15) Plot the graph of the dependence of lg СОrg on lg СH2O by analogy

with Fig. 5. Determine by the graphical method the mean distribution coefficient, K, and find the index, n, from the tangent of the line slope angle.

16) Calculate the coefficient of distribution of acetic acid between organic solvent and water for each investigated system by the formula,

Record the obtained results in Table 3.6.

17) Draw conclusions, in which the following issues are addressed: a) how the concentration of the prepared aqueous acetic acid solution affects the value of the distribution coefficient, and its behavior is compared with the theoretical data; b) the value of the index, n, is analyzed, and assumptions about possible association and dissociation of acetic acid in water and in organic solvent are made; c) extraction abilities of water and organic solvent are compared with respect to acetic acid, and a better extracting agent for acetic acid is chosen.

LABORATORY EXERCISE 8.

STUDY OF THE EXTRACTION PROCESS OF IODINE

The aim of the present lab exercise is to determine the amount of the extracted iodine upon single and multiple extraction by the same volume of extracting agent (water) in the triple J2–H2O–CCl4 system.

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, 4 pieces; 100 or 250mL conic flasks, 3 pieces; 50-mL test tubes, 2 pieces; 30-mL graduated cylinders; 10-mL graduated pipettes; 25-mL titration burette, 2 piece.

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

Laboratory Procedure

1) Transfer two 5-mL portions of the initial iodine solution in СCl4 with

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a concentration of 0.05 mol/L into two bottles with screw caps.

2)Add to the first bottle 15 ml and to the second one 45 mL of distilled water, tightly close them with caps, and bring the mixtures to thermodynamic equilibrium by shaking the bottles in a lab shaker for 30 min.

3)Take 2 mL of the initial solution of I2 in ССl4 by a pipette and transfer into a titration flask, and add 20 mL of water and 3 to 4 drops of a starch solution to the same flask. Perform the titration by a sodium thiosulphate solution under intensive stirring until disappearance of dark blue color. Repeat the titration two times and calculate the mean result. A difference between the two titrations should be no more than 0.2 mL. Record the results of analyses in Table 3.7.

4)After the end of extraction, transfer the mixtures to the numerated test tubes, and let them stand for several min until separation of the liquid layers.

5)Using a pipette, carefully separate the top aqueous layer from the first test tube into a separate flask. Transfer the organic layer which has remained in the test tube into a bottle with a screw cap, add 15 mL of distilled water into the bottle, and place it in a shaking machine for 30 minutes.

6)By a pipette, take a 10 mL sample of the aqueous solution (upper layer) from the second test tube, transfer it to a titration flask, and titrate by a sodium thiosulphate solution in the presence of 4 to 5 drops of a starch solution until disappearance of dark blue coloring. Repeat the titration two times and calculate the mean result. The difference between two titration results should be no more than 0.2 mL. Record the experimental results in Table 7.

7)Separate the organic layer again over 30 min after the second extraction of the solution, repeat operations of the 5th step and perform the third extraction. Combine the aqueous layers from the first and second extractions.

8)Separate the aqueous layer from the 3rd extraction after 30 min of agitation on a shaking machine and combine it with the aqueous layers obtained at the first and second extraction.

9)Determine the concentration of I2 in the combined aqueous layer after the triple (fractional) extraction by a Na2S2O3 solution in the presence of 4 to 5 drops of a starch solution. Repeat the titration 2 times and calculate the

mean result, V 3 . Record the titration results in Table 3.7.

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10) Calculate the initial iodine concentration, Сinit , in carbon

tetraperchloride from with the titration results (Table 7) by the following formula:

where СТ is the concentration of a titrant Na2S2O3 solution (СТ = 0.001

mol/L), V init is the mean volume of the Na2S2O3 solution consumed for the titration of the initial iodine solution in mL, and Vsampleinit is the volume of the initial solution of iodine in carbon tetrachloride taken for the titration

(Vsampleinit = 2 mL).

11) Calculate the mass of iodine in the sample volume of the initial solution taken for extraction by the following formula:

initial solution of iodine in carbon tetrachloride taken for the extraction

(VCCl4 = 5 mL).

12) Calculate the concentration of iodine in the aqueous layer after the single extraction by the following formula:

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where V 1 is the mean volume of the Na2S2O3 solution consumed for the titration in mL, and Vsample1 is the volume of the aqueous layer after the

single extraction taken for the titration (Vsample1 = 10 mL).

13) Determine the mass of iodine that has remained in the organic layer after the single extraction by the following formula:

(VH2O = 45 mL).

14) Calculate the distribution coefficient of iodine between carbon tetrachloride and water using the following formula:

15) Calculate the mass of iodine remained in the organic layer after the fractional extraction by the following formula:

16) Record the obtained results in Table 3.8. Calculate the degree of extraction, Ri, of iodine from carbon tetrachloride at the single and fractional extractions using the formula,

where the index, i, corresponds to either the single or triple extraction.

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