pHYSICAL cHEMISTRY - II

Experiment 2

Aim of the Experiment

To establish the partition co-efficient of iodine in distilled water (d.w.) and carbon tetrachloride.

Principle

When a solute is shaken with 2 immiscible solvents it gets partitioned between the solvents. This distribution of solute in 2 solvents depends upon the solubility of the solute in two solvents. At the distribution equilibrium, the ratio of concentrations of solute in both solvents is constant at a given temperature. The constant is called the partition co-efficient (K) or the distribution coefficient of the solute between the two solvents.

Materials required

Chemicals: Distilled water, Iodine crystals,Carbon tetrachloride, 0.01N Na₂S₂O₃ ,0.1 N Na₂S₂O₃ ,starch indicators

Apparatus: Separating funnels, Conical flask, Pipette, Burette, Reagent bottles

Procedure

Approximately prepare 110 mL of saturated solution of iodine in carbon tetrachloride for use as stock solution.

Prepare the following mixtures in separating funnels :

Set I : 50 mL water + 40 mL stock solution + 10 mL of CCl4

Set II : 50 mL water + 30 mL stock solution + 20 mL of CCl4

Set III : 50 mL water + 20 mL stock solution + 30 mL of CCl4

Set IV : 50 mL water + 10 mL stock solution + 40 mL of CCl4

Shake the mixture in separating funnel forcefully for about 30 minutes for the iodine to get distributed between the two solvents and to reach the distribution equilibrium. Let the flasks to stand for about 10 minutes so that two clear layers are separated. Now remove stopper of the separating funnel and keep its mouth open during this period to facilitate the separation. Release the lower layer which is aqueous into 4 different stoppered dry bottles. (Discard the intermediate layer between both the phases). Organic layer stays in the separating funnels. Using a dry pipette withdraw 10 mL of organic layer to a conical flask. Titrate it against 0.1N Na₂S₂O_3. Starch is used as an indicator. End point Blue colour will disappear. Withdraw 10 mL of the aqueous layer using a dry pipette and titrate it against 0.01N Na₂S₂O₃ solution starch as an indicator. End point : Blue colour will disappear. Repeat the process for all the mixtures.

Calculation

Set-I

1. For Organic layer

Normality of Na₂S₂O₃ (N₁ = 0.1 N)

Volume of organic layer pipetted (V₂) = 10 mL

N₁V₁ (Na₂S₂O₃₎ = N₂V₂ (Organic layer)

N₂ = 0.1 * V1 /10 = (N_org)

Similarly calculate concentration of iodine in organic layer of all other sets.

2. For aqueous layer

Normality of Na₂S₂O₃ (N1 = 0.01N)

Volume of organic layer pipetted (V₂) = 10 mL

N₁V₁ (Na₂S₂O₃₎ = N₂V₂ (Aqueous layer)

N₂ =0.01 * V1 /10 = (N_aq)

Similarly calculate concentration of iodine in of all other sets.

Graph : Plot the graph of log C_aq Vs log C_org

Partition coefficient K=

Log C_aq = (1/n) log C_org + log K

Above equation, is equation of straight line which is y = mx + c )

Result from the graph:

Slope (m) =

Therefore n is nearly =

Substituting the value of slope of line in the equation

Log(1/n) C_aq = (1/n)log C_org + log K

Therefore,

Log K =

K =

Result

Partition co-efficient of iodine in distilled water and carbon tetrachloride __________ by calculation and ______________ by graph.

Precautions

(i) While withdrawing the layers care should be taken to avoid the contamination of one layer by the other.

(ii) While titrating the non-aqueous layer, the titration flask must constantly be shaken, otherwise the equivalence point may pass without the disappearance of purple colour of non-aqueous layer. The addition of about 10% KI solution is must. The addition of aqueous KI solution facilitates extraction of iodine into the aqueous solution during titration due to the formation of unstable complex ion I₃^- as:

K⁺ + I^- + I₂ ⇌ K⁺ + I₃^-

Questions

1.Give the definition of Partition co-efficient.

2.To follow Partition law,Which factor is necessary to reach equilibrium?

Reference Material

1. Practical Physical Chemistry Google EBOOK Link.

2. E Journal on relevant topic.

3. More HN, HAjare AA. Practical Physical Pharmacy, Career publications; 2010:70-73