- Email: [email protected]
Conductivities of calcium chloride and calcium chloride-sodium chloride solutions saturated in calcium carbide and unsaturated in calcium oxide
CONDUCTIVITIES OF CALCIUM CHLORIDE AND CALCIUM CHLORTDE-SODIUM CHLORIDE SOLUTIONS SATURATED IN CALCIUM CARBIDE AND UNSATURATED IN CALCIUM OXIDE C. GENTAZ, M. PARODI and A. BONOMI Battelle, Geneva Research Center, 7, route de Drize, 1227 Carouge, Switzerland (Received
19 December
1974; rrvisrd uarsion 13 June 1975)
Abstract-The specific conductivity variation of the named solutions DS temperature in the range 750-950°C and vs the sodium chloride molar fractions have been measured. The specific conductivity increases sharply with increasing sodium chloride M fraction and reaches a maximum when the M fraction ratio of sodium chloride to calcium carbide is nearly two. This experimental result has been interpreted by assuming interactions between calcium carbide and sodium chloride, and constitutes a verification of a previous thermodynamic result obtained by the same authors.
1. INTRODUCTION
cell is made of two, pure iron, cylindrical electrodes each placed in a hollow alumina tube (Degussa Al 23) 5 mm dia and 35 mm longer than the iron electrode in order to lengthen the effective cell length and hence increase the resistance. The resistance was measured with a conventional ac bridge fed by a sinusoidal oscillator of 1000 Hz frequency in order to avoid polarization of the electrades. This frequency has been chosen since the conductivity is independent of frequency within the range 500 and 5000 Hz. ductivity
In 1962, Barber and Sloan[l] showed that technical grade calcium carbide was soluble to some extent in alkaline-earth and lithium halides. On the basis of these results various authors studied the properties of molten salt solutions of calcium carbide. Investigation methods have mostly been electrochemical (chronopotentiometry, galvanic cells)[2-51. The results confirm that calcium carbide dissolved in molten salt probably gives as a dissociated product, C:- anionic species, which is readily oxidizable either chemically or electrochemically. In other investigations dealing aspects of calcium chloride with some thermodynamic and calcium chloride+odium chloride-technical grade calcium carbide solutions[6-71, preferential interactions between the sodium cation and carbide anion C$- was demonstrated; these solutions exhibited a significant variation in the thermodynamic excess functions. The purpose of the present study was to verify, by measuring the specific conductivity, the hypothesis concerning the ionic interactions.
2 EXPERIMENTAL 2.1. Experimental
2.3. Conductivity
measurements
350 g of powdered analytical grade calcium chloride, compacted in 50 g pellets in order to reduce the dead space, was placed iu the crucible and melted under argon. The temperature was held at 820°C and the molten salt dehydrated under vacuum for about 2 h. Technical grade calcium carbide, about X0”/, in weight of CaC,, was then added, still under argon, and the conductivity cell introduced. Conductivity measurements were made for each increase in temperature. After measurements, the temperature was lowered to 800°C. Then analytical grade sodium chloride was added, under an argon atmosphere, by SUG cessive additions through the valve; the conductivity cell was still immersed in the molten salt. As soon as the sodium chloride was added, measurements were made under a static argon atmosphere, as it has been shown that argon sweeping and, u fortiori, a primary vacuum favour sodium metal in the CaC2CaCl,-NaCl-CaO system; conductivity measurements were again made for each increase in temperature.
PROCEDURE
technique
The two-capillary dip cell[8] has been chosen since saturated solutions with calcium carbide in excess necessarily contained solid suspensions (carbon, uadissolved calcium carbide, etc.) which can interfere with the conductivity measurements. The disadvantage of this type of cell is the parasitic conductance through the capillary walls. This defect cannot be overcome as there is no material presenting both electrical insulation and chemical inertness in the molten salt solutions studied.
2.4. Determination of the cell constant-standardization A standardization in molten KC1 gave 32.6 cn- ’ for the cell constants. It was verified that the positioning of the cell in the alumina crucible did not modify the cell constant if the lower ends of the alumina pipes were more than I cm away from the bottom of the alumina crucible: at less than 1 cm the cell
2.2. Apparafus The molten salt is contained in an alumina crucible which is placed in an inconel cell. This assembly is covered by a controlled argon atmosphere. The con93
C. GENTAZ, M. PARQDI and A. BONOMI
94
constant increased showing that the current distribution between the two alumina pipes had changed. It was also shown that the current distribution was affected by the temperature and hence by the conductivity. The cell constant varied according to the temperature and standardization was necessary. Finally, it was noted that the cell constant varied significantly only after 16 h of experiments; -this variation was mainly due to a parasitic conductance through the alumina wall pipes, a carbon deposit being observed on the external alumina walls. Thus, alumina is not chemically inert vis-ci-vis the calcium carbide dissolved in molten salts. For these reasons, cells were discarded after 8 h use. 2.5. Chemicul analysis The calcium carbide and calcium oxide concentration were measured at the beginning and at the end of an experiment. Calcium carbide analysis The analytical method is the gas-volumetric procedure previously employed[6]. Calcium oxide analysis Calcium oxide is the only strong base present in the bath. The bath samples were dissolved in water, the dissolution being enhanced by the addition of a standard solution of hydrochloric acid in excess, and the calcium oxide was determined by a back-titration with a standard solution of sodium hydroxide. The measured value corresponds to the total calcium oxide and the equivalent calcium oxide, corresponding to the calcium carbide initially present in the sample, must be subtracted in order to obtain the quantity of the calcium oxide dissolved in the bath. 3.RF.SULTS The conductivity measurements were made on bin-
ary calcium chloride-sodium chloride in which the M fraction of sodium chloride varied between 0+6 inclusive. For each composition and temperature, the molten salt was saturated with calcium carbide and the experiments carried out under a static argon pressure of one atmosphere. It has been verified that the calcium carbide concentrations corresponding to saturated solutions followed the laws previously determined[6]. The nominal concentration of calcium oxide was the same in all experiments. However, it decreased from the initial 4*?++5% by wt to 3.&3-l% at the end of the experiment. This decrease is entirely due to the dilution caused by the introduction of sodium chloride. The number of calcium oxide moles, about 0.2 mole, was kept constant during the experiment. However, at the end of every series of experiments, a decrease of the calcium carbide concentration was noticed. As the number of moles of calcium oxide remained constant, it was concluded that the calcium carbide was consumed by chemical reaction with the sodium chloride leading to the formation of sodium metal which has been characterized by analysis of bath samples. The average sodium concentration in the bath was 0.3% in wt (about 0.05 mole or 1% M fraction}. In the following calculations, on the systems for
!C A
Fig. 1. CaSCaCzPCaC1, specific conductivity us temperature (nCa0 2 0.2 mole). 0, CaCI, pure (9); x, CaC1,-CaC,PCaO. which the sodium chloride M fraction varies between (M.5. the sodium chloride, calcium chloride and calcium carbide M fractions have been defined by the initial state; this is, however, inaccurate due to the finite displacement of the equilibrium towards sodium metal production. Only for a sodium chloride M fraction equivalent to 0.6, of the above mentioned M fractions, has the calculation been with reference to the final state of the systems. A DISCUSSION
4.1. System CaGCaC,-CaCll The specific conductivity of the system CaO-CaCzCaCl* is slightly less below 900°C and slightly more above 900°C (Fig. 1) than for calcium chloride[9]. The experimental points were adjusted according to the method of least squares to an equation of the form: y=a+bt the result of which was: y (W’. cm-‘) = - 2.314 + 5-401 10-j
t(“C).
Even with a margin of error the straight lines, giving the specific conductivities of pure calcium chloride and calcium chloride-calcium oxide-calcium carbide mixture, intersect around 900°C. It is expected that the mobility of the dicarbide ion, a complex ion of ionic radii (10) near that of a chloride ionrl 11.would be less than that of chloride ion. This would justify the fact that the specific conductivity of the system CaO-CaC,-CaCl, is slightly less than that of pure calcium chloride at least for temperatures of below 900°C. On the other hand, above 9OO”C, a new conduction mechanism must be considered to explain the higher conductivity of the system CaOCaC,-CaCl,. According to Aksanaran et al.[12] calcium carbide can dissociate to calcium metal and carbon. The calcium will give rise to electronic conductivity. L
_/
4.2. System CaO-CaC,-CaCl,-NaC1 The introduction of sodium chloride leads to a noticeable increase of the specific conductivity of the solution, about 50% (Fig. 2); the specific conductivity of the calcium chloridt+sodium chloride binary[13] has been shown for comparison. Finally, as in a pre-
Conductivities in calcium carbide and calcium oxide
0
0.2
XNaCIdared lo
0.L the binary
0.6
NoCLCaCt2
Fig. 2. Ca@CaC,-CaCl,-NaCI specific conductivity IIS sodium chloride molar fraction for various temperatures. ~30 = P2 mole + XCaC, between O-055and 0.1069 (see Ref. 13.
v;ous study[77], the variation of the specific conductivity of the solution us the M fraction ratio XY.&XCnCI is shown in Fig. 3, which also shows the SPCC~~~C conductivity of sodium-sodium chloride[l4] and calciumlcium chloride systems[151 vs M fraction of metal. Two remarkable results appear, one being the increase of the specific conductivity in the presence of sodium chloride, the other the existence of a maximum conductivity for the ratio XNac,/XcaC2of about two. The only possible hypothesis remaining after careful examination of the various alternatives to explain these two facts is that based upon a sodium chloride-calcium carbide interaction. When sodium chloride is introduced, a substitution of calcium ion by sodium ions can take place according to: CaZ+ J+
2Na+ 4 Na’ -C
95
= C- Na+ + Ca2+.
(1)
Thus the specific conductivity reaches a maximum when all the calcium ions are substituted by sodium ions. This would explain the observed specific conductivity maximum when the ratio XNnCI/Xcnc2is around two corresponding to the stoichiometry of thk reaction : CaC, + 2NaC1+NazCz + CaCl,. (2)
0
2
4
=eg t 6
Fig. 3. Caa CaC,-CaCl,-NaC1 specific conductivity US the ratlo p = [(X,,,, ,)/X(,, z] for various temperatures.
Values obtained are about 10m5 and are characteristic of a strong interaction. However, owing to the inaccuracy of the thermodynamic data, this result only gives an indication of interactions between sodium and dicarbide ions. Furthermore, the explanation of the observed phenomena by the presence of sodium metal is incomplete, as specific conductivity of the sodium-sodium chloride system only varies by 20”/, in the range @-1% sodium M fraction[14] and as specific conductivities higher than 7E I cm-’ were obtained when the system was under a primary vacuum at 900°C. Finally, even if the interaction between sodium chloride and calcium carbide explains the specific conductivity maximum, it does not explain the high values of conductivity. A further mechanism could be proposed which would involve electronic exchange, either between sodium metal and sodium ion, or between carbon and dicarbide ions. Unfortunately, the lack of data has not allowed the application of the models already used by Grantham and Yosim[lS] to explain electronic conductivity of the CuCILCuCI, system. 5. CONCLIJSlON
In spite of the inherent difficulty in obtaining accurate measurements on unstable systems such as CaOCaC,-CaCl,-NaC1, it appears that the addition of When the sodium chloride molar fraction increases sodium chloride to the ternary CaCLCaC,~aCl, beyond this stoichiometry, a dilution effect takes place modifies the structure of the solution which exhibits which could justify the slight decrease observed in higher conductivities with reference to the CaO-the specific conductivity. CaC,-CaCI, systems. This phenomenon has been atRecent data published on the free energy of formatributed both to an interaction between dicarbide ions tion of sodium dicarbide[16] make it possible to caland sodium ions, the latter substituting the calcium culate the free energy of the reaction (2). Assuming ions and to a possible electronic conduction. that the value of the M fractions ratio, p = XNaCI/ Furthermore, it has been noticed that these interactions are at a maximum when the ratio p = Gac,/ Xc acz, which is still less than two, at the maximum of, specific conductivity represents the degree of adapproaches two, corresponding to the stoichiXX, vancement of reaction (2), it has been possible to estiometry of the reaction: mate, from previous results[17], the activity coeffiCaC2 + 2NaCl+Na,C, + CaCl,. cient of Na2C2.
C. GFINTAZ,
96
This verifies the previous thermodynamic
M.
PARODI
interpre-
tations. REFERENCES
1.
W. A. Barber and C. L. Sloan, J. phys. Chem. 65, 2026
(1961). Intrma2. D. R. Morris and J. R. Harry, Proceedings tional Conference industrial Carbon and Graphite, p. 36. Society of Chemical Industry, London (1965). 3. S. H. White, Internal Report Dept. of Chem. Eng. Univof New Brunswick, Canada (1970). 4. D. R. Morris and toll. J. ebctrochem. Sot. 170, 570 (1973). 5. k. Mitchell, Trans metall. Sac. A.I.M.E. 242 (1948). 6. C. Gentaz, G. Bienvenu and A. Bow&a, Rev. Int. Htes. Temp. Refract. (1973). C. Genta and A. Boussiba, Rev. Inf. 7. G. Bienvenu. /!i?‘\. TiW/‘. /7<$Yrcr I I. 61 71 (1974).
and A. BONOMI
R. Winand, Elccwochim Actu 3, 106 (1960). 9. Molten salts-l’ol 1 NSRDS-NBS 15 p. 48 10. S. A. Miller. Acetvlene-Its Pronerties. Manufacture ’ ” and Usrs. V& I, p: 167 (I 965). 11. L. Paulina Nnture of the Chemical Bond. D. 346. Cornell University Press (1445). 12. C. Aksanaran, V. Dosaz and D. R. Morris, Can. J. Chem. 49, 12 (1971). 13. J. B. Story and J. T. Clarke, J. Metals 9, 1449 (1957). 14. H. R. Bronstein and M. A. Bredig. J. Am. them. Sot. SO. 2077 (1958). 15. A.. S. Dworkin, H. R. Bronstein and M. A. Bred&. Discuss. Faraday Sot. No 32, pp. 188-196 (1961). 16. Johnson, Darell et al. J. &em. Thermodynamics 1, 5.57.71 (1973). Battelle/ 17. G Bienvenu, C. Gentaz and A. Boussiba. Geneva. Internal Report Sept. 1972. 18. L. F. Grantham and S. J. Yosim, Molten Salts (Edited by Manantov). p. 409.
8.
19 December
1974; rrvisrd uarsion 13 June 1975)
Abstract-The specific conductivity variation of the named solutions DS temperature in the range 750-950°C and vs the sodium chloride molar fractions have been measured. The specific conductivity increases sharply with increasing sodium chloride M fraction and reaches a maximum when the M fraction ratio of sodium chloride to calcium carbide is nearly two. This experimental result has been interpreted by assuming interactions between calcium carbide and sodium chloride, and constitutes a verification of a previous thermodynamic result obtained by the same authors.
1. INTRODUCTION
cell is made of two, pure iron, cylindrical electrodes each placed in a hollow alumina tube (Degussa Al 23) 5 mm dia and 35 mm longer than the iron electrode in order to lengthen the effective cell length and hence increase the resistance. The resistance was measured with a conventional ac bridge fed by a sinusoidal oscillator of 1000 Hz frequency in order to avoid polarization of the electrades. This frequency has been chosen since the conductivity is independent of frequency within the range 500 and 5000 Hz. ductivity
In 1962, Barber and Sloan[l] showed that technical grade calcium carbide was soluble to some extent in alkaline-earth and lithium halides. On the basis of these results various authors studied the properties of molten salt solutions of calcium carbide. Investigation methods have mostly been electrochemical (chronopotentiometry, galvanic cells)[2-51. The results confirm that calcium carbide dissolved in molten salt probably gives as a dissociated product, C:- anionic species, which is readily oxidizable either chemically or electrochemically. In other investigations dealing aspects of calcium chloride with some thermodynamic and calcium chloride+odium chloride-technical grade calcium carbide solutions[6-71, preferential interactions between the sodium cation and carbide anion C$- was demonstrated; these solutions exhibited a significant variation in the thermodynamic excess functions. The purpose of the present study was to verify, by measuring the specific conductivity, the hypothesis concerning the ionic interactions.
2 EXPERIMENTAL 2.1. Experimental
2.3. Conductivity
measurements
350 g of powdered analytical grade calcium chloride, compacted in 50 g pellets in order to reduce the dead space, was placed iu the crucible and melted under argon. The temperature was held at 820°C and the molten salt dehydrated under vacuum for about 2 h. Technical grade calcium carbide, about X0”/, in weight of CaC,, was then added, still under argon, and the conductivity cell introduced. Conductivity measurements were made for each increase in temperature. After measurements, the temperature was lowered to 800°C. Then analytical grade sodium chloride was added, under an argon atmosphere, by SUG cessive additions through the valve; the conductivity cell was still immersed in the molten salt. As soon as the sodium chloride was added, measurements were made under a static argon atmosphere, as it has been shown that argon sweeping and, u fortiori, a primary vacuum favour sodium metal in the CaC2CaCl,-NaCl-CaO system; conductivity measurements were again made for each increase in temperature.
PROCEDURE
technique
The two-capillary dip cell[8] has been chosen since saturated solutions with calcium carbide in excess necessarily contained solid suspensions (carbon, uadissolved calcium carbide, etc.) which can interfere with the conductivity measurements. The disadvantage of this type of cell is the parasitic conductance through the capillary walls. This defect cannot be overcome as there is no material presenting both electrical insulation and chemical inertness in the molten salt solutions studied.
2.4. Determination of the cell constant-standardization A standardization in molten KC1 gave 32.6 cn- ’ for the cell constants. It was verified that the positioning of the cell in the alumina crucible did not modify the cell constant if the lower ends of the alumina pipes were more than I cm away from the bottom of the alumina crucible: at less than 1 cm the cell
2.2. Apparafus The molten salt is contained in an alumina crucible which is placed in an inconel cell. This assembly is covered by a controlled argon atmosphere. The con93
C. GENTAZ, M. PARQDI and A. BONOMI
94
constant increased showing that the current distribution between the two alumina pipes had changed. It was also shown that the current distribution was affected by the temperature and hence by the conductivity. The cell constant varied according to the temperature and standardization was necessary. Finally, it was noted that the cell constant varied significantly only after 16 h of experiments; -this variation was mainly due to a parasitic conductance through the alumina wall pipes, a carbon deposit being observed on the external alumina walls. Thus, alumina is not chemically inert vis-ci-vis the calcium carbide dissolved in molten salts. For these reasons, cells were discarded after 8 h use. 2.5. Chemicul analysis The calcium carbide and calcium oxide concentration were measured at the beginning and at the end of an experiment. Calcium carbide analysis The analytical method is the gas-volumetric procedure previously employed[6]. Calcium oxide analysis Calcium oxide is the only strong base present in the bath. The bath samples were dissolved in water, the dissolution being enhanced by the addition of a standard solution of hydrochloric acid in excess, and the calcium oxide was determined by a back-titration with a standard solution of sodium hydroxide. The measured value corresponds to the total calcium oxide and the equivalent calcium oxide, corresponding to the calcium carbide initially present in the sample, must be subtracted in order to obtain the quantity of the calcium oxide dissolved in the bath. 3.RF.SULTS The conductivity measurements were made on bin-
ary calcium chloride-sodium chloride in which the M fraction of sodium chloride varied between 0+6 inclusive. For each composition and temperature, the molten salt was saturated with calcium carbide and the experiments carried out under a static argon pressure of one atmosphere. It has been verified that the calcium carbide concentrations corresponding to saturated solutions followed the laws previously determined[6]. The nominal concentration of calcium oxide was the same in all experiments. However, it decreased from the initial 4*?++5% by wt to 3.&3-l% at the end of the experiment. This decrease is entirely due to the dilution caused by the introduction of sodium chloride. The number of calcium oxide moles, about 0.2 mole, was kept constant during the experiment. However, at the end of every series of experiments, a decrease of the calcium carbide concentration was noticed. As the number of moles of calcium oxide remained constant, it was concluded that the calcium carbide was consumed by chemical reaction with the sodium chloride leading to the formation of sodium metal which has been characterized by analysis of bath samples. The average sodium concentration in the bath was 0.3% in wt (about 0.05 mole or 1% M fraction}. In the following calculations, on the systems for
!C A
Fig. 1. CaSCaCzPCaC1, specific conductivity us temperature (nCa0 2 0.2 mole). 0, CaCI, pure (9); x, CaC1,-CaC,PCaO. which the sodium chloride M fraction varies between (M.5. the sodium chloride, calcium chloride and calcium carbide M fractions have been defined by the initial state; this is, however, inaccurate due to the finite displacement of the equilibrium towards sodium metal production. Only for a sodium chloride M fraction equivalent to 0.6, of the above mentioned M fractions, has the calculation been with reference to the final state of the systems. A DISCUSSION
4.1. System CaGCaC,-CaCll The specific conductivity of the system CaO-CaCzCaCl* is slightly less below 900°C and slightly more above 900°C (Fig. 1) than for calcium chloride[9]. The experimental points were adjusted according to the method of least squares to an equation of the form: y=a+bt the result of which was: y (W’. cm-‘) = - 2.314 + 5-401 10-j
t(“C).
Even with a margin of error the straight lines, giving the specific conductivities of pure calcium chloride and calcium chloride-calcium oxide-calcium carbide mixture, intersect around 900°C. It is expected that the mobility of the dicarbide ion, a complex ion of ionic radii (10) near that of a chloride ionrl 11.would be less than that of chloride ion. This would justify the fact that the specific conductivity of the system CaO-CaC,-CaCl, is slightly less than that of pure calcium chloride at least for temperatures of below 900°C. On the other hand, above 9OO”C, a new conduction mechanism must be considered to explain the higher conductivity of the system CaOCaC,-CaCl,. According to Aksanaran et al.[12] calcium carbide can dissociate to calcium metal and carbon. The calcium will give rise to electronic conductivity. L
_/
4.2. System CaO-CaC,-CaCl,-NaC1 The introduction of sodium chloride leads to a noticeable increase of the specific conductivity of the solution, about 50% (Fig. 2); the specific conductivity of the calcium chloridt+sodium chloride binary[13] has been shown for comparison. Finally, as in a pre-
Conductivities in calcium carbide and calcium oxide
0
0.2
XNaCIdared lo
0.L the binary
0.6
NoCLCaCt2
Fig. 2. Ca@CaC,-CaCl,-NaCI specific conductivity IIS sodium chloride molar fraction for various temperatures. ~30 = P2 mole + XCaC, between O-055and 0.1069 (see Ref. 13.
v;ous study[77], the variation of the specific conductivity of the solution us the M fraction ratio XY.&XCnCI is shown in Fig. 3, which also shows the SPCC~~~C conductivity of sodium-sodium chloride[l4] and calciumlcium chloride systems[151 vs M fraction of metal. Two remarkable results appear, one being the increase of the specific conductivity in the presence of sodium chloride, the other the existence of a maximum conductivity for the ratio XNac,/XcaC2of about two. The only possible hypothesis remaining after careful examination of the various alternatives to explain these two facts is that based upon a sodium chloride-calcium carbide interaction. When sodium chloride is introduced, a substitution of calcium ion by sodium ions can take place according to: CaZ+ J+
2Na+ 4 Na’ -C
95
= C- Na+ + Ca2+.
(1)
Thus the specific conductivity reaches a maximum when all the calcium ions are substituted by sodium ions. This would explain the observed specific conductivity maximum when the ratio XNnCI/Xcnc2is around two corresponding to the stoichiometry of thk reaction : CaC, + 2NaC1+NazCz + CaCl,. (2)
0
2
4
=eg t 6
Fig. 3. Caa CaC,-CaCl,-NaC1 specific conductivity US the ratlo p = [(X,,,, ,)/X(,, z] for various temperatures.
Values obtained are about 10m5 and are characteristic of a strong interaction. However, owing to the inaccuracy of the thermodynamic data, this result only gives an indication of interactions between sodium and dicarbide ions. Furthermore, the explanation of the observed phenomena by the presence of sodium metal is incomplete, as specific conductivity of the sodium-sodium chloride system only varies by 20”/, in the range @-1% sodium M fraction[14] and as specific conductivities higher than 7E I cm-’ were obtained when the system was under a primary vacuum at 900°C. Finally, even if the interaction between sodium chloride and calcium carbide explains the specific conductivity maximum, it does not explain the high values of conductivity. A further mechanism could be proposed which would involve electronic exchange, either between sodium metal and sodium ion, or between carbon and dicarbide ions. Unfortunately, the lack of data has not allowed the application of the models already used by Grantham and Yosim[lS] to explain electronic conductivity of the CuCILCuCI, system. 5. CONCLIJSlON
In spite of the inherent difficulty in obtaining accurate measurements on unstable systems such as CaOCaC,-CaCl,-NaC1, it appears that the addition of When the sodium chloride molar fraction increases sodium chloride to the ternary CaCLCaC,~aCl, beyond this stoichiometry, a dilution effect takes place modifies the structure of the solution which exhibits which could justify the slight decrease observed in higher conductivities with reference to the CaO-the specific conductivity. CaC,-CaCI, systems. This phenomenon has been atRecent data published on the free energy of formatributed both to an interaction between dicarbide ions tion of sodium dicarbide[16] make it possible to caland sodium ions, the latter substituting the calcium culate the free energy of the reaction (2). Assuming ions and to a possible electronic conduction. that the value of the M fractions ratio, p = XNaCI/ Furthermore, it has been noticed that these interactions are at a maximum when the ratio p = Gac,/ Xc acz, which is still less than two, at the maximum of, specific conductivity represents the degree of adapproaches two, corresponding to the stoichiXX, vancement of reaction (2), it has been possible to estiometry of the reaction: mate, from previous results[17], the activity coeffiCaC2 + 2NaCl+Na,C, + CaCl,. cient of Na2C2.
C. GFINTAZ,
96
This verifies the previous thermodynamic
M.
PARODI
interpre-
tations. REFERENCES
1.
W. A. Barber and C. L. Sloan, J. phys. Chem. 65, 2026
(1961). Intrma2. D. R. Morris and J. R. Harry, Proceedings tional Conference industrial Carbon and Graphite, p. 36. Society of Chemical Industry, London (1965). 3. S. H. White, Internal Report Dept. of Chem. Eng. Univof New Brunswick, Canada (1970). 4. D. R. Morris and toll. J. ebctrochem. Sot. 170, 570 (1973). 5. k. Mitchell, Trans metall. Sac. A.I.M.E. 242 (1948). 6. C. Gentaz, G. Bienvenu and A. Bow&a, Rev. Int. Htes. Temp. Refract. (1973). C. Genta and A. Boussiba, Rev. Inf. 7. G. Bienvenu. /!i?‘\. TiW/‘. /7<$Yrcr I I. 61 71 (1974).
and A. BONOMI
R. Winand, Elccwochim Actu 3, 106 (1960). 9. Molten salts-l’ol 1 NSRDS-NBS 15 p. 48 10. S. A. Miller. Acetvlene-Its Pronerties. Manufacture ’ ” and Usrs. V& I, p: 167 (I 965). 11. L. Paulina Nnture of the Chemical Bond. D. 346. Cornell University Press (1445). 12. C. Aksanaran, V. Dosaz and D. R. Morris, Can. J. Chem. 49, 12 (1971). 13. J. B. Story and J. T. Clarke, J. Metals 9, 1449 (1957). 14. H. R. Bronstein and M. A. Bredig. J. Am. them. Sot. SO. 2077 (1958). 15. A.. S. Dworkin, H. R. Bronstein and M. A. Bred&. Discuss. Faraday Sot. No 32, pp. 188-196 (1961). 16. Johnson, Darell et al. J. &em. Thermodynamics 1, 5.57.71 (1973). Battelle/ 17. G Bienvenu, C. Gentaz and A. Boussiba. Geneva. Internal Report Sept. 1972. 18. L. F. Grantham and S. J. Yosim, Molten Salts (Edited by Manantov). p. 409.
8.