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Determination of the activities in the solid solutions {xMnO + (1 − x)CaO} by a solid-electrolyte galvanic cell in the temperature range 900 K to 1100 K
M-2583 j. Chem. Thermodynanlics 1991, 23, 547 550
Determination of the activities in the solid solutions { x M n O + (1 - x ) C a O } by a solid-electrolyte galvanic cell in the temperature range 900 K to 1100 K G. ROG, A. KOZLOWSKA-ROG, K. ZAKULA, and W. PYCIOR Institute of Materials Engineering, Academy of Mining and Metallurgy, 30-059 KrakOw, Poland (Received 19 October 1990)
The activities of MnO in {xMnO + (l-x)CaO} solid solutions have been determined in the temperature range 900 K to 1100K by a solid-electrolytegalvanic cell. The activitycoefficients of MnO and CaO and the activitiesof CaO have been calculated. The solutions exhibit positive deviations from ideal behaviour. They can be regarded as regular solutions.
1. Introduction The mixture {xMnO + ( 1 - x ) C a O } has been reported to exhibit complete solid solubility at high temperatures, m The thermodynamic quantities published so far indicate nearly ideal behaviour of the solution. (1'2) However, accurate values of thermodynamic quantities are not yet available. The present paper deals with activity measurements of MnO in {xMnO + ( 1 - x ) C a O } solid solutions in the temperature range 900 K to 1100 K. A solid-electrolyte galvanic cell method was applied. Manganese-lY'-alumina was used as a solid electrolyte. This electrolyte belongs to the family of divalent-ion-exchanged sodium-13-aluminas/3~ Recently manganese-13"-alumina was applied successfully in the determination of the standard molar Gibbs free energies of formation of manganese silicates/~) The cell employed in the present investigation was of the type: PtlAr+ Ozl{xMnO +(1 -x)CaO}(ss)lMn 2+ + lY'-aluminaIMnOIAr+ O21Pt.
(1)
2. Experimental All the materials used were of reagent-grade purity. Manganese-lY'-alumina was prepared from MgO-stabilized sodium-lY'-alumina by ion exchange in molten MnC1 z. The preparation procedure was described in detail previously. (4) Manganese oxide was obtained from manganese sulphate by first preparing the oxalate and then decomposing the oxalate in air to black manganese oxide, the latter being reduced in hydrogen at 1270 K to obtain MnO. 0021 9614/91/060547+04 $02.00/0
© 1991 Academic Press Limited
ACTIVITIES IN {xMnO+(l-x)CaO}(s.s.)
548
The samples of ( x M n O + ( 1 - x ) C a O } solid solution were prepared in the following way: from calcium and manganese nitrates a mixture of calcium and manganese carbonates was precipitated. The preparation was carefully washed and then dried at 380 K. After calcination at 1070 K for 2 h the preparation was heated at 1270 K for 24 h. All the heating experiments were performed in an atmosphere of oxygen-free argon. Five solid solutions of different composition were obtained. The solid solutions were then analysed by the chemical wet method. For all the samples X-ray powder diagrams were performed. The lattice parameters calculated by us did not differ significantly from those determined by Schenck et al. ~1) Vergard's law appeared to be fulfilled for all compositions of the solid solution. The solid solution as well as pure M n O powders were pressed into pellets 2 mm thick and 10 mm in diameter. The half-cells and solid-electrolyte pellets were assembled in a simple springloaded alumina holder according to the galvanic-cell arrangement. The cell was placed in the furnace and heated to the measurement temperature. The e.m.f.s were monitored with a digital voltmeter (Unitra 1321, internal resistance 10 l° f~). Five independent series of e.m.f, measurements were performed for each composition of the solid solution in the temperature range 900 K to 1100 K. A purified-argon flow was passed through the furnace during the e.m.f, measurements. The partial pressure of oxygen in the argon gas did not exceed the equilibrium oxygen partial pressure for MnO. The time taken for the cell to attain equilibrium was no longer than 0.5 h.
3. Results and discussion The overall cell reaction may be represented by MnO(s) = MnO(ss),
(2)
where ss denotes a solid solution. The work of the cell consists, then, in transferring MnO to the solid solution. The experimental values of the e.m.f. E measured for five compositions of solution, changed linearly with temperature. The equations of the least-squares regression lines are given in table 1. From the values of e.m.f, the activity a(MnO) of MnO and the activity coefficient f(MnO) for given x and temperature were calculated: a(MnO) = f ( M n O ) , x = e x p ( - 2 F E / R T ) ,
(3)
where F is Faraday constant. Knowing the activity coefficient f(MnO) as a function of x, the values of the activity coefficient f(CaO) for all compositions investigated TABLE 1. Dependence of the e.m.f, on temperature at different compositions of { x M n O + (1 -x)CaO} solid solution: E / m V = a + b . T / K x
a
102"b
0.10 0.24 0.32 0.50 0.66
68.3±1.9 --48.5±1.3 38.6±1.5 --20.6±1.3 --9.5±1.0
9.93±0.24 6.15±0.16 4.93±0.15 2.96±0.13 1.77±0.11
549
G. ROG, A. K O Z L O W S K A - R O G , K. Z A K U L A , A N D W. PYCIOR 1.Or~
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F I G U R E 1. Activities of M n O and CaO in {xMnO - - - , Ideal solution; O, MnO; O, CaO.
0.6 + (l-x)CaO}
solid solutions at 1100 K.
were d e t e r m i n e d by the m e t h o d described in reference 5. T h e f o l l o w i n g e q u a t i o n was derived: l n { f ( C a O ) } = - c~(MnO) • (1 - x ) -
I
(1 x)
(4)
c~(MnO) • d(l - x),
do
TABLE 2. Values of the activity coefficients of M n O and CaO in { x M n O + (1 - x ) C a O } solid solution at temperatures 900 K, 1000 K, and 1100 K
900 K 0.10 0.24 0.32 0.50 0.66
5.80 3.49 2.69 1.72 1.28
f(MnO) 1000 K 4.86 3.08 2.44 1.63 1.26
1100 K
900 K
f(CaO) 1000 K
1100 K
4.21 2.78 2.25 1.56 1.23
1.02 1.13 1.25 1.73 2.62
1.02 1.12 1.22 1.64 2.38
1.01 1.11 1.20 1.55 2.21
550
ACTIVITIES IN {xMnO+(1-x)CaO}(s.s.) 14
I
I
I
I
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__10
g,
2
0
J
0.2
0.4 0.6 (l-x) 2
I
0.8
FIGURE 2. RT'ln{J~MnO)} against (l--x) z for {xMnO + (1 x)CaO} solid solutions at 1100 K. where ~(MnO) = ln{f(MnO}/(1 - x) 2.
(5)
Then, the activity a(CaO) of C a O was calculated: a(CaO) = J f C a O ) . (1 - x).
(6)
The results obtained for f ( M n O ) and f ( C a O ) are given in table 2 for temperatures of 900 K, 1000 K, and 1100 K. The activities of M n O and C a O in the solid solution as functions of composition at 1100 K are shown in figure 1. Similar plots were also obtained at other temperatures. As can be seen in figure 1 the { x M n O + ( 1 - x ) C a O } solid solutions show significant positive deviations from Raoult's law. In figure 2 a plot of R T . ln{f(MnO)} against ( 1 - x ) z at 1100 K is presented. The plot is a straight line, which indicates regular behaviour of the solutions under study. Similar plots were obtained for other temperatures. The strong positive deviations from ideality could be related to a tendency towards immiscibility in the solid solution at lower temperatures. REFERENCES 1. 2. 3. 4. 5.
Schenck, H.; Frohberg, M. G.; Nfinnighoff, R. Archiv Eisenhiittenwes. 1964, 35, 269. Driessens, F. C. M. Ber. Bunsenges. phys. Chem. 1968, 72, 764. Farrington, G. C.; Dunn, B. Solid State Ionic's 1982, 7, 267. R6g, G.; Pycior, W. J. Chem. Thermodynamics 1987, 19, 381. Darken, L. S.~ Gurry, R. W. Physical Chemist13' gfMetals. McGraw-Hill: New York, 1972, pp. 264 to 266.
Determination of the activities in the solid solutions { x M n O + (1 - x ) C a O } by a solid-electrolyte galvanic cell in the temperature range 900 K to 1100 K G. ROG, A. KOZLOWSKA-ROG, K. ZAKULA, and W. PYCIOR Institute of Materials Engineering, Academy of Mining and Metallurgy, 30-059 KrakOw, Poland (Received 19 October 1990)
The activities of MnO in {xMnO + (l-x)CaO} solid solutions have been determined in the temperature range 900 K to 1100K by a solid-electrolytegalvanic cell. The activitycoefficients of MnO and CaO and the activitiesof CaO have been calculated. The solutions exhibit positive deviations from ideal behaviour. They can be regarded as regular solutions.
1. Introduction The mixture {xMnO + ( 1 - x ) C a O } has been reported to exhibit complete solid solubility at high temperatures, m The thermodynamic quantities published so far indicate nearly ideal behaviour of the solution. (1'2) However, accurate values of thermodynamic quantities are not yet available. The present paper deals with activity measurements of MnO in {xMnO + ( 1 - x ) C a O } solid solutions in the temperature range 900 K to 1100 K. A solid-electrolyte galvanic cell method was applied. Manganese-lY'-alumina was used as a solid electrolyte. This electrolyte belongs to the family of divalent-ion-exchanged sodium-13-aluminas/3~ Recently manganese-13"-alumina was applied successfully in the determination of the standard molar Gibbs free energies of formation of manganese silicates/~) The cell employed in the present investigation was of the type: PtlAr+ Ozl{xMnO +(1 -x)CaO}(ss)lMn 2+ + lY'-aluminaIMnOIAr+ O21Pt.
(1)
2. Experimental All the materials used were of reagent-grade purity. Manganese-lY'-alumina was prepared from MgO-stabilized sodium-lY'-alumina by ion exchange in molten MnC1 z. The preparation procedure was described in detail previously. (4) Manganese oxide was obtained from manganese sulphate by first preparing the oxalate and then decomposing the oxalate in air to black manganese oxide, the latter being reduced in hydrogen at 1270 K to obtain MnO. 0021 9614/91/060547+04 $02.00/0
© 1991 Academic Press Limited
ACTIVITIES IN {xMnO+(l-x)CaO}(s.s.)
548
The samples of ( x M n O + ( 1 - x ) C a O } solid solution were prepared in the following way: from calcium and manganese nitrates a mixture of calcium and manganese carbonates was precipitated. The preparation was carefully washed and then dried at 380 K. After calcination at 1070 K for 2 h the preparation was heated at 1270 K for 24 h. All the heating experiments were performed in an atmosphere of oxygen-free argon. Five solid solutions of different composition were obtained. The solid solutions were then analysed by the chemical wet method. For all the samples X-ray powder diagrams were performed. The lattice parameters calculated by us did not differ significantly from those determined by Schenck et al. ~1) Vergard's law appeared to be fulfilled for all compositions of the solid solution. The solid solution as well as pure M n O powders were pressed into pellets 2 mm thick and 10 mm in diameter. The half-cells and solid-electrolyte pellets were assembled in a simple springloaded alumina holder according to the galvanic-cell arrangement. The cell was placed in the furnace and heated to the measurement temperature. The e.m.f.s were monitored with a digital voltmeter (Unitra 1321, internal resistance 10 l° f~). Five independent series of e.m.f, measurements were performed for each composition of the solid solution in the temperature range 900 K to 1100 K. A purified-argon flow was passed through the furnace during the e.m.f, measurements. The partial pressure of oxygen in the argon gas did not exceed the equilibrium oxygen partial pressure for MnO. The time taken for the cell to attain equilibrium was no longer than 0.5 h.
3. Results and discussion The overall cell reaction may be represented by MnO(s) = MnO(ss),
(2)
where ss denotes a solid solution. The work of the cell consists, then, in transferring MnO to the solid solution. The experimental values of the e.m.f. E measured for five compositions of solution, changed linearly with temperature. The equations of the least-squares regression lines are given in table 1. From the values of e.m.f, the activity a(MnO) of MnO and the activity coefficient f(MnO) for given x and temperature were calculated: a(MnO) = f ( M n O ) , x = e x p ( - 2 F E / R T ) ,
(3)
where F is Faraday constant. Knowing the activity coefficient f(MnO) as a function of x, the values of the activity coefficient f(CaO) for all compositions investigated TABLE 1. Dependence of the e.m.f, on temperature at different compositions of { x M n O + (1 -x)CaO} solid solution: E / m V = a + b . T / K x
a
102"b
0.10 0.24 0.32 0.50 0.66
68.3±1.9 --48.5±1.3 38.6±1.5 --20.6±1.3 --9.5±1.0
9.93±0.24 6.15±0.16 4.93±0.15 2.96±0.13 1.77±0.11
549
G. ROG, A. K O Z L O W S K A - R O G , K. Z A K U L A , A N D W. PYCIOR 1.Or~
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F I G U R E 1. Activities of M n O and CaO in {xMnO - - - , Ideal solution; O, MnO; O, CaO.
0.6 + (l-x)CaO}
solid solutions at 1100 K.
were d e t e r m i n e d by the m e t h o d described in reference 5. T h e f o l l o w i n g e q u a t i o n was derived: l n { f ( C a O ) } = - c~(MnO) • (1 - x ) -
I
(1 x)
(4)
c~(MnO) • d(l - x),
do
TABLE 2. Values of the activity coefficients of M n O and CaO in { x M n O + (1 - x ) C a O } solid solution at temperatures 900 K, 1000 K, and 1100 K
900 K 0.10 0.24 0.32 0.50 0.66
5.80 3.49 2.69 1.72 1.28
f(MnO) 1000 K 4.86 3.08 2.44 1.63 1.26
1100 K
900 K
f(CaO) 1000 K
1100 K
4.21 2.78 2.25 1.56 1.23
1.02 1.13 1.25 1.73 2.62
1.02 1.12 1.22 1.64 2.38
1.01 1.11 1.20 1.55 2.21
550
ACTIVITIES IN {xMnO+(1-x)CaO}(s.s.) 14
I
I
I
I
I
I
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g,
2
0
J
0.2
0.4 0.6 (l-x) 2
I
0.8
FIGURE 2. RT'ln{J~MnO)} against (l--x) z for {xMnO + (1 x)CaO} solid solutions at 1100 K. where ~(MnO) = ln{f(MnO}/(1 - x) 2.
(5)
Then, the activity a(CaO) of C a O was calculated: a(CaO) = J f C a O ) . (1 - x).
(6)
The results obtained for f ( M n O ) and f ( C a O ) are given in table 2 for temperatures of 900 K, 1000 K, and 1100 K. The activities of M n O and C a O in the solid solution as functions of composition at 1100 K are shown in figure 1. Similar plots were also obtained at other temperatures. As can be seen in figure 1 the { x M n O + ( 1 - x ) C a O } solid solutions show significant positive deviations from Raoult's law. In figure 2 a plot of R T . ln{f(MnO)} against ( 1 - x ) z at 1100 K is presented. The plot is a straight line, which indicates regular behaviour of the solutions under study. Similar plots were obtained for other temperatures. The strong positive deviations from ideality could be related to a tendency towards immiscibility in the solid solution at lower temperatures. REFERENCES 1. 2. 3. 4. 5.
Schenck, H.; Frohberg, M. G.; Nfinnighoff, R. Archiv Eisenhiittenwes. 1964, 35, 269. Driessens, F. C. M. Ber. Bunsenges. phys. Chem. 1968, 72, 764. Farrington, G. C.; Dunn, B. Solid State Ionic's 1982, 7, 267. R6g, G.; Pycior, W. J. Chem. Thermodynamics 1987, 19, 381. Darken, L. S.~ Gurry, R. W. Physical Chemist13' gfMetals. McGraw-Hill: New York, 1972, pp. 264 to 266.