Chemistry Lab Experiment: Determination of distribution coefficient of iodine between organic and inorganic layer

Determination of the Distribution Coefficient of Iodine Between Organic and Aqueous Phases

Introduction
The distribution (or partition) coefficient is the ratio in which a solute like iodine divides itself between two immiscible solvents—here, an organic solvent (CCl₄ or CHCl₃) and water. This experiment determines the distribution coefficient $K = [I_2]_{\text{organic}} / [I_2]_{\text{aqueous}}$ at room temperature.

Objective
To determine the distribution coefficient of iodine between distilled water and carbon tetrachloride or chloroform.

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Principle

When shaken with two immiscible solvents, iodine distributes between them based on its solubility. At equilibrium, the ratio of concentrations remains constant at a given temperature:

$$K = \frac{[I_2]_{\text{organic}}}{[I_2]_{\text{aqueous}}}$$

Titration using sodium thiosulfate and starch as an indicator allows us to determine the iodine concentrations in both layers.

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Reaction During Titration

$$I_2 + 2S_2O_3^{2-} \rightarrow 2I^- + S_4O_6^{2-}$$

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Materials Required

Chemicals

• Iodine crystals

• Carbon tetrachloride (CCl₄) or Chloroform (CHCl₃)

• Distilled water

• 0.1 N and 0.01 N Sodium thiosulfate (Na₂S₂O₃)

• Starch solution

• 10% KI (optional to assist titration)

Apparatus

• Separating funnels

• Pipettes

• Burettes

• Conical flasks

• Reagent bottles

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Experimental Sets

Set	Water (mL)	I₂ Stock (mL)	CCl₄ (mL)
I	50	40	10
II	50	30	20
III	50	20	30
IV	50	10	40

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Procedure

1. Prepare a saturated iodine solution in CCl₄.

2. Prepare 4 sets of mixtures as per the table above.

3. Shake each separating funnel for 30 minutes and let stand 10 minutes.

4. Separate both layers carefully.

5. Withdraw 10 mL of each layer and titrate:

   • Organic layer with 0.1N Na₂S₂O₃.

   • Aqueous layer with 0.01N Na₂S₂O₃.

   • Use starch as an indicator (blue to colorless endpoint).

6. Constant shaking during titration is essential.

7. Optionally, add 5 mL of 10% KI to aid iodine extraction in titration.

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Calculation

Using:

$$N_1 V_1 = N_2 V_2$$

Determine concentrations in each layer, then calculate:

$$K = \frac{[I_2]_{\text{organic}}}{[I_2]_{\text{aqueous}}}$$

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Graphical Analysis

Plot:

$$\log [I_2]_{\text{aqueous}} \text{ vs. } \log [I_2]_{\text{organic}}$$

Equation:

$$\log [I_2]_{\text{aq}} = \frac{1}{n} \log [I_2]_{\text{org}} + \log K$$

• Slope = 1/n

• Intercept = log K

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Result Format

Set	Titre (Organic)	Titre (Aqueous)	[I₂]ₒᵣg	[I₂]ₐq	log Org	log Aq
I
II
III
IV

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Conclusion

The distribution coefficient of iodine between CCl₄ and water was calculated both:

• Experimentally from titration values.

• Graphically from the slope and intercept of the plot.

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Schematic Illustration

Aqueous and organic layers forming in separating funnel with pipetting and titration steps.

(Image credit: Adapted from open-access chemistry resources)