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Potentiometric study of molten AgI + KI mixtures
Ekctrochimica
Acta. 1966, VoI. 11 pp. 385 fo 387.
Pergamon Press Ltd.
Prinled in Northern Ir&md
POTE~TIO~ET~C STUDY OF MOLTEN A@ + RI MIXTU~S* S. STERNBERG, I. ADORIAN and I. GALASIU Polytechnical Institute, Dept of Physical Chemistry, Bucharest, Rumania Abstract-The
emf of the cell Ag(s) 1A@(l) + IWO 1I&, 1 am-d,C
Xl
-%
has been measured as a function of temperature and molar fraction. The the~~~~ic excess properties of the components of the molten mixture AgI + KI are calculated. The mixture shows negative deviations from ideal behaviour. Comparison with the chloride and bromide systems is made. fern de la cellule Ag(s) 1AgI(1) f K.I(l) 1Ia(g, 1 atm), C a ttC mesurCe en fonction de la X1 % d’ex&s des tempbature et de la fraction molaire. On en dMuit les propri&tb thermodynamiques mdlanges fondus AgI + KI: les dkviations sont ntgatives par rapport au comportement id&l. Comparaisons avec les syst&mes chlorures et bromures.
R&umLLa
Zusammenfassung-Es
wurde die EMK der Zelle
AgWAgW Xl
+ JW/Mg, x2
1 atd, C
in Funktion von Konzentrationen (x1, xX) und Temperatur gemessen. Die Abweichungen der thermodynamischen Grassen der Komponenten in der geschmolzenen Mischung AgI + KI warden errechnet. Die Mischung zeigt negative Abweichungen vom Idealverhalten. Vergleiche mit dem Chlorid- und Bromid-System werden angestellt.
activities in binary mixtures of molten halides can be determined by means of the galvanic cell method. This method, elaborated by Hildebrand and coworker9 has in the main been successfully applied to binary mixtures of silver or lead halides with alkali or alkaline-earth halides.l-7 However, onIy bromides and chlorides have been studied. To extend this study to mixtures of silver or lead iodide with other iodides, a reversible iodine electrode had to be achieved,* on the basis of the cell THERMODYNAMIC
A&) IAgWl 12(g,1 at@,C. A study of the thermodynamic presented, the emf of the cell
properties
of the AgI + KI mixture is now
A&) IAgW + KW I,@, 1 at@,C Xl
*2
* Presented at the 15th Meeting of CITCE, 18 December 1964.
London,
385
September
1964; manuscript
received
386
S. STERNBERG,I. AD~RIAN and I. GALASILJ
being measured ; x1 and x2 represent the molar fractions. The thermodynamic activity of AgI has been calculated on the basis of Nernst’s equation, E1 = E, - glna,, and the partial molar excess free energy by means of the equation GIE = RTln yl.
The activity coefficients yIiI of KI have been calculated by means of the GibbsDuhem equation x1x log yz = _’ d log yr. s 0 x2 EXPERIMENTAL
TECHNIQUE
To realize the reversible iodine electrode, the cell contains iodine in the gaseous state, from the iodine generator up to its evacuation from the cell. Pyrex-type glass is used. From an iodine-filled container maintained at 23O”C, iodine is introduced into and evacuated from the cell by electrically heated glass tubes. The cathode consists of a 3-mm diameter silver wire; the anode is a graphite rod. The emf of the cell is measured conventially, to &to-l mV. For reproducible emf, it is necessary to saturate the graphite electrode with iodine; preliminary experiments showed that after about 20 h of gaseous iodine bubbling, the emf is constant and reproducible to f l-5 mV. Details are given elsewhere.” RESULTS The measured emf values are given in Table 1, together with the activities, the activity coefficients and the excess partial molar free energy of the AgI and KI It may be noted that this mixture, like the AgCl + KC15 and AgBr + KBr2 mixtures, shows negative deviations from ideal behaviour. In Fig. 1 the values of the partial molar excess free energy of the three mixtures as a function of xS2 are given. The fact that the negative deviations of the AgI + KI mixture are slightly larger than those of the chloride and bromide mixtures may be explained by the tendency of this mixture to form complexes; the phase diagram shows that a compound is formed with TABLE 1. MEASURED
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0.550 0559 0.573 0:5885 0.605 0.622 0.645 0.672 -
V 0.250 x 1O-s (T 0.245 x lo-’ (T 0.235 x lo-’ (T 0.225 x lo-* (T 0.210 x lo-’ (T 0*190 x lo-” (T 0.160 x lo-’ (T 0.140 x lo-’ (T -
EMF AND CALCULATED THERMODYNAMIC AgI + m MIXTURES
873) 873) 873) 873) 873) 873) 873) 873)
1 0.887 0.736 0.599 0.481 0.384 0.282 0.197 -
1 0.985 0.920 0.856 0.802 0.767 0.706 0.657 -
cal/mole 0 -25 -144 -270 -384 -460 -604 -729
-
VALUES,
Cal/mole 0.1 0.2 0.3 0.4 0.5
0.380 0.576 0.720 0.811 b.855
;:; 0.8 0.9 1.0
0.950 0901 0.990 0.998 1
-1678 -959 -571 -364 -271 -181 -88 -17 -3 0
Potentiometric study of molten AgI + KI mixtures
387
an incongruent melting point for xABI= O-77, whereas the AgCl + KC1 and AgBr + KBr mixtures yield on solidification simple eutectic mixtures.g The AgI + KI mixture shows deviations from the regular solution equation (Gr” = BXS2), as may be seen in Fig. 1. The results show that the iodine gas electrode can be used to determine the thermodynamic activities of the components in iodide mixtures, as easily as the chloride and -1000
- 800
z
[
E 1 x
-600 -
-
-4oo-
%
x AgBrtKBr o AgCItKCl
x’2 FIG. 1
bromide activities previously studied. Of particular interest is a study of the reciprocal mixtures of the type Me,1 + MerrX, where X represents the chloride or bromide anion. The thermodynamic activity of the Me,1 component may be determined by means of the cell Mer IMerI(1) + MerrX(l)l 12(g, 1 aW, C, since the potential difference between I,II- and the other halogen half-cells is very large, the equilibrium MerrX + &I2+ MerJ + &X2 being virtually entirely to the left. Investigations of this kind are now being carried out in our laboratory. REFERENCES 1. J. H. HILDEBRAND and G. RUHLE,J. Am. them. Sot. 49,722 (1927). 2. J. H. HILD~RAND and E. SALSTROM, J. Am. them. Sot. 54,4257 (1932). 3. M. F. LANTRATOFF and A. F. ALABISCHEFF, Zh.prikl. Khim., Mask. 27,723 (1954). 4. B. F. MARKOV,I. K. DELIMARSCHI and I. D. PANTACHENKO, Zh. fir. Khim. 29,Sl (1955). 5. I. G. MURGULESCU and S. STERNBERG, Rev. Chim. Acad. R.P.R. 2,251 (1957); 3,55 (1958). 6. I. G. MURGULESCU and D. MARCXUDAN,Rev. Chim. Acad. R.P.R. 3,47 (1958). 7. S. STTRNBERG and S. GHEORGHIU,Srudii Cert. Chim. 7, 107 (1959). 8. S. STERNBERG, I. ADORIANand I. GALASIU,J. Chim. phys., to he published. 9. N. S. DOMBROVSKAIA and Z. A. KOLOSKOVA, Zzv. SFHA 22,178 (1953).
Acta. 1966, VoI. 11 pp. 385 fo 387.
Pergamon Press Ltd.
Prinled in Northern Ir&md
POTE~TIO~ET~C STUDY OF MOLTEN A@ + RI MIXTU~S* S. STERNBERG, I. ADORIAN and I. GALASIU Polytechnical Institute, Dept of Physical Chemistry, Bucharest, Rumania Abstract-The
emf of the cell Ag(s) 1A@(l) + IWO 1I&, 1 am-d,C
Xl
-%
has been measured as a function of temperature and molar fraction. The the~~~~ic excess properties of the components of the molten mixture AgI + KI are calculated. The mixture shows negative deviations from ideal behaviour. Comparison with the chloride and bromide systems is made. fern de la cellule Ag(s) 1AgI(1) f K.I(l) 1Ia(g, 1 atm), C a ttC mesurCe en fonction de la X1 % d’ex&s des tempbature et de la fraction molaire. On en dMuit les propri&tb thermodynamiques mdlanges fondus AgI + KI: les dkviations sont ntgatives par rapport au comportement id&l. Comparaisons avec les syst&mes chlorures et bromures.
R&umLLa
Zusammenfassung-Es
wurde die EMK der Zelle
AgWAgW Xl
+ JW/Mg, x2
1 atd, C
in Funktion von Konzentrationen (x1, xX) und Temperatur gemessen. Die Abweichungen der thermodynamischen Grassen der Komponenten in der geschmolzenen Mischung AgI + KI warden errechnet. Die Mischung zeigt negative Abweichungen vom Idealverhalten. Vergleiche mit dem Chlorid- und Bromid-System werden angestellt.
activities in binary mixtures of molten halides can be determined by means of the galvanic cell method. This method, elaborated by Hildebrand and coworker9 has in the main been successfully applied to binary mixtures of silver or lead halides with alkali or alkaline-earth halides.l-7 However, onIy bromides and chlorides have been studied. To extend this study to mixtures of silver or lead iodide with other iodides, a reversible iodine electrode had to be achieved,* on the basis of the cell THERMODYNAMIC
A&) IAgWl 12(g,1 at@,C. A study of the thermodynamic presented, the emf of the cell
properties
of the AgI + KI mixture is now
A&) IAgW + KW I,@, 1 at@,C Xl
*2
* Presented at the 15th Meeting of CITCE, 18 December 1964.
London,
385
September
1964; manuscript
received
386
S. STERNBERG,I. AD~RIAN and I. GALASILJ
being measured ; x1 and x2 represent the molar fractions. The thermodynamic activity of AgI has been calculated on the basis of Nernst’s equation, E1 = E, - glna,, and the partial molar excess free energy by means of the equation GIE = RTln yl.
The activity coefficients yIiI of KI have been calculated by means of the GibbsDuhem equation x1x log yz = _’ d log yr. s 0 x2 EXPERIMENTAL
TECHNIQUE
To realize the reversible iodine electrode, the cell contains iodine in the gaseous state, from the iodine generator up to its evacuation from the cell. Pyrex-type glass is used. From an iodine-filled container maintained at 23O”C, iodine is introduced into and evacuated from the cell by electrically heated glass tubes. The cathode consists of a 3-mm diameter silver wire; the anode is a graphite rod. The emf of the cell is measured conventially, to &to-l mV. For reproducible emf, it is necessary to saturate the graphite electrode with iodine; preliminary experiments showed that after about 20 h of gaseous iodine bubbling, the emf is constant and reproducible to f l-5 mV. Details are given elsewhere.” RESULTS The measured emf values are given in Table 1, together with the activities, the activity coefficients and the excess partial molar free energy of the AgI and KI It may be noted that this mixture, like the AgCl + KC15 and AgBr + KBr2 mixtures, shows negative deviations from ideal behaviour. In Fig. 1 the values of the partial molar excess free energy of the three mixtures as a function of xS2 are given. The fact that the negative deviations of the AgI + KI mixture are slightly larger than those of the chloride and bromide mixtures may be explained by the tendency of this mixture to form complexes; the phase diagram shows that a compound is formed with TABLE 1. MEASURED
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0.550 0559 0.573 0:5885 0.605 0.622 0.645 0.672 -
V 0.250 x 1O-s (T 0.245 x lo-’ (T 0.235 x lo-’ (T 0.225 x lo-* (T 0.210 x lo-’ (T 0*190 x lo-” (T 0.160 x lo-’ (T 0.140 x lo-’ (T -
EMF AND CALCULATED THERMODYNAMIC AgI + m MIXTURES
873) 873) 873) 873) 873) 873) 873) 873)
1 0.887 0.736 0.599 0.481 0.384 0.282 0.197 -
1 0.985 0.920 0.856 0.802 0.767 0.706 0.657 -
cal/mole 0 -25 -144 -270 -384 -460 -604 -729
-
VALUES,
Cal/mole 0.1 0.2 0.3 0.4 0.5
0.380 0.576 0.720 0.811 b.855
;:; 0.8 0.9 1.0
0.950 0901 0.990 0.998 1
-1678 -959 -571 -364 -271 -181 -88 -17 -3 0
Potentiometric study of molten AgI + KI mixtures
387
an incongruent melting point for xABI= O-77, whereas the AgCl + KC1 and AgBr + KBr mixtures yield on solidification simple eutectic mixtures.g The AgI + KI mixture shows deviations from the regular solution equation (Gr” = BXS2), as may be seen in Fig. 1. The results show that the iodine gas electrode can be used to determine the thermodynamic activities of the components in iodide mixtures, as easily as the chloride and -1000
- 800
z
[
E 1 x
-600 -
-
-4oo-
%
x AgBrtKBr o AgCItKCl
x’2 FIG. 1
bromide activities previously studied. Of particular interest is a study of the reciprocal mixtures of the type Me,1 + MerrX, where X represents the chloride or bromide anion. The thermodynamic activity of the Me,1 component may be determined by means of the cell Mer IMerI(1) + MerrX(l)l 12(g, 1 aW, C, since the potential difference between I,II- and the other halogen half-cells is very large, the equilibrium MerrX + &I2+ MerJ + &X2 being virtually entirely to the left. Investigations of this kind are now being carried out in our laboratory. REFERENCES 1. J. H. HILDEBRAND and G. RUHLE,J. Am. them. Sot. 49,722 (1927). 2. J. H. HILD~RAND and E. SALSTROM, J. Am. them. Sot. 54,4257 (1932). 3. M. F. LANTRATOFF and A. F. ALABISCHEFF, Zh.prikl. Khim., Mask. 27,723 (1954). 4. B. F. MARKOV,I. K. DELIMARSCHI and I. D. PANTACHENKO, Zh. fir. Khim. 29,Sl (1955). 5. I. G. MURGULESCU and S. STERNBERG, Rev. Chim. Acad. R.P.R. 2,251 (1957); 3,55 (1958). 6. I. G. MURGULESCU and D. MARCXUDAN,Rev. Chim. Acad. R.P.R. 3,47 (1958). 7. S. STTRNBERG and S. GHEORGHIU,Srudii Cert. Chim. 7, 107 (1959). 8. S. STERNBERG, I. ADORIANand I. GALASIU,J. Chim. phys., to he published. 9. N. S. DOMBROVSKAIA and Z. A. KOLOSKOVA, Zzv. SFHA 22,178 (1953).