DISTRIBUTION OF A SOLUTE BETWEEN IMMISCIBLE SOLVENTS Ceniza E. A.*, Colonia J. L.*, Galarpe C.*, Rivas N. M.*, Sunga J. L.* * Department of Chemical Engineering, Cebu Institute of Technology – University Cebu City Philippines
Date Submitted: Oct.4,2010
I.
INTRODUCTION
If a solute is introduced into any two phase system which may be gas/solid, gas/liquid, liquid/liquid or liquid/solid, it will become distributed between the two phases. K=CA, aqCA, org (1)
aaqaorg=K
Equation 1 is a mathematical statement of the Nernst Distribution Law, which states that a substance will distribute itself between two solvents until at equilibrium. The ratio of the activities of the substance in the two layers is constant at any given temperature. When When the the solu soluti tions ons are are dilu dilute te,, or when when the the solu solute te beha behave vess idea ideall lly, y, the the activ activit ity y is essentially equal to concentration C. [1] The ratio of the concentration, of the substance A in the organic phase, CA, org, and aqueous phase, CA, aq, at equilibrium gives the distribution coefficient K. (2)
K=CA, aqCA, org
. K depen depends ds on the the natur naturee of the the solu solute te and and the the liqu liquid idss invol involve ved. d. Temperature is another factor affecting the nature of this constant and so is the manner in which the constant is written CACB or CBCA CBCA. The The dist distri ribut butio ion n law law is valid valid only only when when the the solu solute te under undergo goes es no chan change ge such such as dissociation or association. This limitation to the applicability of the distribution law was pointe pointed d out in the conjun conjuncti ction on with with the use of Henry’ Henry’ss law for gases. Distrib Distributi ution on coefficients vary with temperature.
When a substance distributes itself between two solvents without the complication of association, dissociation, or reaction with the solvent it is possible to calculate the weight of the substance which can be removed in a series of extractions. Suppose a solution contains x grams of a substance in V cc of solution and suppose that this solution is shaken repeatedly with v cc samples of pure immiscible second solvent until distribution equilibrium is attained. At the end of n extractions the weight x of solute remaining unextracted will be n
(3)
x = xoKVKV+vn n
Here K=C1C2, when K is known equation 3 may be used to estimate the number of extraction necessary to reduce x to some given value x . Another important assumption that can be made from equation 3 is that if a given volume V of a solvent is available for extraction, greater extracting efficiency can be obtained if this volume is utilized in a number of separate extractions that if it were all used once. Hence, it is better to extract with small volumes of solvent several times than once with a large volume. n
I.
APPARATUS AND MATERIALS
Quantity
Description
1 10 3 2 3 1 3 1 1 1 1 1
50 ml 250 ml 250 ml 10 ml 100 ml 10 ml 100 ml
II.
Articles Burette Beaker Erlenmeyer Flask Pipette Graduated Cylinder Graduated Cylinder Separatory Funnel Aspirator Iron Clamp Iron Stand Test Tube Brush Dropper
Chemicals Phenolphthalein 75 ml Benzene 30 ml CH3COOH 5g NaOH 600 ml Distilled Water
PROCEDURE
One hundred milliliters each of approximately 0.50, 1, and 2N solutions of acetic acid in water were prepared. One hundred milliliters of 0.5 N sodium hydroxide was
prepared to be used as titrant. Twenty-five milliliters of each of the three acetic acid solutions were pipetted into closed separatory funnels. Twenty-five milliliters of benzene was added to each. The solutions were allowed to stand at room temperature for 20 minutes, with frequent shaking. After the solutions have come to equilibrium, 10 ml sample was pipetted out from the upper layers into separate Erlenmeyer flasks. Another 10 ml sample was drained from the lower layers into separate Erlenmeyer flasks. The samples were then titrated with 0.5 N sodium hydroxide solution.
RESULTS
Table 1 Solution
Volume of Sample (ml)
Volume of NaOH used (ml)
Concentration of HAc (ml) UPPER LOWER
Distribution Coefficient
UPPER
LOWER
UPPER
LOWER
K
0.5N HAc + benzene
10
10
0.2
6.4
0.01
0.32
32.00
1N HAc + benzene
10
10
0.4
13
0.02
0.65
32.5
2N HAc + benzene
10
10
0.6
20
0.03
1.00
33.33
Table 1.1 shows the concentration of acetic acid in the upper and lower layers after titrating with 0.5N NaOH. The Distribution coefficient K at different concentration of acetic acid mixed with benzene is almost the same. Fig.1 Distribution Coefficient vs Concentration Figure 1 shows a straight line which signifies that the distribution contant values are almost the same. III.
ANALYSIS
From table 1.1 and Fig.1 it is shown that the values of the distribution constants are near each other or almost the same. This can be justified by the Nernst distribution law which states that a substance will distribute itself between two solvents until at equilibrium and the ratio of the activities of the substance in two layers is constant at any given temperature.[1]. Since the solutions are dilute the activities of the substance in the two layers are essentially equal to their concentrations.[1] Thus equation 2 was used to get the distribution coefficient. Since the distribution constant is a ratio it justifies that it is almost the same in the three solutions regardless of their concentration or normality. It
could also be seen in table 1.1 that the concentration of the acetic acid is greater in the lower layer which is water than in the upper layer which is benzene. Since acetic acid and water are both polar and like dissolves like, it is justifiable that acetic acid naturally dissolves more in the water layer. IV.
CONCLUSION
From the data collected it is safe to say that the acetic acid was distributed into the two layers, water which the lower layer and benzene which is the upper layer, until at equilibrium the ratio of the concentration of the acetic acid in the two layers is constant. It is also safe to say that the equilibrium constant will not vary no matter the concentration or normality of a substance.
V.
REFERENCES
[1.] Maron, Samuel and Lando, Jerome. Fundamentals of Physical Chemistry. New York, USA: Macmillan Publishing Co., Inc., 1951
