Lab manual

1. 1
Determination of partition coefficient or distribution coefficient of acetic acid
between water and n-butanol
Dr. Mausumi Adhya
HOD and Associate Professor
Supreme Knowledge Foundation, West Bengal, India

2. 2
Theory
If a substance (solute) is soluble in a mixture of two immiscible liquids (solvents), at constant
temperature it distributes in the solvents in such a way that the ratio of the substance in the two liquid is
always same at any concentration of substance. The ratio is called distribution or partition coefficient. The
law is called Nernst Distribution Law.
If acetic acid (AcOH) is taken as solute and n-butanol and water are taken as two immiscible solvents,
partition coefficient of AcOH = concentration of AcOH in organic layer (n-butanol)/ concentration of
AcOH in aqueous layer. The concentration of AcOH in n-butanol and water is finding out by sodium
hydroxide (NaOH) solution.
Materials
1. Apparatus: Two stoppered reagent bottle, burette with burette stand, pipette, conical flask, beaker,
volumetric flask and dropper
2. Chemicals: NaOH, AcOH, n-butanol, distilled water, phenolphthalein indicator
Procedure
1. Preparation of reagents
(I) Preparation of 1000 ml N/4 NaOH solution
Gram-equivalent weight of NaOH = molecular weight of NaOH/1 = 40 g
To prepare 1000 ml N/4 NaOH gm-equivalent required=40/4 g = 10g
10g of NaOH is weight out in 1000 ml volumetric flask and dissolved in distilled water with continuous
stirring. The water is added upto the mark. Finally the solution is made uniform by shaking.
(II) Preparation of 100 ml 2M AcOH solution
V1S1=V2S2
Strength of concentrated AcOH 17.4 (M)
V× 17.4= 100× 2 i.e. V= 11.49 ml
11.49 ml concentrated AcOH is taken in a 100 ml measuring cylinder. Distilled water is added up to the
mark. The solution is transferred to a beaker and shaken for making the solution uniform.
(III) Preparation of phenolphthalein indicator
0.5 g of solid is dissolved in a mixture of 50 ml ethanol and 50 ml water.
2. Preparation of solution to find out the partition co-efficient of AcOH in n-butanol and water.
Two stoppered reagent bottle of 250 ml are taken. They are numbered 1 and 2. The following mixture is
kept in them.

3. 3
Bottle
number
Volume of 2M
AcOH (ml)
Volume of n-
butanol (ml)
Distilled water
(ml)
1 25 50 25
2 50 50 0
The bottles are stoppered tightly and shaken vigorously by hand for 1 h or mechanical shaker for 30 min.
Therefore, the bottles are allowed to stand for 30 min so that the two layers become clearly separated.
3. Standardization of aqueous layer and organic layer by NaOH
5 ml aqueous layer is pipetted out in a 100 ml conical flask and 1-2 drops phenolphthalein is added to it.
Therefore the solution is titrated by N/4 NaOH running from burette dropwise with continuous stirring
the conical until the colour of the solution is light pink.
5 ml organic layer is pipetted out in a 100 ml conical flask containing 20 ml distilled water. After vigorous
stirring 1-2 drops phenolphthalein is added to it. The solution is titrated by N/4 NaOH like earlier.
Results
Table 1: Standardization of aqueous layer and organic layer by NaOH
Bottle
no.
Layer
taken
Volume
of layer
(ml)
Burette reading Mean
volume of
NaOH (ml)
Strength of
NaOH (N)
Strength of
layer (N)
Initial Final Difference
1
Organic
layer
5 S1
Aqueous
Layer
5 S2
2
Organic
layer
5 S3
Aqueous
Layer
5 S4

4. 4
Strength of AcOH in any layer (N) = Strength of NaOH × Volume of NaOH required/ Volume of layer
taken
Calculation
Bottle number Partition Coefficient of AcOH Mean
1 S1/ S2
2 S3/ S4
Conclusion
The partition coefficient or distribution coefficient of acetic acid between n-butanol and water is
……………..
Precautions
1. After shaking, the bottle should stand for sufficient time so that the two layers are clearly separated.
2. The layers should pipette out carefully so that a single drop of any layer is not mixed to another layer.
Viva Voce
1. State Nernst distribution law.
2. What is partition co-efficient?
3. How can you determine the partition co-efficient of AcOH in n-butanol and water? Explain.
4. In this experiment n-butanol and water are separated in two layers. Which one is in upper layer and
which one is in lower layer and why?
Ans. n-Butanol is in upper layer and water is in lower layer as density of n-butanol (0.81cg/cm3) is lower
than density of water (1g/cm3)