlayer.Thefreebromineoriodinebeingmoresolublein
chloroformconcentratesintothelowerlayer,makingitredfor
bromineandvioletforiodine.

12. 5)Determination of Association
Whenasubstanceisassociated(orpolymerized)insolventA
andexistsassimplemoleculesinsolventB,theDistributionlaw
ismodifiedas
n√Ca/Cb= k
when n is the number of molecules which combine to form an
associated molecule.

13. (6) Determination of Dissociation
Suppose a substance X is dissociated in aqueous layer and exists as single
molecules in ether.
If x is the degree of dissociation (or ionisation), the distribution law is modified
as
C1 /C2 )(1-x) = K
where C1 = concentration of X in benzene
C2 = concentration of X in aqueous layer
Thevalueofxcanbedeterminedfromconductivitymeasurements,whileC1
andC2arefoundexperimentally.ThusthevalueofKcanbecalculated.Using
thisvalueofK,thevalueofxforanyotherconcentrationsofXcanbe
determined.

14. (7)DeterminationofSolubility
Supposethesolubilityofiodineinbenzeneistobedetermined.
Iodineisshakenwithwaterandbenzene.Atequilibrium
concentrationsofiodineinbenzene(Cb)andwater(Cw)are
foundexperimentallyandthevalueofdistributioncoefficient
calculated.
Cb/Cw=Kd
Sb/Sw=Kd
whereSb=solubilityinbenzene;andSw=solubilityinwater.

15. THANK YOU