Lab Experiment: Determination of Partition Coefficient
Experiment 4A: Determination of a Partition Coefficient for Benzoic Acid in Methylene Chloride and Water
Experiment 4B: Solvent Extraction I: Acid-Base Extraction Using the System Benzoic Acid, Methylene Chloride, and Sodium Bicarbonate Solution
Objective
A: To accustom participants (students) to general procedures that are used to obtain a partition coefficient at the microscale level. We will gain experience in such practices as the transfer of microliter volumes of solutions with a Pasteur filter pipet, the use of a vortex mixer, and the use of automatic delivery pipets.
B: To illustrate an extensively used extraction technique where a reversible reaction is employed to alter solubility characteristic of the substance of interest.
Experimental Procedure
Experiments 4A and 4B were followed as described in Mayo, pages 144-147, with the modifications listed in the Blackboard document. Also, instead of using a sand bath, we used a heating block instead to recover the benzoic acid and we used a boiling stone to do this, which was not mentioned in the lab book.
Reaction Scheme
A:
B:
Data and Results
Expt
Benzoic acid initial (mg)
CH2Cl2
(mL)
H2O (mL)
10% NaHCO3 solution (mL)
Benzoic acid in CH2Cl2 (mg)
Benzoic acid remaining in aqueous (mg)
Benzoic acid KpCH2Cl2/H2O or KpCH2Cl2/aq. NaHCO3*
4A
79
1.0
1.0
NA
54
37
1.5
4B
91
1.0
NA
1.0
7.0
84
.083
*Benzoic acid has been modified in experiment B, making the Kp value not entirely accurate. However, it is still useful to demonstrate how benzoic acid separates in the two phases.
Observations
In experiment A, after the addition of methylene chloride to the Benzoic Acid, there was a pungent smell wafting from the vial. The methylene chloride layer formed a defined convex layer at the bottom of the vial, showing detailed separation between the methylene chloride and water layers with benzoic acid dissolved in each.
In B, there was very little residue recovered in test tube of dichloromethane after heating.
Experiment 4B: Solvent Extraction I: Acid-Base Extraction Using the System Benzoic Acid, Methylene Chloride, and Sodium Bicarbonate Solution
Objective
A: To accustom participants (students) to general procedures that are used to obtain a partition coefficient at the microscale level. We will gain experience in such practices as the transfer of microliter volumes of solutions with a Pasteur filter pipet, the use of a vortex mixer, and the use of automatic delivery pipets.
B: To illustrate an extensively used extraction technique where a reversible reaction is employed to alter solubility characteristic of the substance of interest.
Experimental Procedure
Experiments 4A and 4B were followed as described in Mayo, pages 144-147, with the modifications listed in the Blackboard document. Also, instead of using a sand bath, we used a heating block instead to recover the benzoic acid and we used a boiling stone to do this, which was not mentioned in the lab book.
Reaction Scheme
A:
B:
Data and Results
Expt
Benzoic acid initial (mg)
CH2Cl2
(mL)
H2O (mL)
10% NaHCO3 solution (mL)
Benzoic acid in CH2Cl2 (mg)
Benzoic acid remaining in aqueous (mg)
Benzoic acid KpCH2Cl2/H2O or KpCH2Cl2/aq. NaHCO3*
4A
79
1.0
1.0
NA
54
37
1.5
4B
91
1.0
NA
1.0
7.0
84
.083
*Benzoic acid has been modified in experiment B, making the Kp value not entirely accurate. However, it is still useful to demonstrate how benzoic acid separates in the two phases.
Observations
In experiment A, after the addition of methylene chloride to the Benzoic Acid, there was a pungent smell wafting from the vial. The methylene chloride layer formed a defined convex layer at the bottom of the vial, showing detailed separation between the methylene chloride and water layers with benzoic acid dissolved in each.
In B, there was very little residue recovered in test tube of dichloromethane after heating.
Cited: 1. Mayo, D. W.; Pike, R. M.; Forbes, D. C. Microscale Organic Laboratory with Multistep and Multiscale Syntheses, 5th ed.; John Wiley & Sons, Inc., 2011; pp 141-147. 2. Modifications for Expts 4A and 4B: Partition Coefficient Determination and Solvent Extraction I, Blackboard document.