Thermodynamic study of the liquid-liquid equilibrium water-chloroform-acetic acid

American Journal of Physical Chemistry 2013; 2(6): 117-121 Published online November 30, 2013 (http://www.sciencepublishinggroup.com/j/ajpc) doi: 10.11648/j.ajpc.20130206.11 Thermodynamic study of the liquid-liquid equilibrium water-chloroform-acetic acid Silio Lima de Moura, José Aroldo Viana dos Santos, Francisco Carlos Marques da Silva Bioelectrochemistry Laboratory, Federal University of Piauí, Teresina 64 049 550, Brazil Email address: siliosilicio@hotmail.com(s. L. Moura) Tocite thisarticle: Silio Lima de Moura, José Aroldo Viana dos Santos, Francisco Carlos Marques da Silva. Thermodynamic Study of the Liquid-Liquid Equilibrium Water-Chloroform-Acetic Acid. American Journal of Physical Chemistry. Vol. 2, No. 6, 2013, pp. 117-121. doi: 10.11648/j.ajpc.20130206.11 Abstract: Experimental liquid-liquid equilibria of the water-chloroform-acetic acid system were studied at temperature of 298.15 K. Complete phase diagrams were obtained by determining solubility and tie-line data. Data for the construction of ties-lines were determined by preparing mixtures of known mole fraction of components in the region of formation of two phases. Separation factors were evaluated for the immiscibility region. Keywords: Liquid-Liquid Equilibrium, Microstructure, Ternary System 1. Introduction Recovery of organic acids from dilute solutions resulting from fermentation processes is important, and many solvents have been tried in attempts to improve recovery [1-19]. Pure liquids when mixed in appropriate proportions at certain temperatures and pressures, do not form only one homogeneous liquid phase, but two liquid phases with different compositions. This fact is due to the two-phase state to be more stable than the monophasic state. If these phases are in equilibrium, then the phenomenon is called liquid-liquid equilibrium (LLE) [20]. All systems seek thermodynamic equilibrium. The thermodynamic stability criterion provides that must be satisfied by providing that at a constant temperature and pressure, a steady state is that in which present a minimum of the Gibbs free energy (Equation 1) [21,22]:, 0 (1) When two or more substances are mixed, dgis defined as the difference between the Gibbs free energy of the solution and the pure compounds. If dg 0, forms a stable single phase solution, but if dg 0, the homogeneous solution is unstable and the system is forced to split into two or more phases in order to minimize the Gibbs free energy. This way or two-phase systems are formed by multiphasic. System comprising two or more phases, thus a heterogeneous system is a closed system and each phase within this system is a open homogeneous system in the closed total system. Such system, in which no chemical reaction occurs, will be in equilibrium in relation to the process of heat transfer, displacement of the border and mass transfer. The mechanical and thermal equilibrium, it s necessary that the pressure and temperature within the system are uniform throughout all phases π, to achieve the mechanical and thermal equilibrium. If µi is the chemical potential, it is also expected to have a uniform value across all phases composing the heterogeneous system. This was proved by Gibbs in 1875, and the results for a closed heterogeneous system in equilibrium without chemical reaction with respect to the processes mentioned are [21, 22]:. (2). (3).. (4) where the superscripts represent the phases and the subscripts represent the components. The status of each phase of a steady state can be characterized with n+2 variables: pressure, temperature, and chemical potential of each of the n components in phase. However, not all of these variables are independent, however, the Gibbs-Duhem equation (Equation 5) shows how these variables are related [22]:

118 Silio Lima de Moura et al.: Thermodynamic Study of the Liquid-Liquid Equilibrium Water-Chloroform-Acetic Acid 0 (5) This equation introduces a restriction on simultaneous variation of T, P and chemical potential µi for a single phase. Thus, the n+2 variables that can be used to characterize a phase, only n+1 are independent. The limitation introduced by the equation of Gibbs-Duhem causes one of the variables is dependent. Therefore, it is said that a phase has n+1 degrees of freedom. Thus, if each step of the system is in equilibrium, the total number of independent variables is p(n+1). If the heterogeneous system as a whole is in equilibrium, then there are (p-1)(n +2) equilibrium relationships between the p(n+1) variables given by the equations (2), (3) and (4). Then the number of degrees of freedom, which is the number of intensive variables minus the number of relations or constraints are [22]: 1 1 2 (6) 2.2. Procedure The binodal curve for the water-chloroform-acetic acid ternary system was determined by the solubility method. Binary mixtures of known compositions were shaken in a glass erlenmeyer (Figure 1) equipped with a microburet with an accuracy of 0.005 cm 3. The third component (acetic acid)was progressively added until the transition point was reached. The end point was determined by observing the transition from a heterogeneous to homogenous a mixture until a permanent homogenous could be observed. Ternary mixtures of known overall compositions lying within the two phase region were prepared shaken thoroughly and then allowed to reach equilibrium. Samples were carefully taken from each phase and analyzed to obtain the tie lines. An electronic balance, accurate to ±0.1 mg, was used during the experiments. This equation is the Gibbs phase rule. In this paper, we evaluate acetic acid as an agent for the extraction of chloroform from dilute aqueous solutions, we herein report liquid-liquid equilibrium results at temperatures of 298.15 K for the ternary system of water-chloroform-acetic acid. 2. Experimental 2.1. Materials Acetic acid and butyl acetate with purities of 99.98 % (W/W) and 99.50 % (W/W), respectively, were purchased from Merck. Acetic acid and butyl acetate were used without further purification. Deionized water was further distilled before use. Densities were measured with an Anton Paardensimeter (model 4500), along with some values from the literature (Weast, 1990). Substance Tube Table 1. Mole fraction of each substance. Figure1. Scheme of chloroform-water mixtures prepared for the determination of the phase diagram. 3. Results and Discussion Table 1 shows the mole fractions of all components obtained from the experiments for he acetic acid solvent at 298.15 K. All points shown are the results of measurements obtained in duplicates. 1 2 3 4 5 6 7 8 9 H 2O 0.12 0.26 0.36 0.43 0.49 0.53 0.59 0.62 0.74 CHCl 3 0.54 0.36 0.26 0.20 0.14 0.11 0.08 0.05 0.02 H 3CCOOH 0.37 0.39 0.37 0.36 0.37 0.36 0.34 0.32 0.24 Figure 2 illustrates the composition of each component of Table 1. One observation is that there is a decrease of the molar fractions of chloroform and acetic acid, with the increase of the molar fraction of water. However, an important data shows that for the composition chloroform-water, the acetic acid addition a slight increase and then immediately decrease more and more, meaning that the system reaches equilibrium liquid-liquid with greater ease with lower amount of solvent (acetic acid).the amount of acetic acid is lower at the beginning in relation the amount of chloroform, but this phenomenon is reversed immediately, staying always above the composition of chloroform in the system. Figure 3 shows the increasing of chloroform composition but a much larger statement of the composition of acetic acid, confirmed Figure 2. This can be explained by the fact that acetic acid having a different interaction with chloroform, and this is more effective interaction with water, remembering that this type of analysis same evaluating the behavior of the two components in the system there is the other component water.

American Journal of Physical Chemistry 2013; 2(6): 117-121 119 water and chloroform form microstructures partially miscible, whereas acetic acid (white balls) forms a microstructure completely miscible in each of them in any proportion. Figure2. Curves of the composition of chloroform and acetic acid vs. water composition obtained for the system water-chloroform-acetic acid at 298.15 K. Figure5. Ternary diagram with binodal curve and ties-lines (a, b, c) for the system water-chloroform-acetic acid at 298.15K. Figure3. Binodal curve of system chloroform vs. acetic acid (Mole fraction) obtained for the system water-chloroform-acetic acid at 298.15 K. Figure4. Liquid-Liquid equilibria (Mole fraction) for system water-chloroform vs acetic acid at 298.15 K. ( ) solubility (binodal curve) data for water and ( ) chloroform. In the ternary diagram of Figure 5, the part that lies inside the solubility curve is the region where two phases in equilibrium has a microstructure rich in water (blue balls) but also contains minor amounts of microstructures in other dissolved components and other microstructure rich in chloroform (green balls) microstructures which also contains small amounts of other dissolved. This is because The overall composition of the system with the addition of acetic acid is shown in Figure 4. This graph depicts in a partial diagram of the ternary system, because it can identify nearby where the mole fraction of acetic acid tends to zero, the composition of the other components (water and chloroform) are distant or nearly pure as well as in vertices of the ternary diagram. Above the solubility line has become a single phase region, where the three components form a microstructure completely miscible. The point c'' is defined as the critical point, ie, where the two curve segments finds itself. At this point formed two liquid phases with the same composition and density. The points a and b represent the combined liquid layers in the absence of acetic acid. The total composition of the system is c, so that the lever rule, there is a greater amount than the layer b of the layer. Adding a small amount of acetic acid to the system, the composition varies along the line that joins the vertex c corresponds to acetic acid, the new composition is represented by c'.the addition of acetic acid alters the composition of the two layers for the data values a' and b'. Note that the acetic acid will preferably for the layer b' rich in water, so that the correlation line between the two solutions combined a' and b' does not remain parallel to ab.the relative amounts of a' and b' are given by the lever rule, that is, the ratio of the correlation line segments a'b', this segment is referred ties-line or mooring line. With addition of more acid, the composition varies along the broken line cc; layer rich in water increases in quantity, while the other rich in chloroform decreases. As the correlation lines are not parallel, the point at which the two solutions combined have the same composition is not located on top of the binodal curve but is on one side at the point c'', called lattice point. If the system have the

120 Silio Lima de Moura et al.: Thermodynamic Study of the Liquid-Liquid Equilibrium Water-Chloroform-Acetic Acid composition initial C and will add acetic acid, the composition will vary over cc'', immediately below of c'' the two layers are present in comparable amounts; in c'' the separation surface between the two layers disappears such as the solution becomes homogeneous. If we increase the temperature, the shape and size of the two-phase region will change. 4. Conclusion The study ternary system, water-chloroform-acetic acid at temperature of 298.15 K, may be characterized by an increase in solubility of a solvent in which was obtained a microstructure of liquid-liquid equilibrium. In the study of these microstructures, it can be concluded that the performance of acetic acid specifically in the binodal curve presented points of solubilizing effects which according to the theory of Gibbs are thermodynamically favorable. 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