LABORATORY EXERCISE 6.

DETERMINATION OF THE DISTRIBUTION COEFFICIENT OF IODINE BETWEEN ORGANIC SOLVENT AND WATER

The aim of the present laboratory exercise is to determine the distribution coefficient of iodine between organic solvent and water, and to estimate possible dissociation and association of iodine molecules in these solvents.

The used research method is titration.

Equipment and instruments are a titration unit and laboratory shaking machine.

Glassware: 50-mL bottles with lined screw caps, 3 pieces; 100 or 250mL conic flasks, 6 pieces; 50-mL test tubes, 3 pieces; 30-mL graduated cylinders; 2 and 10-mL pipettes; 25-mL titration burettes, 2 pieces.

Reagents and materials: a 0.05 M (0.1 N) iodine solution in carbon tetrachloride, analytically pure (reagent grade) carbon tetrachloride, 0.05 M and 0.001 M sodium thiosulphate (Na2S2O3) solutions, and a 1% freshly prepared aqueous solution of starch.

Experiment Procedure

1) Prepare several water–organic solvent mixtures containing different amounts of iodine (see Table 3.1) and place them in the numerated bottles with screw caps.

Table 3.1

Aqueous-organic mixtures containing different amounts of iodine

2) Place the bottles with tightly closed caps in a shaking apparatus and shake them for 40 to 45 min until achieving the phase distribution equilibrium of iodine.

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3)Arrange a titration unit. Fill the titration burettes with the corresponding sodium thiosulphate solutions with the known concentrations.

4)After the end of mixing, subsequently pour the content of each bottle into the enumerated test tubes, and let the mixture stand for 10 to 15 min. The mixture separates into two layers due to a difference in the densities of the two solvents. The upper layer in a test tube is aqueous and the bottom layer is organic.

5)In order to determine the concentration of iodine in both layers of the equilibrium system, take the following actions, using a pipette:

•take 2 samples of 10 mL each from the upper aqueous layer, so that no drops of organic liquid get to the pipette, and transfer them to conic flasks for titration; add 4 to 5 drops of the indicator (starch solution) into each flask and titrate with a 0.001 M sodium thiosulphate solution until decoloration of the blue iodine-starch complex;

•take 2 samples of 2 mL each from the bottom organic layer and transfer them to conic flasks for titration; add 20 mL of distilled water in each flask, 4 to 5 drops of a starch solution, and titrate with a 0.05 M sodium thiosulphate solution at vigorous stirring until decoloration of the drops of carbon tetrachloride; the reaction of sodium thiosulphate with iodine proceeds by the following equation:

2Na2S2O3 + J2 = Na2S4O6 + 2NaJ

Water is added because iodine is titrated by a sodium thiosulphate solution only in an aqueous phase. In this connection, a titration flask should be shaken intensively all the time during the analysis for the gradual extraction of iodine into an aqueous layer.

6) Fill Table 3.2 with the results of titration. Calculate the mean V value from the results of titration.

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7) Calculate the equilibrium concentration of iodine in an aqueous layer, CJI2 , and in a layer of organic solvent, CJII2 , by the formula:

where Vsamp is the volume of a sample taken for titration in mL (Vsamp equals 2 or 10 mL depending on which layer is investigated), V is the mean volume of a sodium thiosulphate solution consumed for titration of the sample in mL, and СТ is the concentration of a sodium thiosulphate solution in mol/L. Record the obtained results in Table 3.3.

8) Using a generalized expression for the distribution law (Eq. (19)), calculate the distribution coefficient of iodine in the water – organic solvent system for each investigated mixture, and determine its mean value by the formula:

graphical method, determine the mean distribution coefficient, К, and the index, n, indicating possible association or dissociation of iodine in an aqueous solution and in organic solvent.

The intercept of the linear dependence on the ordinate lg CJII2 axis allows

one to determine lgК and, hence, approximately the mean distribution coefficient, K, while the tangent of the line slope angle gives the value of the index, n, according to the formula:

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