Aim:
1. To determine the phase diagram for three-component liquid system of Water- Ethanol-Toluene
2. To determine the solubility limits in a ternary system of water and two other liquids (ethanol and toluene), one of which is completely miscible (ethanol) and the other is partly miscible with water (toluene)
Introduction:
In making pharmaceutical formulation, often multiple components need to be mixed together and need to be in homogeneous form. This is usually possible by knowing the exact ratio of each component to be mixed with regard of some other condition such as temperature. In this experiment, there are three components of concern which were Ethanol, Toluene and Water. If water and toluene mixed together with ethanol, all three components can achieve homogeneous solution at equilibrium if proper proportion was used. Solutions are homogeneous because the ratio of solute to solvent remains the same throughout the solution even if homogenized with multiple sources, and stable because the solute will not settle out after any period of time, and it cannot be removed by a filter or a centrifuge. This type of mixture is very stable, i.e., its particles do not settle, or separate. There are 3 components but only 1 phase exists. Thus, by using the phase rule, we can find out that the degree of freedom is 4 which are temperature, pressure, and concentrations of the two of the three components. A phase diagram is the number of phases of a system that can exist in equilibrium at any time depends on the conditions of temperature, concentration and composition. A phase diagram is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium.
Each of three corners or apexes of the triangle represent 100% by weight of one component (A, B, or C). As a result, that same apex will represent 0% of the other two components. For example, the top corner point represents 100% B. Each side of the triangle represents a binary mixture where two components mixed together and any line drawn parallel to one of the sides shows the percentage of a particular component. The three lines joining the corner points represent two-component mixture of the three possible combinations of A, B and C. By dividing each line into 100 equal units, the location of a point along the line can be directly related to the percent concentration of one component in a two-component system. The area within the triangle represents all the possible combinations of A, B and C to give three-component system. Line AC , opposite apex B represent system containing A and C (B=0). The horizontal lines running across the triangle parallel to AC denote increasing percentages of B from B=0 (on line AC)to B=100 (at point B).
Solubility differs when there are different components mixing together. When the third component is added to a pair of miscible liquid, it may affect the mutual solubility. If the third component is more soluble in one of the liquids than in the other, then the miscibility between that pair of liquids decreases. But, if the third component is soluble in both components, then the mutual solubility will increase.
Apparatus:
Burette, pipette, retort stand, conical flask, measuring cylinder, test tube, conical flask stopper
Material:
Toluene, ethanol, distilled water
Procedure:
1. Mixtures of ethanol and toluene in sealed containers measuring 100cm³was prepared containing the following percentages of ethanol (in percent): 10,25,35,50,65,75,90 and 95.
2. 20mL of each mixture was prepared by filling a certain volume using a burette (accurately).
3. Each mixture was titrated with water until cloudiness is observed due to the existence of a second phase.
4. A little water was added and shakes well after each addition.
5. The room temperature was measured.
6. The percentage based on the volume of each component was calculated when the second phase starts to appear/separate.
7. The points were plotted onto a triangular paper to give a triple phase diagram at the recorded temperature.
8. A few more measurements had been done if necessary. The determination in the experiment had been done twice.
Result:
% ethanol (v/v)
|
Volume of Water Used (mL)
|
Average
| |
Titration I
|
Titration II
| ||
10
|
1.3
|
1.1
|
1.2
|
25
|
1.6
|
1.0
|
1.3
|
35
|
1.1
|
1.9
|
1.5
|
50
|
1.9
|
2.1
|
2.0
|
65
|
2.7
|
2.7
|
2.7
|
75
|
4.3
|
3.9
|
4.1
|
90
|
10.0
|
10.4
|
10.2
|
95
|
16.8
|
14.2
|
15.5
|
Calculation:
| Total volume |
Water
|
Toluene
| Ethanol | |||
| (x+20mL) | Volume (mL) | % | Volume (mL) | % | Volume (mL) | % |
21.2
|
1.2
|
5.7
|
18.0
|
84.9
|
2.0
|
9.4
|
21.3
|
1.3
|
6.1
|
15.0
|
70.4
|
5.0
|
23.5
|
21.5
|
1.5
|
7.0
|
13.0
|
60.4
|
7.0
|
32.6
|
22.0
|
2.0
|
9.1
|
10.0
|
45.45
|
10.0
|
45.45
|
22.7
|
2.7
|
11.9
|
7.0
|
30.8
|
13.0
|
57.3
|
24.1
|
4.1
|
17.0
|
5.0
|
20.7
|
15.0
|
62.3
|
30.2
|
10.2
|
33.8
|
2.0
|
6.6
|
18.0
|
59.6
|
35.5
|
15.5
|
43.7
|
1.0
|
2.8
|
19.0
|
53.5
|
Questions:
1. Will a mixture containing 70% ethanol, 20% water and 10% toluene remain clear or form two phases?
At these concentrations, the mixture will remain clear and form one liquid phase.
2. What will happen if you dilute 1 part of the mixture with 4 parts of (a) water; (b) toluene; (c) ethanol?
1 part mixture x 20% water = 1 x 20/100 = 0.2 part of water
1 part mixture x 10% toluene = 1 x 10/100 = 0.1 part of toluene
1 part mixture x 70% ethanol = 1 x 70/100 = 0.7 part of ethanol
Therefore, there are 0.7 part of ethanol; 0.2 part of water; 0.1 part of toluene in the mixture.
(a) 1 part of mixture + 4 parts of water:
Water = (0.2+4 / 1+4) x 100% = 84%
Toluene = (0.1 /1+4) x 100% =2%
Ethanol = (0.7/1+4) x 100% =14%
From the phase diagram, this mixture is under the area of the binomial curve. Therefore, a 2 phase is formed.
(b) 1 part of mixture + 4 parts of toluene:
Water = (0.2 / 1+4) x 100% = 4%
Toluene = (0.1+4 / 1+4) x 100% =82%
Ethanol = (0.7 / 1+4) x 100% =14%
From the phase diagram, this mixture is outside the area of the binomial curve. Therefore, a clear single liquid phase of solution is formed.
(c) 1 part of mixture + 4 parts of ethanol:
Water = (0.2/ 1+4) x 100% = 4%
Toluene = (0.1 / 1+4) x 100% =2%
Ethanol = (0.7+4 / 1+4) x 100% =94%
From the phase diagram, this mixture is outside the area of the binomial curve. Therefore, a clear single liquid phase of solution is formed.
Discussion:
A ternary phase diagram has three components. The sum of these subtracted from the total will give the concentration of the third component and because of this in the three-component system, only two concentration term is required to describe the system. In ternary system, which contains three component usually composed of elements as well as included pressure and temperature also. This ternary phase diagram also known as triangular diagram.
Toluene is soluble in ethanol but in the case of water and toluene, they usually form a two phase system because they are only slightly miscible. However, ethanol is completely miscible with both toluene and water. Thus, the addition of sufficient amount of ethanol to the toluene-water system would produce a single liquid phase as these components were mixed until certain proportion, all the components would be completely miscible and definitely will form homogenous mixture. This experiment is carried out by first making a solution of ethanol and toluene which will be completely miscible and addition of water where at first it will make up two phase. As we continue to add more water until appropriate amount, the mutual solubility of the liquid pair will be increased until at one point, homogenous mixture is formed. The region under the graph shows that there are two phase system form which consist of water and toluene since the solubility of water with toluene is weaker than that with ethanol. Meanwhile the region above the graph shows homogenous mixture.
From the diagram, each corner represent 100 % of ethanol, 100 % of toluene and 100 % of water while the other two components are 0 %. The curve of the plotted graph is termed a binodal curve or binodal. The two-phase area is represented by the region bounded by the curve whereas the region of the graph that is not bounded by the binodal curve represents the one–phase region. Mixture with composition contained within region A are cloudy in appearance due to the phase separation due the amount of ethanol is not sufficient for homogenous mixture to be produced. Mixture with composition that falls into the region of the graph that is not bounded by the binodal curve is clear and homogenous. . For these mixtures, the amount of ethanol is sufficient to produce a single liquid phase.
A nice and complete binomial curve
The result obtained were not to the most accuracy as when the data have been plotted it should have look something like the graph shown before this. The line which separate the single-phase system and the two-phase system were the point at which supposed to be obtained through this experiment. There are some errors that were formed when conducting this experiment which lead to this inaccuracy. One of it is the parallax error. Parallax error may occur while taking the reading of the measuring cylinder or the burette. Then, another error that lead to the inaccuracy is the degree of cloudiness is not sure. This is due to the wide range of cloudiness that make exact point when to stop water titration is wrongly determined. As the result, an excess amount or insufficient amount of water is happened which lead to the inaccuracy during conducting the experiment. Besides the volatility of the chemicals also leads to the error. This is because the mixture of toluene and ethanol may vaporize if it is left longer and unsealed. Lastly, the temperature in the laboratory might not constant also affect the system by causing an immiscibility.
Precautionary steps should be taken while carrying out the experiment to reduce the possibility of error to occur in order to get a better result in this experiment. First of all, the apparatus that want to use should clean and wipe properly and dry before use to prevent the chemicals from being contaminated. Next, it is very important to ensure eye of the observer to be perpendicular to the scale of the measuring cylinder or burette to avoid parallax error as well as the conical flask must be shaken well after every addition of water. The same observer should be assigned to determine the cloudiness of mixture in order to get a more accurate results. Finally, the mixture of toluene and ethanol should be titrated immediately to prevent vaporization or it should be sealed that can lead to the inaccuracy during conducting the experiment.
Conclusion:
Ethanol,toluene and water system is a ternary system with one pair of partially miscible liquid ( toluene and water). The addition of sufficient amount of ethanol to the toluene-water system would produce a single liquid phase in which all the three components are miscible and the mixture is homogenous. From the experiment, as the number of volume of ethanol by percentage increase and number of volume of toluene by percentage decrease, the volume of water will increase. When the solution is stirred, the transition from one region to another can be observed by appearance (or disappearance) of cloudiness or turbidity in the solution. The turbidity results from scattering of light by the large number of very small “oily” droplets of the second phase that are produced when the system is stirred.
Reference
1. http://www.chm.davidson.edu/vce/phasechanges/PhaseDiagram.html
2. Physicochemical Principles of Pharmacy , 3rd edition (1998) . A.T. Florence and D.Attwood. Macmillan Press Ltd.
3. Physical Pharmacy: Physical Chemistry Principles in Pharmaceutical Sciences, by Martin, A.N.
4. http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Phase_Transitions/Phase_Diagrams
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