DETERMINATION OF THE DISTRIBUTION CONSTANT OF IODINE BETWEEN WATER AND CYCLOHEXANE

Objective

To determine the distribution coefficient (the equilibrium concentration ratio) of iodine between the immiscible solvents water and cyclohexane (C₆H₁₂). This is a study of equilibrium. The distribution coefficient will be derived from data obtained by performing a chemical analysis for iodine in the water layer.

Apparatus and Materials

50 mL buret, two test tubes (25 x 200 mm), two No. 4 rubber stoppers, Pasteur pipet, ring stand, buret clamp, 120 mL aqueous iodine solution, 75 mL Na₂S₂0₃ solution, 13 mL C₆H₁₂, starch solution, potassium iodide.

Discussion

When a solute (the iodine in this case) is mixed with two immiscible liquids in contact with each other (the water and cyclohexane), the solute dissolves in each solvent to some extent, and so distributes itself between the layers. When equilibrium has been reached, the ratio

K_d = molar concentration of solute in solvent l / molar concentration of solute in solvent 2

is constant, independent of the two individual concentrations but dependent upon the temperature and the nature of the specific solute and solvents involved. K_d is an equilibrium constant called the distribution coefficient. For the iodine-water-cyclohexane system

K_d = [1₂] in C₆H₁₂ / [I₂] in H₂O

where K_d is now specifically the distribution coefficient (or molar concentration ratio) of iodine between water and cyclohexane. This type of procedure is used during the common industrial separation technique known as "extraction." During extraction, compounds are separated by taking advantage of their different solubilities in each of a pair of immiscible solvents. Drugs and compounds of plutonium are examples of substances separated and purified using extraction techniques.

In this experiment, the molar distribution coefficient at room temperature will be determined by chemical analysis of the iodine in a water solution before and after shaking it with cyclohexane. The analysis will be done by titrating these aqueous solutions with sodium thiosulfate solution using starch indicator. The equation for the reaction between iodine and the sodium thiosulfate is

I₂ + 2Na₂S₂0₃ = Na₂S₄0₆ + 2NaI

(blue with (colorless) (colorless) (colorless)

starch)

In the presence of starch (a small amount of which is added to the solution just before the end of the titration)7 all the reactants and products are colorless except I₂, which forms a deep blue complex. The titration is continued until the end-point indicated by the disappearance of the blue color. Solid potassium iodide (KI) is added during the titration to prevent loss of iodine vapor from the open flask. Iodide ion from the KI combines with iodine to form the nonvolatile triiodide ion.

Procedure

1. Obtain about 120 mL of aqueous iodine solution and about 75 mL of sodium thiosulfate solution. Rinse a buret with several 2-3 mL portions of sodium thiosulfate solution and then fill the buret with this solution.

2. Carefully measure 10.0 mL of the iodine solution in a graduated cylinder and transfer to a 250 mL Erlenmeyer flask. Rinse the graduated cylinder with a few milliliters of deionized water and pour the water into the flask. Add a few crystals of potassium iodide (KI). Swirl to dissolve the solid KI.

3. Read the volume of liquid in the buret. Titrate with the thiosulfate solution until the brown color of iodine has become barely perceptible. Add about 1 mL of starch solution and continue adding the thiosulfate solution until the blue color of the starch-iodine complex just disappears. Again read the volume of the liquid in the buret.

4. Clean and dry a large test tube. Carefully measure, with a 25 mL graduated cylinder, 50.0 mL of iodine solution into the test tube. Add 5.0 mL of cyclohexane measured in the same graduated cylinder without washing. Tightly stopper the test tube with a rubber stopper and shake the mixture for about 1 minute, and then allow it to stand for about five minutes.

5. Repeat Step 4 with a second 50.0 mL sample of iodine solution and 8.0 mL of cyclohexane

6. Remove the cyclohexane layer (upper layer) with a Pasteur pipet [CAUTION: its tip is very sharp] from the first test tube into a beaker. Then pour 25.0 mL of the water layer into a clean, dry graduated cylinder. Transfer into a 250 mL Erlenmeyer flask, add a few crystals of KI, and titrate as before with sodium thiosulfate solution. The starch may be added at the beginning of the titration.

7. Repeat with the water layer from the second test tube.

8. Pour the cyclohexane solutions from steps 6 and 7 into the bottle on the side bench labeled ORGANIC WASTE.

For Your Report

Before writing your report, review the Guidelines for Laboratory Reports. Your instructor will provide more details on how your results are to be incorporated into your report. Some things you should specifically do for this report are:

1. The thiosulfate solution has been prepared so that 1.00 mL of solution will react with 0.000127 g (5.00 x 10^-7 mole) of iodine. Calculate from the volumes of thiosulfate solutions used:

a) the number of moles of iodine in 50 mL of the aqueous iodine solution before shaking with cyclohexane. b) the number of moles of iodine in 50 mL of the aqueous iodine solution after shaking with 5 mL of cyclohexane, and from this the molar concentration of iodine in the water layer. c) same as b) after shaking with 8 mL of cyclohexane. d) the number of moles of iodine in the 5 mL cyclohexane layer and in the 8 mL cyclohexane layer, and the molar concentration of iodine in each of these layers.

For part a), remember that you analyzed a 10 mL sample; for parts b) and c), you analyzed a 25 mL sample.

The results of b) and c) above yield two values for the concentration of iodine in the water layer, and the results of d) yield two values for the concentration of iodine in the cyclohexane layer. From these calculate K_d for the two cases.

Report for each case: the volume of the aqueous layer; the volume of the cyclohexane layer; the concentration of iodine in the aqueous layer; the concentration of iodine in the cyclohexane layer; and the distribution coefficient.

2. Show sample calculations for the work in part 1 above.

3. Review the syllabus and answer the advance study asignment questions for next week's laboratory exercise. Include these answers with your report.

Sample Calculation

145 mmols of a solute is added to a mixture of 10.0 mL of water and 30.0 mL of cyclohexane at 25°C. After equilibrium has been established, analysis of the water layer shows it to contain 34.5 mmols of the solute. Calculate K_d for this solute.

Moles solute in aqueous layer = 34.5 mmol

Moles solute in cyclohexane layer = 145 - 34.5 = 110.5 mmol

molarity = (moles solute)/(volume of solution in liters)

Molarity of solute in aqueous layer = (34.5 x 10-3)/0.0100

= 3.45 molar

Molarity of solute in cyclohexane layer = (110.5 x 10-3)/0.0300

= 3.68 molar

K_d = molar concentration of solute in solvent 1 / molar concentration of solute in solvent 2

K_d = 3.68/3.45 = 1.07 [no units]

Advance Study Assignment

1. Explain how the molar distribution of iodine between two immiscible liquids will be determined in this experiment.

2. Write a balanced chemical equation to show the oxidation of sodium thiosulfate by molecular iodine.

3. Write short definitions (25 words or less) for each of the following: a. extraction, b. distribution coefficient, c. equilibrium.

Check the course syllabus for specific schedule information for your lab section.

Adapted with permission from Glanville and Russell, "Experiments in General Chemistry" Virginia Tech Department of Chemistry, 1995.

These pages are maintained by Dr. Nicholas H. Snow. For further information or to comment, please send email.

(nhs) 1/27/97