DISTRIBUTION OF A SOLUTE BETWEEN IMMISCIBLE SOLVENTS
This experiment was conducted conducted to determine the distribution coefficient coefficient K. Two immiscible liquids were mixed in a separatory funnel and formed two layers. The upper layer consisted of acetic acid and the lower layer consisted of diethyl ether. After establishing equilibrium, samples of each layer were obtained and titrated with sodium hydroxide using phenolphthalein as its indicator. The value of K was calculated after determining the concentration of each layer.
1. INTRODUCTION When two immiscible solids come in contact with each other, a dissolved substance will distribute itself between the two according to a definite equilibrium. The principle of the equilibrium distribution of a substance (solute) between two immiscible phases, one of which is usually a solvent is the bases for extraction in technical sense. Crystallization, purification, and isolation (may only be restricted to a solid) are insufficient ways to separate mixtures of compounds. Extraction is the recovery of a substance from a mixture by bringing it into contact with a solvent, which dissolves the desired material. Unlike recrystallization and distillation, it does not yield a pure product; thus, the former techniques may be required to purify a product isolated b y extraction.
For a reaction carried by an aqueous solution at the end of the reaction there may be a presence of unreacted material or unwanted side products. Solvent extraction may be used to obtain the pure product. The aqueous solution is shaken with another solvent that does not mixed with water. Such solvents are said to be immiscible with water, could be, for example, ether, CCl4. The immiscible solvent is chosen such that the substances that are wanted will dissolve in it, leaving the other substance in the water layer. The solvent are separated and if necessary, the procedure is repeated by adding more of the organic solvent to the aqueous layer, shaking the solutions together, separating them and running of the solution containing the required product.
2. MATERIALS AND METHODS 2.1 Materials: Three 100 ml separatory funnels Three 100 ml Erlenmeyer Flasks 100 ml volumetric flask 25 ml pipette 10 ml pipette 1.0 N acetic acid or trichloroacetic acid or other acid
Glacial acetic acid Carbon tetrachloride Ether 0.5 N sodium hydroxide 0.01 N sodium hydroxide
2.2 Methods: 100 ml of 0.50 N, 1 N, and 2 N solutions of acetic acid in water were prepared. 25 ml of each were pipette into 100 ml separatory funnels. Each were added 25 ml of diethyl ether. The separatory funnels were covered with closed rubber tubes to keep water out of the thermostat and set in a thermostat at 25° at 20 minutes. Shake frequently. The solutions were allowed to come to equilibrium before removing from the thermostat. The lower layers were run into beakers taking careful care that the upper layers were not included. 10 ml samples of the drained lower layers were titrated with sodium hydroxide using phenolphthalein as indicator. Samples of the upper layer were taken by a pipette and titrated with sodium hydroxide using phenolphthalein as indicator. A second set of experiments were carried out using 25 ml aliquots of carbon tetrachloride instead of ether.10 ml of the aliquots were measured immediately since carbon tetrachloride is very volatile. The phases were titrated with 0.01 N sodium hydroxide.
3. RESULTS CH3COOH
Sample Used
Normality
(ml)
Volume
Concenrattion
K c
NaOh used (ml)
0.5 N
10
8.7
0.0435 N
14.5
1.0 N
10
16.1
1.61 N
14.64
2.0 N
10
28.4
5.78 N
12.52
K ave= 13.4 Table 3.1.Tabulated Results of the Upper layer
CH3COOH
Sample Used
Normality
(ml)
Volume
Concenrattion
K c
NaOh used (ml)
0.5 N
10
0.6
0.03 N
14.5
1.0 N
10
11
0.11 N
14.64
2.0 N
10
2.3
0.46 N
12.52
K ave= 13.4 Table 3.2. Tabulated Results in Lower Layer
4. DISCUSSION In the first solution, diethyl ether, water and 0.5N acetic acid. On the second , is a mixture of diethyl ether, water and 1.0N acetic acid. And lastly, solution 3 is a mixture of diethyl ether, water and 2.0N acetic acid. The liquid formed two layers. The upper layer of the solution contains the organic-rich layer which is diethyl ether and the lower
layer
contains the aqueous-rich layer which is water and acetic acid. There is a definite ratio of distribution of solute in immiscible liquids, making the volume of liquids below its saturation point. The ratio is represented by a constant k known as the distribution coefficient. In table, it is observed that the molarity and volume of NaOH used for the titration increases with the increase of concentration of acetic acid. It is also observed that the more volume of NaOH were used in the aqueous layer than that of the organic layer leading to a decreasing value for K as the concentration of the solute increases. The concentration of the upper layer over the concentration of the lower layer denotes K. As the concentration of acetic acid is increased, the distribution coefficient, K, decreases. The lower layer is more concentrated than the upper layer. This means that more moles of acetic acid are present in the aqueous phase than in the organic phase.
5. CONCLUSSION When a substance is added to a mixture which soluble to lesser or greater extent in both immiscible liquids, by shaking and settling to constant temperature, the concentration in each layer becomes constant but the solute will continually interchange
itself between the two layers. The layer that has most of the desired product in it can be separated from the other. The higher the distribution coefficient the more solute will be transferred to the extraction solvent. The upper layer of CH 3COOH and CCl 4 has a greater concentration of HoAc and higher distribution coefficient so more of its solute were distributed into the upper layer which the CCl 4. While of CH3COOH and diethyl ether, the concentration values of HoAc is greater in lower layer but still the amount of distributed solute in the upper layer is much higher than the upper layer of CH 3COOH and CCl4. The HoAc is more soluble with diethyl ether and it will be better to use it as its extraction solvent.
APPENDICES Equations: Eq (A.1) Distribution Constant (k):
K=
CAQ CORG
Where: CAQ is the concentration of aqueous layer CORG is the concentration of the organic layer
Sample Computations: 0.5 N CH3COOH and ether Lower Layer: (.)(.6)
=
= 0.03
Upper Layer:
= =
(.)(8.7) . .
= 0. 435
= 14.5
6. REFERENCES http://academics.wellesley.edu/Chemistry/chem211lab/Orgo_Lab_Manual/Appendix/Tec hniques/Extraction/extraction_n.html http://www.chem.ucla.edu/~bacher/Specialtopics/extraction.html http://www.mendelset.com/articles/685/extraction_and_determination_distribution_coeffi cient_kd
