Planning and Designing Chemistry Lab – To determine the partition coefficient of monocarboxylic acid between water and diethyl ether using extraction.
As a young chemist, supplied with 15g of a solid monocarboxylic acid, of molar mass 122 and the solvent diethyl ether; plan and design and experiment to determine the partition coefficient of the acid between water and diethyl ether. You are free to choose the bits of apparatus necessary to achieve the objective of the design, and if needed one other chemical apart from water.
The monocarboxylic acid is more soluble in water than in diethyl ether.
To determine the partition coefficient of monocarboxylic acid between water and diethyl ether using extraction.
Polar and ionic substances generally dissolve more readily in water than in organic solvents and vice versa. If a solute therefore, dissolves in two immiscible solvents, then it will partition itself between the two solvents such that the ratio of its concentration in one solution to its concentration in the other solvent is constant when the liquid mixture is in equilibrium (at a fixed temperature).
Determining the partition coefficient of a particular solute leads to an understanding of its solubility as well as it’s the hydrophilic-hydrophobic nature. Knowing the partition coefficients of various solutes allows us to separate chemicals efficiently by extraction.
- Manipulated Variables: Solute and Solvents Chosen
- Controlled Variable: Mass of Solute (Carboxylic Acid)
- Responding Variables: Partition Coefficient and concentration of acid in each solution
Materials and Apparatus:
Electronic balance, Measuring cylinder , beakers, thermometer , separatory funnel and stopper, retort Stand, burette, pipette, conical flask, distilled water, diethyl ether, monocarboxylic acid
- Weigh an empty dry beaker on the balance and record its mass.
- Add a small amount of monocarboxylic acid (solute) to the beaker and record the mass of the beaker and solid.
- Subtract the mass of the empty beaker from the mass of the beaker and solid in order to calculate the mass of the solid.
- Repeat the process until 5.00g of the solute is in the beaker.
- Measure 80cm3 of diethyl ether as well as 80cm3 of distilled water (solvents) and pour the solvents into the separatory funnel.
- Add the solute and shake carefully (by inversion) for a prolonged period of time to ensure full agitation of the mixture (pause occasionally and open tap for a few seconds).
- Secure the separatory funnel in the retort stand.
- Allow the liquid mixture to equilibrate.
- After the liquid mixture has settled (two distinct layers can be seen).
- Place a beaker below the funnel.
- Remove the cover and release the tap so that the solution runs into beaker. Stop at the interface layer.
- Rinse and fill the burette with dilute sodium hydroxide solution of known concentration and record the initial burette reading.
- Using pipette filler rinse the pipette with the solution in the beaker and carefully transfer 25cm3 of the solution to a clean conical flask.
- Add 2 drops of phenolphthalein indicator.
- Run the sodium hydroxide solution from the burette into the flask with swirling until pale pink color is seen.
- Record the burette reading.
- Repeat the process until consistent results are obtained.
Treatment of Results:
- To determine the mass of solute being used subtract the mass of the empty beaker from the mass of the beaker and the solute.
- To determine the volume of sodium hydroxide used, subtract the initial burette reading from the final burette reading.
- Add three consecutive values obtained for volume used and divide the sum by 3 to obtain the mean liter. The mean liter is the average amount of sodium hydroxide used in the titration.
- In order to calculate the amount of solute which dissolved in each solvent:
- Determine the number of moles of NaOH used in the titration.
- Determine the number of moles of acid which dissolved in the aqueous layer.
- Convert the moles of acid obtained above to mass/grams.
- Subtract the mass of acid obtained in the previous calculation from the total mass of acid added to the solvents to obtain the amount of solute which dissolved in layer 2.
- Calculate the partition coefficient:
Interpretation of Results:
A partition coefficient value of less than one shows that the monocarboxylic acid is more soluble in water than in diethyl ether. However, a partition coefficient value of more than one shows that the monocarboxylic acid is more soluble in diethyl ether than in water. If any mass of carboxylic acid is placed in any volume of a liquid mixture of diethyl ether and water the will partition or distribute themselves according to the partition coefficient.
Sources of Error/Limitation
- Some of the interface layer may mix with the aqueous layer during release.
- Environmental factors may cause the temperature in the room to change. Therefore each successive trial may occur at different temperature affecting the results.
- Cover of the separating funnel was removed before the tap was opened to prevent air from flowing in and causing the layers to mix. Similarly, during agitation of the liquid mixture, the funnel’s stopper was removed intermittently to relieve the buildup of pressure.