Determination of Partition of Coefficient of Benzoic Acid

Introduction:

How a solute is distributed between two immiscible solvents is described by the partition coefficient. This provides a precise way to measure the accuracy of atomistic force in various solvent environment.¹ Partition coefficient or also known as the distribution coefficient can be a valuable method in studies in natural aquatic environments.It can be used as a method of estimating the degree of contaminants from particles to water in aquatic environments.² In this experiment, we will demonstrate a method of determining the partition coefficient, specifically benzoic acid.This experiment uses the separation technique of acid-base extraction as well as liquid to liquid extraction to partition benzoic acid between equal amounts of methylene chloride and water by taking advantage of the ionizability.

Reaction:

Part A: Liquid to Liquid Extraction

Part B: Acid-Base Extraction

Chemical Table:

Procedure:

This lab was separated into 2 lab periods. In the first lab period, a simple liquid to liquid extraction was done for the determination of a partition coefficient of benzoic acid. 50mg of benzoic acid was added to a 3mL conical vial. With an Eppendorf pipette, 600µL of methylene chloride and 600µL of water was added to the conical vial. The vial was then shaken until all the solid material dissolved. The two layers were observed to have separated into 2 layers. The lower methylene chloride was drawn and transferred into a clean vial with a pipette. 100mg of anhydrous sodium sulfate was then added to the methylene chloride and stirred to facilitate “drying”. The solution was observed to still be cloudy with the sodium sulfate also being “gooey”. An additional 100mg of sodium sulfate and 0.5mL methylene chloride was added till the solution turned clear. After the solution appeared dry, it was transferred into a clean pre-weighed dry vial. The sodium sulfate was washed with an additional of 0.5mL of methylene chloride where the solution is added into the pre-weighed vial. The solvent from the sample was then evaporated using the rotary evaporator. After the evaporation, the vial was reweighed to determine the weight of the benzoic acid extracted into the original methylene chloride layer (40.9mg). The melting point of the benzoic acid was then collected as well (120.2°C).

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The second lab period was an acid-base extraction of benzoic acid. 50mg of benzoic acid was added onto a 5mL conical vial fitted with a screw cap. 600µL of methylene chloride was then added into the vial with an Eppendorf pipette. The vial was then capped and shaken to dissolve the solid. The vial was then carefully vented and 300µL of 10% aqueous sodium bicarbonate was added to the methylene chloride solution with an Eppendorf pipette. The vial was then capped and shaken again where the two layers separating was observed. With a Pasteur pipette, the aqueous layer was then transferred into a 20mL beaker. The extraction was then repeated with more 300µLof sodium bicarbonate where the aqueous layer was combined in the same beaker. 6M of HCl was then added into the beaker until the pH is between 2 and 3. To test the pH, the solution was stirred around with a spatula in which the spatula then touched the pH paper (Do not dip the pH paper into the solution). White solid precipitate was then observed from this solution, which should be benzoic acid. The precipitate was then collected using a Hirsh funnel fitted with filter paper that was pre-moistened with cold distilled water. The beaker was then rinsed with cold distilled water and filtered to collect any additional benzoic acid. The benzoic acid was then dried in the funnel. After it is completely dry, the weight (45mg) and the melting point (119°C) of the benzoic acid was collected.

Results:

Part A:

Weight of Benzoic Acid in 600µL organic layer: 40.9mg

Weight of Benzoic Acid in 600µL aqueous layer: 9.1mg

Partition Coefficient: 4.5

Melting Point: 120.2°C

Part B:

Collected: 4.5mg of white solid

Percent Recovery: 9%

Melting Point: 119.3°C

Discussion:

In the first section of the lab, we successfully determined the partition coefficient through a liquid to liquid extraction. The partition coefficient was determined to be 4.5 as the benzoic acid in the organic layer was 40.9mg and the benzoic acid in the aqueous layer was 9.1mg. The literature melting point is reported to be 122°C. The experimental value of the extracted benzoic acid was 120.2°C with a percent error of -1.5%. In the second section of the lab, we only successfully extracted 4.5mg of the benzoic acid from the acid-base extraction. The percent recovery for this extraction is only 9%. The melting point for this was 119.4°C with a percent error of -2.1%. In this experiment, the partition coefficient for this method of extraction would be the same regardless of which method of extraction is used. By taking advantage of the ionizability, we were able to perform an extraction to partition benzoic acid between equal amounts of methylene chloride and water where the partition coefficient was determined to be 4.5 for part A, which would be the same for part B.

Works Cited:

1. Bannan, C. C.; Calabró, G.; Kyu, D. Y.; Mobley, D. L. Calculating Partition Coefficients of Small Molecules in Octanol/Water and Cyclohexane/Water. Journal of Chemical Theory and Computation 2016, 12 (8), 4015–4024.
2. Gormley-Gallagher, A. M.; Douglas, R. W.; Rippey, B. The Applicability of the Distribution Coefficient, KD, Based on Non-Aggregated Particulate Samples from Lakes with Low Suspended Solids Concentrations. Plos One 2015, 10 (7).