where the a and b values should be taken in the same scale units (Fig. 3.5.).

Fig. 3.5. The graphical method of determination of the n and K values from the experimental equilibrium concentrations of iodine in the aqueous and organic layers.

10)Pour the remaining organic layers into a special vessel for collecting organic wastes under a hood.

11)Draw conclusions from the performed lab experiments; based on the analysis of the value of the index, n, make assumptions about possibility of the processes of association or dissociation of iodine in the aqueous and organic layers. Compare the obtained results with the literature data.

LABORATORY EXERCISE 7.

DETERMINATION OF THE DISTRIBUTION COEFFICIENT OF ACETIC ACID BETWEEN ORGANIC SOLVENT AND WATER

Work objective: is to determine the distribution coefficient of acetic acid between organic solvent and water, and to estimate possible dissociation and association of acetic acid in these solvents.

The used research method is titration.

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

Glassware: 100-mL bottles with lined screw caps, 5 pieces; 200-mL titration conic flasks, 9 pieces; 50-mL test tubes, 5 pieces; 30-mL cylinders; 5, 10, and 25-mL pipettes; 25-mL titration burettes, 2 pieces.

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Reagents and materials: a 2 M aqueous solution of acetic acid, common organic solvents such as hexane, toluene, carbon tetrachloride, petroleum ether, etc., 0.1 and 0.5 M sodium hydroxide (NaOH) solutions, and an phenolphthalein solution in ethanol (1%).

Laboratory Procedure

1)Following Table 4, prepare 5 aqueous solutions of acetic acid with various concentrations by dilution of the initial aqueous acetic acid solution with a concentration of 2 mol/L.

2)Transfer 25 mL of each prepared solution by a pipette to the separate numerated 100-mL bottles. Add 25 ml of organic solvent (under instructions of the teacher) by a pipette to each bottle. Tightly close the bottles by screw caps.

3)Place the bottles in a laboratory shaker, and extract acetic acid from aqueous solutions to organic solvent until establishing the equilibrium for 25 to 30 min.

4)In the meantime while the extraction goes, determine exact concentration of acetic acid in the prepared solutions by the titration method.

5)Prepare a titration set. Fill the burette by an aqueous NaOH solution with a known concentration. Such a value of the concentration of a NaOH solution should be chosen, so it would be less than the concentration of the prepared acetic acid solution. For the titration of acetic acid solutions under numbers 1 and 2 (see Table 3.4), an NaOH solution with a concentration of 0.1 mol/L should be used, and for the titration of solutions

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under numbers from 3 to 5 an NaOH solution with a concentration of 0.5 mol/L.

6) Take 5 mL of the prepared aqueous solution of acetic acid from the flask under no. 1 by a pipette and transfer to a conic titration flask, add 15 to 20 mL of distilled water, 1 to 2 drops of the phenolphthalein indicator, and titrate with an NaOH solution until appearing the steady pale pink color. Repeat the titration 2 to 3 times until obtaining nearly the same V1 and V2 volumes of an NaOH solution. Record the titration results Table 3.5.

Calculate the mean titration volume, V .

7)Perform similar titrations for the other prepared acetic acid solutions with various concentrations. Record the obtained results in Table 3.5.

8)Turn off the shaking machine upon expiration of the extraction time. Pour the contents of each flask into the corresponding numerated test tubes, and wait several minutes until separation of the aqueous layer from the organic. Transfer a 5-mL sample of the aqueous layer from a test tube into a titration flask by a pipette, so that no drops of the organic layer get to the sample. If organic solvent is less denser than water, then the upper layer in a test tube is an organic solution (hexane, toluene, pentanol, ethyl acetate, etc.), and the bottom layer is an aqueous solution. Such solvents as chloroform and carbon tetrachloride are denser than water and remain in the bottom layer.

9)Titrate two samples of each equilibrium aqueous acetic acid solution (after extraction) by a NaOH solution using the method described in the 5th

and 6th steps. The obtained titration results (V1, V2, and V ) record in Table 3.5.

10) Pour residual organic layers into a special vessel for collecting organic wastes under the hood.

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11) Calculate the initial concentrations of acetic acid,

СinitH2O (CН3СООН), in aqueous solutions (before extraction) from the titration results (Table 5) by the formula,

where СТ(NaOH) is the molarity of an NaOH solution in mol/L, V (NaOH) is the mean volume of an NaOH solution consumed for the titration in mL, and V(CH3COOH) is the volume of a СН3СООН solution taken for the titration in mL. Record the obtained results in Table 3.6.

12) Calculate the equilibrium concentrations of acetic acid СH 2O (СН3СООН) in aqueous solutions after extraction from the titration

results (Table 3.5) using Eq. (3.43). Record the obtained results in Table 3.6.

13) Calculate the equilibrium concentrations of acetic acid in an organic solution, СОrg (СН3СООН), by the formula,

СОrg (СН3СООН) = СinitH 2O (СН3СООН) – СH2O (СН3СООН), (3.44)

which is valid for the case when equal volumes of the aqueous solution and organic solvent are taken for the extraction. Record the obtained results in Table 3.6.

Table 3.6

The Equilibrium Concentrations (mol/L) of Acetic Acid

14) Calculate logarithms of the equilibrium concentrations of СН3СООН in the aqueous (lg СH2O ) and in organic (lgСОrg ) solutions. Record the obtained results in Table 6.

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