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Acta Chim.Slov. 1999, 46(4), pp. 463-470
PHASE EQULIBRIA IN THE (WATER + 1,4-DIOXANE + MAGNESIUM CHLORIDE) SYSTEM*
M. Bester Rogač and D. Dolar
Faculty of Chemistry and Chemical Technology, University of Ljubljana,
Aškerčeva 5, 61000 Ljubljana, Slovenia
(Received 28.5.1999)
ABSTRACT
The solubilities of magnesium chloride in (water+1,4-dioxane) and the corresponding phase diagram at the temperature 298.15 K were determined. To investigate the phase separation Gibbs free energy accompanying the dilution of the saturated two-phase system is given.
INTRODUCTION
Water and 1,4-dioxane are miscible in all proportions. The addition of some salt to (water+1,4-dioxane) causes at certain composition a phase separation, giving two conjugate solutions[1,2,3]. For this phenomenon the Gibbs free energy change[4] has been determined for the system (water+1,4-dioxane+sodium chloride). In this contribution the results of similar investigation are presented for
*                                                                                                                                                  th
Dedicated to Professor Drago Leskovšek on the occasion of his 80 "birthday
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the (water+1,4-dioxane+magnesium chloride) system. The activities of water, 1,4-dioxane and of magnesium chloride in mixed solvent from the vapour pressure data have been reported[5]. By using of some of these results the Gibbs free energy accompanying the phase separations was obtained.
EXPERIMENTAL
Magnesium chloride, analytical-reagent grade (Kemika Zagreb, Croatia) and spectroscopically pure 1,4-dioxane, C4H8O2, (Kemika Zagreb, Croatia) were used without further purification. Distilled water of resistivity greater than 0.3 MQ-cm was used in all experiments.
To determine the solubility of MgCl2 as a function of the solvent composition the saturated or very concentrated water solution was prepared in the thermostated cell at (298.15±0.05) K. 1,4-dioxane was added until the solid phase was precipitated. The solution was mixed so that the equilibrium was reached. The salt content of the solution was checked by potentiometric titration of Cl - with AgNO3, using Cl - ion-selective electrode and a saturated mercury sulphate electrode as the reference. The composition of solvent was determined from a calibration diagram[6], representing the dependence of refractive index on concentration of salt for different mixtures.
To determine the phase diagram, 1,4-dioxane was added to the water solutions of MgCl2 until the phase separation occurs. The composition of solutions, obtained by phase separation, was determined as described above. Some of them were used as stock solution for preparing solutions for vapour-pressure measurements by dynamic gas-saturated method, as described previously[5].
To check the solid phase in equilibrium with saturated solutions or two-phase system, some X-ray powder samples were measured. The solid phase was unambiguously identified as complex MgCl2-C4H8O2-6H2O, which has been described previously[7].
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TREATMENT OF RESULTS
The Gibbs free energy of a homogenous solution consisting of amounts of substance n1 of H2O, n2 of C4H8O2, and n3 of MgCl2, is given by
G = n1\i1 + n2\i2 +n3\i3 .
m, |i2 and I-L3 are the corresponding chemical potentials. The chemical potential of component i in solution at constant temperature is
\li =\lf +RT-lnai ,                                                                             2.
where \if is the standard chemical potential and ai the activity of component i, obtained from the relation
ai=fi/fi .
fi is the fugacity of the component in solution and   fi°   the fugacity in the standard state. The activity a3 of the electrolyte is related to the molality m by
3    (m±y±\3        m3       3 a3=a±   =   ------—    =4—y±
V    m°    J        m°
y± is the activity coefficient of MgCl2 and mO is the standard molality, 1 mol-kg-1. The fugacities f1 of H2O and f2 of C4H8O2 were calculated as[8]
f1=p 1-exp[{(p-p*)-(B 11-V 1) + 2^2-py}/RT] ,                                5.
f2 =p2 ' exp[{(p ~p2) ' (B 22 _V2*)+ 2812 •p• (1 -y ) } / RT]               6.
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where
ö12=B12-(B11+B22)/2 .                                                                        7.
pi is the experimental partial pressure of H20, p 2 that of C4H802 and p the total
*                                                        *
vapour pressure. pi   is the vapour pressure and V    the molar volume of pure
*                                                     *
liquid water, p2 is the vapour pressure and V2 the molar volume of pure liquid 1,4-dioxane, and y is the mole fraction of C4H802 in the vapour. The constants Bu, B22 and Bu are the second virial coefficients, reported in the literature[9,10]
From activities of the components in the mixed solvent the activities of the salt could be obtained by applying of the Gibbs-Duhem theorem as has already been done[5].
As there is no pure MgCl2 in equilibrium with saturated solutions neither in water nor in mixed solvents, it can not be chosen as an initial state. So we decided to consider the process, where the two-phase system with an average concentration of solvent (0.677H20 +0.323C4H802), saturated with MgCl2, is diluted by adding the solvent with the same composition. The reduced Gibbs free energy, accompanying this process is given by
A G/RT = nfinfr/f*) + m fln(fi/fi T) + m^ntf^*) + n2 Tln(f2/f2 T)
+n 3{(AiiP(m)-Aii3^m))/RT+5ln(my±/(m*Y±*))}.                        8.
The fugacities of H20 and C4H802, fi and/2, molality m and the mean activity
(ti coefficient y± are taken from the literature[5]. The superscript    refers to the
saturated solution and the superscript ( )   to the solvent without salt. Because the
standard chemical potential JI3 in initial and final solution is not known, the differences between the standard chemical potential of MgCl2 in mixed solvent and in water A/i3° and Aß3* are applied, giving ji3°(m)-ßs1*™ )=Aii30<m)-Aii3^m) in molal scale[5].
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RESULTS AND DISCUSSION
Figure 1 shows the dependence of the solubility of MgCl2 on solvent composition. It is evident that the phase separation occurs in the region where the mole fraction of C4 H8O2 in the solvent is between 0.137 and 0.823. The top phases are rich in C4 H8O2 and low in salt while the converse is true for the bottom phases.

0.0
0.2
0.4
0.6
0.8
1.0
x
FIGURE 1. The solubility   of MgCl2   in {(1-x) H2O + x C4H8O2 )} and the phase diagram at the temperature 298.15K.
The mixtures, saturated with electrolyte, split into two conjugate solutions, connected in the Figure 1 with the upper line. The average concentrations can be obtained from the mass balance as follows
m '-m
D* - D'
m  -m''     D''-D
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where fri and m ' ' denote the molalities of the electrolyte in the bottom and in the top phase respectively. m refers to the average concentration of the hypothetical solutions. D' , D'' and D are the corresponding weight per cent of 1,4-dioxane in the solvent. In Table 1 all for the calculations of the Gibbs free energy demanded quantities are collected[5]. Because the conjugate solutions are in equilibrium only the more precise data for the heavier phases are given.
Table 1. The weight percent of 1,4-dioxane, D´; the molality, m´, the reduced difference of standard chemical potential of MgCl2 in mixed solvent and in water in the molal scale, A/i3°(m)/RT; the mean activity coefficient of MgCl2, y+ ; and the fugacities of water and of the 1,4-dioxane, fi   and /2; all in the bottom phase. m    is   the average (hypothetical) molality in the
(0.677 H2O +0.323 C4H8O2 ) = 70 wt. % solvent.
D'                 m'               Afl3                      y±                    fi                 /2                  tn
	mol kg'1	RT		Pa	Pa	mol kg'1
43.8	1.8531	11.947	0.2897	1931	4193	0.922
47.8	1.2187	13.385	0.0403	2197	4074	0.632
52.9	0.8395	15.305	0.0102	2295	3993	0.4653
64.5	0.4086	19.920	0.00313	2411	3907	0.3061
70.0	0.2805	22.075	0.00071	2455	3963	-
70.0	0.2019	22.075	0.00096	2488	3857	-
70.0	0.1084	22.075	0.00176	2530	3743	-
70.0	0.0513	22.075	0.00503	2549	3636	-
In the mixed solvent (0.677 H2O+0.323C4H8O2) a binodal point on the phase diagram is almost reached at m( MgCl2 ) » 0.3 mol kg-1, corresponding to x3 » 0.01. At this composition a discontinuity in the Gibbs free energy occurs, see Figure 2.
469
H       5-	^ o				
CĆ.	O	o o			
co		o			
C			o	o	
-'~v      n					a
co      u					V
X					
,t					
y)         -5co				e •	
^	^"\	e	e		
^ -10-	ö e	e ^	•		
¦o					
<	••	• •			
-15-		i	¦       i		1             '
0.00
0.01
0.02
xo
0.03
0.04
FIGURE 2. DG/n^RT accompanying the dilution of two-phase system with an average composition of solvent (0.677H2O + 0.323C4H 8O2) saturated with Mg C lj_ , and solvent with the same composition. (O), the contribution of the solvent; (O) the contribution of the electrolyte; and (O) the total change. x3 is the mole fraction of MgCl2 in the solution.
Acknowledgement.    The authors wish to thank Professor Ivan Leban for X-ray measurements.
REFERENCES
[1]   H. F. Bogardus, C. C. Lynch, J. Phys. Chem. 1943, 47, 650-654.
[2]   A. N. Campbell, E. M. Kartzmark, W.B. Maryk, Can. J. Chem. 1966, 44, 935-937.
[3]   A. N. Campbell, S.Y. Lam, Can. J. Chem. 1972, 50, 3388-3390.
[4]   M. Bešter, D. Dolar, J. Chem. Thermodynamics 1991, 23, 809-816.
[5]   M. Bešter, D. Dolar, Acta Chim. Slov. 1997, 44/3, 289-301.
[6]   M. Bešter, D. Us, D. Dolar, Fizikalna kemija raztopin (C1-0517-103-90),
Poročilo o delu, Katedra za fizikalno kemijo FNT, Ljubljana, 1990, 52-57. [7]   J. C. Barnes, T. J. R.Weakley, J. Chem. Soc. Dalton Trans. 1972, 1786-1788. [8]   J. Taraszewska, M. Klon-Palczevska, D. Wyrzykowska-Stankiewicz,
Fluid Phase Equilibria 1979, 3,  13-22. [9]   F. G.J.Keyes, Chem. Phys. 1925, 15, 611-615. [10] A. L. Bacarella, A. Finch, E.J.Grunwald, Phys. Chem. 1956, 60, 573-576.
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POVZETEK
Dol očil i smo topnost in fazni diagram za sistem (voda+l,4-dioxan+MgCl      2). Iz podanih vrednosti parnih tlakov komponent v mešanem topilu in aktivnosti MgCl2 smo izračunal i Gibbsovo prosto energijo razredčenja nasičenega dvofaznega sistema pri konstantni sestavi topila.