Determination of Partition Coefficient via Extraction of Benzoic Acid and its Conjugate Base from Two Immiscible Phases Abstract

Determinationof Partition Coefficient via Extraction of Benzoic Acid and itsConjugate Base from Two Immiscible Phases


Thepurpose of this experiment was to determine the partition coefficientof Benzoic Acid in two immiscible phases: an organic phase and anaqueous phase. Part A involved the liquid-liquid extraction of asolution of Benzoic acid in CH2Cl2 as the organic phase and distilledwater as the aqueous phase. Part B involved the same liquid-liquidextraction but with Sodium Bicarbonate as the aqueous layer. Theprocedure involved 50 mg of benzoic acid pipetted and added to the5.0ml reaction vial and the record of the weight of benzoic acidnoted. 600 µL of methylene chloride added to dissolve benzoic acidand water 60µL of water being added. The benzoic acid was capped andshaped to dissolve for 4 minutes. The two layers were then separatedand Methylene chloride solution dried for 10 minutes, transferred toclean culture tube and weight determined. The partition coefficientwas obtained by dividing the recovered mass of Benzoic acid in theorganic layer by the mass left in the aqueous layer. In thisexperiment, the partition coefficient determined in part A is 24while the partition coefficient recovered in part B is 0.02.

Statementof purpose

Theprincipal aim of the undertaken experiment was to come up with thepartition coefficient where two methods of liquid-liquid extractionare used but as for the part Two there is the use of SodiumBicarbonate that act as the aqueous layer. In this leads to thereaction between Benzoic acid and sodium bicarbonate causing massrecovery of Benzoic acid in organic layer that form the basis forcalculating the partition coefficient.

Statementof results

Initial mass


Amount remaining

in CH2Cl2


Amount remaining

in water


Partition Coefficient


Benzoic acid (Part A)





Benzoic Acid (Part B)





Calculations:PART A

Initialmass of Benzoic acid: 50 mg

Recoveredmass of Benzoic acid in organic layer: 48 mg

Recoveredmass of Benzoic acid in aqueous layer: 2 mg

Partitioncoefficient (Kp)= Massin organic layer (mg) =48mg = 24

Mass in aqueous layer (mg) 2 mg


Initialmass of Benzoic acid: 50 mg

Recoveredmass of Benzoic acid in organic layer: 1 mg

Recoveredmass of Benzoic acid in aqueous layer: 49 mg

PartitionCoefficient (Kp) = Massin organic layer (mg) =1mg= 0.02

Massin aqueous layer (mg)49 mg


Inthis experiment part, one was to separate the Methylene chloride fromthe mixture that it was in. This involves the aspect of solventextraction where the compounds are separated based on the relativesolubility in two different immiscible liquids, that is, theseparation of a substance from a mixture by preferentially dissolvingin a suitable solvent. The benzoic acid reacts with the SodiumBicarbonate causing the formation of Sodium Benzoate salt. It isfound that this reaction is soluble in water than in CH2Cl2 whichbrings about the basis of separation. The partition coefficient isdetermined using the value obtained.


Liquid-Liquidextraction is a separation and extraction technique used to separatecompounds based on their solubility in different liquid media, oftenan organic solvent and water. It is also the process of extracting acompound from a solvent where on is miscible and the other one isnot. When two immiscible liquids are mixed together, two distinctlayers formed. The liquid arrangement is based on their density. Inpart, A of this experiment the CH2Cl2 (density = 1.33g/cm3) is on thebottom while H2O (density 1.00g/cm3) is on top. In part B, the CH2Cl2is on the bottom while the Na2CO3 solution (containing 92% H2O) is ontop. Benzoicacidisweakly polar that has 2 groups: benzoic ring (non polar) &ampcarboxylic (polar), Benzoic Acid is weak that is less soluble inwater but more soluble in methylene chloride.Benzoic acid ends up in the bottom layer because it is more solublein CH2Cl2 (an organic compound) and it has very limited solubility inwater. In Part B however, due to the formation of the Sodium Benzoatesalt (ionic compound) from the reaction of Benzoic acid and sodiumbicarbonate which is very soluble in water and almost insoluble inCH2Cl2, it ends up in the top aqueous layer. The reaction in Part Bis reversible even though equilibrium favors the forward reactionbecause carbonic acid formed is capable of donating a proton to theanion of the benzoate salt to give benzoic acid and sodiumbicarbonate.Kp is less because salt is form –&gt the sodium benzoate ioniccompound. Highly soluble in water and not soluble in methylenechloride…try to talk about the principles in the discussion.&nbspThe Partition coefficient obtained in part A is 24 while in part B is0.02 which are different from the theoretical value which should be53.5 in part A (Benzoicacid).

Sourcesof error

Theseincludenot drying appropriately lead to miss up the exp results, notshaking tube in part B lead to wrong result, and not opening thevial after shaking equally lead to bad result.

Mixturesalso may not be separated properly leading to sodium bicarbonatebeing extracted instead of benzoic acid.

Alittle bit of recovered Benzoic acid was lost in the process ofweighing that affected the value of the recovered mass.