Partition co-efficient of Benzoic Acid in Benzene and Water

BACKGROUND

Benzoic Acid

Aim: To determine the partition co-efficient of benzoic acid between benzene and water.

When a solute is shaken with two immiscible solvents it gets distributed between the solvents. This distribution of solute in two solvents depends upon the solubility of the solute in two solvents.

At the distribution equilibrium, the ratio of concentration of the solute in the two solvents is constant at a given temperature. The constant is called the partition coefficient (K) or the distribution coefficient of the solute between the two solvents.¹

REQUIREMENTS

Chemicals:    

• Saturated solution of benzoic acid in benzene
• Benzene
• 0.01N NaOH
• 0.1N NaOH
• Distilled water

Apparatus:    

• Separating funnel: 250 ml
• Conical flask
• Pipette
• Burette
• Stoppered bottles

PROCEDURE

Prepare the following mixtures in separating funnels:

•     Set I: 25ml water + 25 ml of saturated solution of benzoic acid in benzene
•     Set II: 25 ml water + 20 ml saturated solution of benzoic acid in benzene + 5 ml benzene
•     Set III: 25ml water + 15 ml saturated solution of benzoic acid in benzene + 10 ml benzene

Shake the mixture in the separating funnel vigorously for about 30 minutes so that the benzoic acid gets distributed between the two solvents and the distribution equilibrium is reached.

Allow the flasks to stand for 10 minutes to separate into two clear layers (remove the stopper of the separating funnel and keep its mouth open during this period to facilitate the separation).

Drain off the lower aqueous layers in 3 different stoppered dry bottles. (Discard the intermediate layer between the two phases). Benzene layer remains in the separating funnels.

Using a dry pipette take 5 ml of organic layer (Benzene) into a conical flask containing 10ml of water and titrate against 0.1 N NaOH using phenolphthalein as an indicator.

End point: Colorless to pink.Pipette out 10 ml of the aqueous layer using a dry pipette and titrate it against 0.01 N NaOHsolution using phenolphthalein as an indicator. End point will be colorless to pink.

Observation

Set No.	Vorg	Vaq	Norg=Corg	Naq=Caq	K	log Corg	log Caq
I
II
III

Mean Partition coefficient K =

Where,

V_org= Volume in ml of 0.1N Sodium hydroxide per 5ml of organic layer

V_aq= Volume in ml of 0.01N Sodium hydroxide per 10ml of aqueous layer

N_org= Normality of organic layer

N_aq= Normality of aqueous layer

C_org= Concentration of organic layer in g mole/lit = Normality (N_org)

C_aq= Concentration of aqueous layer in g mole/lit = Normality (N_aq)

K= C_aq/C_org^½= Partition coefficient of benzoic acid in water and benzene

Calculations

Set I

1.For Organic layer

        Normality of Sodium hydroxide (N₁= 0.1N)

        Volume of organic layer pipetted (V₂) = 5ml

        N₁V₁ (Sodium hydroxide) = N₂V₂ (Organic layer)

Similarly calculate concentration of benzoic acid in organic layer of setsII and III.

2. For aqueous layer

        Normality of Sodium hydroxide (N₁= 0.01N)

        Volume of organic layer pipetted (V₂) = 10 ml

        N₁V₁ (Sodium hydroxide) = N₂V₂ (Aqueous layer)

Similarly calculate concentration of benzoic acid in aqueous layer of setsII and III.

Graph

        Plot the graph of log C_aqVs log C_org
      

        Above equation, is equation of a straight line (i.e. y = mx+c)        

Result from the graph

        Therefore n is nearly =

        Substituting the value of slope of line in the equation

        LogCaq = log C_org + log K

           = log C_org + log K

        Therefore,

        LogK=

             K = __________

CONCLUSION

1. Partition co-efficient of benzoic acid between distilled water and benzene is ____________ by calculation and ____________ by graph.
2. Since C_aq/C_org^½ is practically constant benzoic acid exists as a dimer (n=2) in benzene.
3. Molecular condition of benzoic acid in benzene is 1/slope = n = ____ , molecules of benzoic acid associate in benzene.

REFERENCES

1. More HN, Hajare AA. Practical Physical Pharmacy, Career publications, 2010: 73-76.

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