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The NaF-AlF3-Al2O3-Na2O system—I standard free energy of formation of α-aluminium oxide from emf measurements
THE STANDARD x-ALUMTNIUM
NaF-AIFs-A1,03-Na20 FREE ENERGY OXIDE FROM ,&.STERTEN,
of Jndustrial
Division
Electrochemistry,
S. HAUGEN
The
University
SYSTEM-l FORMATION OF EMF MEASUREMENTS OF
and
K. HAMBERG
of Trondheim,
N-7034,
NTH,
Trondheim,
Norway
Abstract-The behaviour of oxygen and alummium electrodes was studied by means of emf measurements in Na,AlF, melts saturated with oc-AlzO, in the temperature range 1233-1310K. The results indicate that the electrodes are reversible when they are separated by an a-Al,O, diaphragm. However, the alumimum electrode reacts with the melt to a certain extent. This means that the standard free energy data for a-Al,O, are not easily derived from emJ measurements in Na3AlF, melts saturated with a-Al,O,. However, the emf for the following cell (J’t)O~ILi~AlF~,,,,
AIF,,,,.
AlZ03,..,//Li~Al~~,,,
was in excellenr aereement with values derived from in the JANAF TaYbles in the range 102(X1275 K.
INTRODUCTION
with
assumed
cell
=I,
AlxO,,,
.>/O,W)
(1)
(Pt)%,
.,,,,,,/Na,AlF,, /Na,AlF,,
alloy
Ar(Pt)
formation
Ah..,,
)_
Al,O,,,,,,,/OdPt)
(IV)
Al&L,..,, J
AlF3,h.,,,, AL03,..,,,/OAPt)
(VI
The description of cell (IT) and (IV) above is not strictly correct as will be discussed below. The saturation limit for AIFj in Li,AlF, has been determined by Holm[ 171.
2. EXPERIMENTAL Figure 1 shows the main features of a typical set up for cell (IV). This figure will also be used to describe the construction of the other cells (II), (III) and (IV). The cell container consisted in all the runs of either AUC graphite (Umon Carblde) or sintered r-Al,Oa (99,7x,). The melt for the cells (II). (III) and (IV) was made up of natural Greenland cryolite which was saturated with reagent grade a-A120,. Certified LiF from Fisher and sublimed A1F3 were used as electrolyte in cell (V). The bottom of the cell container was in all runs covered with a layer of r-AlzO, powder with a thickness of about 1 cm. The electrodes were separated into two compartments by means of tither an open or a closed rx-Al,O, tube embedded in the solid powder on the bottom of the crucible. Experience shows that the electrical resistivity of this kind of diaphragm immersed in cryolite melts 1s of such a magnitude that one wrll expect direct melt contact between the compartments through pores[14]. Besides, it is found that the transference number of the Na * ions is close to unity under these condltionsL I 5- 16 J. The arrangement described should electively reduce the possibility of any influence of either dissolved metal on the O,-(Pt) electrode or dissolved oxygen on the Al-electrode. The O,-(Pt) electrode was located inside the alumina tube described above. It consisted of a Pt-foil
(11)
Al,O,,..,,,1 A1zO,,..,,/Oz,
for z-Al,O,
All%,,,,,/
/Li3AlFb.
reaction
or Al-Cu
data
/Na3AlF,.
(1)
I ) + 3/2 O,,,, = *-AlzOx~,.,
Al,O~,..,,,/AI
free energy
Al/Li.AF,.
has been the object of several studies [l-4]. The results from these measurements show potential values lower than those calculated from thermodynamic data given in the JANAF Tables [S]. Experimental results from direct en!{ measurements have been listed by Thonstad[d] together with decomposition potentials obtained by extrapolation of current/voltage curves[2.7-91. However. the extrapolated values are not in agreement with the JANAF data either. rhe discrepancy may be due to different kinds of overvoltage on the electrodes and depolarization effects caused by dissolved metal. it is well known that there is some metal solubility in Na3A1Fh-A1203melts in contact with A1[1&13]. The dissolved metal appears to consist of sodium as well as aluminium or as reduced species. In order to eliminate the influence of dissolved metal on the enlf for cell (I) the electrodes should be separated from each other by means of a diaphragm. The present workers found it necessary to give this point some attention in an attempt to repeat the enzf measurements for cell (I). The purpose of the present investigation was (1) to examtne the stabiltty. and reversibility of the Al and Oz electrodes in Na,AlF,,-Al,O,-melts and (2) if possible to obtain reliable standard free energy data for x-AIZOJ. The following cells were applied in the present study AI/Na,AIF,.
,. AIF, ,,,. Al103,,,/Al
Al/Na,AlF,,
Accurate CW$ measurements in molten cryolitic melts have been difficult to achieve because of the lack of reliable electrodes. container,and diaphragm materials. The ctlrfof the following cell Al,, ,/Na,AlF,,,,.
standard
(III) 589
590
A.
STERTEN. S. HAUGEN and K. HAMBERG and 0.24 respectively at 1295°K. These values seem to be in reasonable agreement with activity data given by Wilder[17). In spite of these results the Al or Al Cu alloy electrode may not be used without some precaution, since the electrodes react with the melt to a certain degree. This point is discussed below. The open-circuit ernJ‘ of the oxygen concentration cell (III) was measured as a function of the oxygen partial pressure. The theoretical c~?nf‘for this cell is: E =
-
RT In POL.
4F
Figure 2 shows the experimental emf as a function of the partial pressure of oxygen. These data are in good agreement with theoretical values from equation (2) shown as a straight line in the figure. ‘The r,?lf for cell (111) remained constant to within f 0.5 mV for scvcral hours at a given tcmpcrature and pressure. The cell potential was also independent of the depth of immersion of the electrodes into the melt. The rerelsibility of the measured CWJ’ was someFig. 1. Schematic diagram of an expermxntal cell: A-P1 and Pt-lO:,Rh wires in contact with the Pt foil G, B-m four bore alumina tube through which oxygen gas 1s supplied to the Pt foil electrode; C--alumina tube: D-T‘a wire; E--alumina sheath; F-graphite or alumina crucible: H- Fluoride melt saturated with alumina : J-AI electmde: K-alumina crucible.
wrapped around the lower part of a four bore alumina protection tube containing a Pt-lO’$/o Rh/Pt thermocouple in contact with the foil. High purity O1 (>99 9973 was supplied to the P(-foil through the bores of this protection tube. The Al (99 9980,:) electrode was placed in a small a-Al,O, crucible in the other electrode compartment. The electrical contact to Ihe Al-electrode was provided by means of l-mm Ta wire shielded by an a-Al,03 tube. Purified argon gas was passed over the meta electrode compartment in the temperature controlled furnace. The actual cell temperature was determmed by means of a separate thermocouple located inside a closed-end CY-Al,03 protection tube immersed in the melt (not shown in Fig. 1). The oxygen electrodes in cell (III) were placed in two identical compartments like the one described above. The emf as a function of the partial pressure of oxygen was studied by diluting the oxygen with argon supplied to one of the electrodes. Two identical Al-clectrodcs or one of them diiuled with Cu were used in cell (II). The diaphragm was not employed in these measurements. 3. RESULTS
AND
times checked by shorting the cell for about IO s. The emf always returned to the original value within
a minute. The results
given indicate that the O,-(Pt) electrode and reversible reference electrode in Na,AlF‘,-Al,O?,,,-melts when it is surrounded by an stAIZO, diaphragm as described above. is
a
reliable
The en?f of cell (IV) was independent of which of the two types of diaphragm was used. This result indicates that neither dissolved metal nor dissolved oxygen have any influence on the Oz(Pt) and Al-clectrade reactions respectively. However, a drastic drop was observed in the cell voltage when the tube-type diaphragm was lifted somewhat so that free convection between the electrode compartments occurred.
DISCUSSION
The rrnf of uell (II) between identical Al-electrodes was usually within 0 + 2 mV during the runs which lasted for several hours at constant tcmpcraturc. The activity of Al in liquid Al-Cu alloys with molar ratio xAI = 0,66 and xA, = 0,49 was found to be 0.53
Fig. 2. EMJ’ as a function of the parrial pressure of oxygen for cell (Ill) at 1271 K. The straight line is the theoretical one according to equation (2).
The NaF-AlF,-AI,O,-NaZO
591
system-1
tion (1) can be attributed to the aluminium electrode compartment. This assumption is reasonable since we know that there is some metal solubility in the melt. The following solution equilibria may be suggested 2Al 22l
> ._
2.1’
E
2.1,
Fig. 3. Typical enlf-temperature data from three separate runs from cell (IV). The data are corrected for the thermal r~zgenerated between the different leads employed. Placing the diaphragm properly into the alumina sludge again the r~rfretained its original value within a few min. Introduction of oxygen into the Al-electrode compartment did not on the other hand have any noticeable influence on the emf of cell (IV). These results show clearly that it is the gas electrode which must be protected from action of dissolved metal. Typical emf-temperature data for cell (IV) from three different runs are shown in Fig. 3. Usually. a straight line plot was &tamed for each run. The data was recorded at a cooling rate of less than 1 K/min from about 130°K to the eutectic temperature of 1236 K. Emftemperature data obtained at a heating rate of about 1 K/min were practically identical with those described above, allhough Lhc: scatter of lht: points was somewhat larger. The slope of c,nlfltemperaturc plol for cell (IV) was found to be -0.55 t 0.02 mV/K which corresponds approximately to the derived JANAF value of -0.566 mV/K. The average value of the f>nf at 1273 K for cell (IV) from 15 runs was found to be 2.183 V with a standard deviation of about f 3 mV. The corresponding value derived from the JANAF Table is 2.195 V, which seems to be the correct value ab shown below. Before discussing the discrepancy of 12 mV we will describe some other experimental results. In 5 runs the (Pt)-0, electrode was replaced with an (Au)-0, electrode in cell (IV). Firstly, it is important to note that this replacement did not change the cmf‘ within the limits of cxperimcntal uncertainties. Secondly. the slope of the relationship between the wnf and the oxygen pressure at constant temperature for cell (IV) seems to agree fairly well with the theoretical slope derived from equation (2). These results are illustrated in Fig. 4 for both types of oxygen electrodes combined with the Al-electrode. Although no definite conclusions can be drawn these results indicate that the failure to reach the correct emf for reac-
+
AIFS =
3AlF
(3)
Al + 3NaF
=
AlFS
+
~Nz+,,~,,
(4)
Al +
= AIF,
+
3Na,F.
(5)
6NaF
Although the nature of dissolved metal is unknown the net effect will be an increased activity of aluminium fluoride and a reduced activity of sodium fluoride in the Al-electrode compartment. This condition means that cell (IV) is not merely a simple formation cell as described by reaction (1). In addition there will be a small contribution in the end arismg from the different activities in the two compartments. In the followmg it is assumed that the transport number of the Na+ ion is equal to unity[lS, 161. The change in free energy when passing a current of six Furadays in cell (IV) from the Al-electrode to the O,-electrode can then he expressed by the following equation
where jr’ and f?’ are chemical potentials on the left and the right sides oi the cell. pAI and mz arc neglected since they are equal to zero. The en~f for the cell (IV) may from equation (6) be written E = where
EC.,,,,
E,,,,L. +
E%p,
(7)
rn?f’arising from the concentration is the standard rrnf for CI - Al,O,. (6) and (7) it is easily derived
is the
cell and E&,o, From equations
which should correspond to minus 12 mV as discussed above. This value is compatible with a change in the NaF/AlF, ratio Cram 3.0 to 2.9[15]. The solubility limit of aluminium in the melt can then be estimated from the change in the NaF/AlF3 ratio and
Cl.4
-‘-a Fig.
PO,
4. The relative change in emj as a function of partial pressure of oxygen at 1273 K for cell (TV)
the
592
A. S~ERTEN. S. HAUGEN and
Fig. 5. Standard rnzfdata as a function of the temperature (IPTS-68) for the formation of a-A120a from celI (V). The line corresponds to free energy data for a-A1203 taken fronl the JANAF Table[5]. reaction (4) which is used for the sake of simplicity. The total solubility at 1273 K is in this way catculated to be 0.19 wt “/uAl. This value may be somewhat too high, although recent measurements of the solubility in Na3AIF,-A120, melts show values ranging from 0.05 to 0.70 wt “/, Al[ll-131. In order to derive the real standard free energy of formation for CY-AI,O~ we decided to use cell (V) since it was believed that the metal sotubility was very small in this melt. The end‘ for this cell as a function of the temperature is shown in Fig. 5. The individual points are average values from several runs. The emf had to be measured at constant temperature in cell (V) in order to obtain reproducible results. Once a constant cell enzf was established for a given temperature, the set point of the temperature controller was changed to a new value and the sequence of measurements was repeated. To obtain reliable rr~$ values below approximately I 1OC”K it turned out to be necessary to polarize the electrodes with a small current for a few minutes before the open circuit rrnf was allowed CO stabilize. Data below 950°K are neglected since it was difficult to obtain reproducible results. The reason for this failure is not known. Despite some uncertainties as indicated above it IS believed that the ernf data glvcn in Fig. 5 are very accurate. The majority of the points in the figure are less than + 200 cal away from the line which corresponds to the standard free energy of formation of r-Al,O, in the JANAF TabIes[S].
4. CONCLUSION An Al-electrode constructed as described in present paper seems to be a reversible electrode
the in
K. HAMBER~
Na,AlF, melts saturated with a-A1,03. However, it reacts to some extent with the electrolyte resulting in an increased AlF, activity and a reduced NaF activity. (Pt)O, and (Au)O, electrodes enclosed into z-alumina tubes seem to be reversible and reliable electrodes in Na,AlF, -Al,O,-melts. Standard emf values for or-Al,O, are difficult to achieve in Na,AlF,-Al,Os melts. To avoid the influence of dissolved metal on the oxygen electrode the cell must be separated into two chambers. Consequently, the composition of the electrolytes will be different in the two compartments. This means that reliable standard eflf values for cr-Al,03 cannot be obtained without a correction term taking into account this difference in composition. In Li,AlF, melts saturated with A1F3 and cc-Al,O, it seems possible to obtain reliable rnd data for the formation of r-AI,OJ. In this case the melt reacts probably very little with the Al-elcctrodc. Ackl,awlrdg~nlent-We wish @an Council for Industrial financial support.
to thank the and Scientific
Royal NorweResearch for
REFERENCES
W. D. Treadwell and L. Tercbesi, Hrlt-. chim. Acra 16. 922 (1933). Z. Elecrrochem. 40. 605 (1934). 2. P. Drossbach, and A. A. Rcvazyan, J. appl. Chem. 3. V. P. Mashovets USSR 30. 1069 (1957). 4. M. Rey. C.r. hehd. Se’a~~c. Acad. Sci., Paris 260. 5528 (1965). Thermochemical Tables, NSRDS-NBS 37. 5. JANAF 2nd cd. (lY71). Elrcrrochim. Acra 13. 449 (1968). 6. J. Thonstad. and R. G. Seyl. 7‘run.s. L’lecftwchem 7. M. K. Thompson sot. 64. 321 (1933). and J. Waddington, 7‘ra,,s. p-arrrtlo_,a 8. _I. W. Cuthbertson Sot. 32. 745 (1934). 9. K. Grjotheim, Conirrburiun lo thr Theory qf (he Aiuminiurn ~k’ctro~J~.~~s, Det. Kgl. Norskr Videnskabers Selskaps Sknfter No. 5, p. 67. Brun, Trondheim (19.56). IO. J. Thonstad, Cnn. J. Clzmm. 43. 3429 (I 965). Mrt. ‘Frm~s. 3. 1817 I I. K. Yoshida and E. W. Dewing, 1.
(1972).
J. Gerlach, W. Schmidt and H. Schmidt, Er,,rc~all 20. 11I (1967). 13. A. M. Arthur, Met. Trans. 5. 1225 (1974). 14. K. Yoshida and E. W. Dewing, MEW. Trawls. 3. 683 (1972). 12
A. Stertcn and K. Hamberg, Light Metals 1976. S. R. Leavitt (Ed.) Proc. 105 AIME Annual Meeting, Vol. I. p. 203, Las Vegas (1976). W. B. Frank and L. M. Foster, J. phys. Chem. 61, I h. 1531 (1957). Thermodynamic Properties of Molten 17. J. L. Helm. Cryolite and other Fluoride Mixtures, Dr. Thesis, The Univ. of Trondheim. NTH, Norway (1971). 15
NaF-AIFs-A1,03-Na20 FREE ENERGY OXIDE FROM ,&.STERTEN,
of Jndustrial
Division
Electrochemistry,
S. HAUGEN
The
University
SYSTEM-l FORMATION OF EMF MEASUREMENTS OF
and
K. HAMBERG
of Trondheim,
N-7034,
NTH,
Trondheim,
Norway
Abstract-The behaviour of oxygen and alummium electrodes was studied by means of emf measurements in Na,AlF, melts saturated with oc-AlzO, in the temperature range 1233-1310K. The results indicate that the electrodes are reversible when they are separated by an a-Al,O, diaphragm. However, the alumimum electrode reacts with the melt to a certain extent. This means that the standard free energy data for a-Al,O, are not easily derived from emJ measurements in Na3AlF, melts saturated with a-Al,O,. However, the emf for the following cell (J’t)O~ILi~AlF~,,,,
AIF,,,,.
AlZ03,..,//Li~Al~~,,,
was in excellenr aereement with values derived from in the JANAF TaYbles in the range 102(X1275 K.
INTRODUCTION
with
assumed
cell
=I,
AlxO,,,
.>/O,W)
(1)
(Pt)%,
.,,,,,,/Na,AlF,, /Na,AlF,,
alloy
Ar(Pt)
formation
Ah..,,
)_
Al,O,,,,,,,/OdPt)
(IV)
Al&L,..,, J
AlF3,h.,,,, AL03,..,,,/OAPt)
(VI
The description of cell (IT) and (IV) above is not strictly correct as will be discussed below. The saturation limit for AIFj in Li,AlF, has been determined by Holm[ 171.
2. EXPERIMENTAL Figure 1 shows the main features of a typical set up for cell (IV). This figure will also be used to describe the construction of the other cells (II), (III) and (IV). The cell container consisted in all the runs of either AUC graphite (Umon Carblde) or sintered r-Al,Oa (99,7x,). The melt for the cells (II). (III) and (IV) was made up of natural Greenland cryolite which was saturated with reagent grade a-A120,. Certified LiF from Fisher and sublimed A1F3 were used as electrolyte in cell (V). The bottom of the cell container was in all runs covered with a layer of r-AlzO, powder with a thickness of about 1 cm. The electrodes were separated into two compartments by means of tither an open or a closed rx-Al,O, tube embedded in the solid powder on the bottom of the crucible. Experience shows that the electrical resistivity of this kind of diaphragm immersed in cryolite melts 1s of such a magnitude that one wrll expect direct melt contact between the compartments through pores[14]. Besides, it is found that the transference number of the Na * ions is close to unity under these condltionsL I 5- 16 J. The arrangement described should electively reduce the possibility of any influence of either dissolved metal on the O,-(Pt) electrode or dissolved oxygen on the Al-electrode. The O,-(Pt) electrode was located inside the alumina tube described above. It consisted of a Pt-foil
(11)
Al,O,,..,,,1 A1zO,,..,,/Oz,
for z-Al,O,
All%,,,,,/
/Li3AlFb.
reaction
or Al-Cu
data
/Na3AlF,.
(1)
I ) + 3/2 O,,,, = *-AlzOx~,.,
Al,O~,..,,,/AI
free energy
Al/Li.AF,.
has been the object of several studies [l-4]. The results from these measurements show potential values lower than those calculated from thermodynamic data given in the JANAF Tables [S]. Experimental results from direct en!{ measurements have been listed by Thonstad[d] together with decomposition potentials obtained by extrapolation of current/voltage curves[2.7-91. However. the extrapolated values are not in agreement with the JANAF data either. rhe discrepancy may be due to different kinds of overvoltage on the electrodes and depolarization effects caused by dissolved metal. it is well known that there is some metal solubility in Na3A1Fh-A1203melts in contact with A1[1&13]. The dissolved metal appears to consist of sodium as well as aluminium or as reduced species. In order to eliminate the influence of dissolved metal on the enlf for cell (I) the electrodes should be separated from each other by means of a diaphragm. The present workers found it necessary to give this point some attention in an attempt to repeat the enzf measurements for cell (I). The purpose of the present investigation was (1) to examtne the stabiltty. and reversibility of the Al and Oz electrodes in Na,AlF,,-Al,O,-melts and (2) if possible to obtain reliable standard free energy data for x-AIZOJ. The following cells were applied in the present study AI/Na,AIF,.
,. AIF, ,,,. Al103,,,/Al
Al/Na,AlF,,
Accurate CW$ measurements in molten cryolitic melts have been difficult to achieve because of the lack of reliable electrodes. container,and diaphragm materials. The ctlrfof the following cell Al,, ,/Na,AlF,,,,.
standard
(III) 589
590
A.
STERTEN. S. HAUGEN and K. HAMBERG and 0.24 respectively at 1295°K. These values seem to be in reasonable agreement with activity data given by Wilder[17). In spite of these results the Al or Al Cu alloy electrode may not be used without some precaution, since the electrodes react with the melt to a certain degree. This point is discussed below. The open-circuit ernJ‘ of the oxygen concentration cell (III) was measured as a function of the oxygen partial pressure. The theoretical c~?nf‘for this cell is: E =
-
RT In POL.
4F
Figure 2 shows the experimental emf as a function of the partial pressure of oxygen. These data are in good agreement with theoretical values from equation (2) shown as a straight line in the figure. ‘The r,?lf for cell (111) remained constant to within f 0.5 mV for scvcral hours at a given tcmpcrature and pressure. The cell potential was also independent of the depth of immersion of the electrodes into the melt. The rerelsibility of the measured CWJ’ was someFig. 1. Schematic diagram of an expermxntal cell: A-P1 and Pt-lO:,Rh wires in contact with the Pt foil G, B-m four bore alumina tube through which oxygen gas 1s supplied to the Pt foil electrode; C--alumina tube: D-T‘a wire; E--alumina sheath; F-graphite or alumina crucible: H- Fluoride melt saturated with alumina : J-AI electmde: K-alumina crucible.
wrapped around the lower part of a four bore alumina protection tube containing a Pt-lO’$/o Rh/Pt thermocouple in contact with the foil. High purity O1 (>99 9973 was supplied to the P(-foil through the bores of this protection tube. The Al (99 9980,:) electrode was placed in a small a-Al,O, crucible in the other electrode compartment. The electrical contact to Ihe Al-electrode was provided by means of l-mm Ta wire shielded by an a-Al,03 tube. Purified argon gas was passed over the meta electrode compartment in the temperature controlled furnace. The actual cell temperature was determmed by means of a separate thermocouple located inside a closed-end CY-Al,03 protection tube immersed in the melt (not shown in Fig. 1). The oxygen electrodes in cell (III) were placed in two identical compartments like the one described above. The emf as a function of the partial pressure of oxygen was studied by diluting the oxygen with argon supplied to one of the electrodes. Two identical Al-clectrodcs or one of them diiuled with Cu were used in cell (II). The diaphragm was not employed in these measurements. 3. RESULTS
AND
times checked by shorting the cell for about IO s. The emf always returned to the original value within
a minute. The results
given indicate that the O,-(Pt) electrode and reversible reference electrode in Na,AlF‘,-Al,O?,,,-melts when it is surrounded by an stAIZO, diaphragm as described above. is
a
reliable
The en?f of cell (IV) was independent of which of the two types of diaphragm was used. This result indicates that neither dissolved metal nor dissolved oxygen have any influence on the Oz(Pt) and Al-clectrade reactions respectively. However, a drastic drop was observed in the cell voltage when the tube-type diaphragm was lifted somewhat so that free convection between the electrode compartments occurred.
DISCUSSION
The rrnf of uell (II) between identical Al-electrodes was usually within 0 + 2 mV during the runs which lasted for several hours at constant tcmpcraturc. The activity of Al in liquid Al-Cu alloys with molar ratio xAI = 0,66 and xA, = 0,49 was found to be 0.53
Fig. 2. EMJ’ as a function of the parrial pressure of oxygen for cell (Ill) at 1271 K. The straight line is the theoretical one according to equation (2).
The NaF-AlF,-AI,O,-NaZO
591
system-1
tion (1) can be attributed to the aluminium electrode compartment. This assumption is reasonable since we know that there is some metal solubility in the melt. The following solution equilibria may be suggested 2Al 22l
> ._
2.1’
E
2.1,
Fig. 3. Typical enlf-temperature data from three separate runs from cell (IV). The data are corrected for the thermal r~zgenerated between the different leads employed. Placing the diaphragm properly into the alumina sludge again the r~rfretained its original value within a few min. Introduction of oxygen into the Al-electrode compartment did not on the other hand have any noticeable influence on the emf of cell (IV). These results show clearly that it is the gas electrode which must be protected from action of dissolved metal. Typical emf-temperature data for cell (IV) from three different runs are shown in Fig. 3. Usually. a straight line plot was &tamed for each run. The data was recorded at a cooling rate of less than 1 K/min from about 130°K to the eutectic temperature of 1236 K. Emftemperature data obtained at a heating rate of about 1 K/min were practically identical with those described above, allhough Lhc: scatter of lht: points was somewhat larger. The slope of c,nlfltemperaturc plol for cell (IV) was found to be -0.55 t 0.02 mV/K which corresponds approximately to the derived JANAF value of -0.566 mV/K. The average value of the f>nf at 1273 K for cell (IV) from 15 runs was found to be 2.183 V with a standard deviation of about f 3 mV. The corresponding value derived from the JANAF Table is 2.195 V, which seems to be the correct value ab shown below. Before discussing the discrepancy of 12 mV we will describe some other experimental results. In 5 runs the (Pt)-0, electrode was replaced with an (Au)-0, electrode in cell (IV). Firstly, it is important to note that this replacement did not change the cmf‘ within the limits of cxperimcntal uncertainties. Secondly. the slope of the relationship between the wnf and the oxygen pressure at constant temperature for cell (IV) seems to agree fairly well with the theoretical slope derived from equation (2). These results are illustrated in Fig. 4 for both types of oxygen electrodes combined with the Al-electrode. Although no definite conclusions can be drawn these results indicate that the failure to reach the correct emf for reac-
+
AIFS =
3AlF
(3)
Al + 3NaF
=
AlFS
+
~Nz+,,~,,
(4)
Al +
= AIF,
+
3Na,F.
(5)
6NaF
Although the nature of dissolved metal is unknown the net effect will be an increased activity of aluminium fluoride and a reduced activity of sodium fluoride in the Al-electrode compartment. This condition means that cell (IV) is not merely a simple formation cell as described by reaction (1). In addition there will be a small contribution in the end arismg from the different activities in the two compartments. In the followmg it is assumed that the transport number of the Na+ ion is equal to unity[lS, 161. The change in free energy when passing a current of six Furadays in cell (IV) from the Al-electrode to the O,-electrode can then he expressed by the following equation
where jr’ and f?’ are chemical potentials on the left and the right sides oi the cell. pAI and mz arc neglected since they are equal to zero. The en~f for the cell (IV) may from equation (6) be written E = where
EC.,,,,
E,,,,L. +
E%p,
(7)
rn?f’arising from the concentration is the standard rrnf for CI - Al,O,. (6) and (7) it is easily derived
is the
cell and E&,o, From equations
which should correspond to minus 12 mV as discussed above. This value is compatible with a change in the NaF/AlF, ratio Cram 3.0 to 2.9[15]. The solubility limit of aluminium in the melt can then be estimated from the change in the NaF/AlF3 ratio and
Cl.4
-‘-a Fig.
PO,
4. The relative change in emj as a function of partial pressure of oxygen at 1273 K for cell (TV)
the
592
A. S~ERTEN. S. HAUGEN and
Fig. 5. Standard rnzfdata as a function of the temperature (IPTS-68) for the formation of a-A120a from celI (V). The line corresponds to free energy data for a-A1203 taken fronl the JANAF Table[5]. reaction (4) which is used for the sake of simplicity. The total solubility at 1273 K is in this way catculated to be 0.19 wt “/uAl. This value may be somewhat too high, although recent measurements of the solubility in Na3AIF,-A120, melts show values ranging from 0.05 to 0.70 wt “/, Al[ll-131. In order to derive the real standard free energy of formation for CY-AI,O~ we decided to use cell (V) since it was believed that the metal sotubility was very small in this melt. The end‘ for this cell as a function of the temperature is shown in Fig. 5. The individual points are average values from several runs. The emf had to be measured at constant temperature in cell (V) in order to obtain reproducible results. Once a constant cell enzf was established for a given temperature, the set point of the temperature controller was changed to a new value and the sequence of measurements was repeated. To obtain reliable rr~$ values below approximately I 1OC”K it turned out to be necessary to polarize the electrodes with a small current for a few minutes before the open circuit rrnf was allowed CO stabilize. Data below 950°K are neglected since it was difficult to obtain reproducible results. The reason for this failure is not known. Despite some uncertainties as indicated above it IS believed that the ernf data glvcn in Fig. 5 are very accurate. The majority of the points in the figure are less than + 200 cal away from the line which corresponds to the standard free energy of formation of r-Al,O, in the JANAF TabIes[S].
4. CONCLUSION An Al-electrode constructed as described in present paper seems to be a reversible electrode
the in
K. HAMBER~
Na,AlF, melts saturated with a-A1,03. However, it reacts to some extent with the electrolyte resulting in an increased AlF, activity and a reduced NaF activity. (Pt)O, and (Au)O, electrodes enclosed into z-alumina tubes seem to be reversible and reliable electrodes in Na,AlF, -Al,O,-melts. Standard emf values for or-Al,O, are difficult to achieve in Na,AlF,-Al,Os melts. To avoid the influence of dissolved metal on the oxygen electrode the cell must be separated into two chambers. Consequently, the composition of the electrolytes will be different in the two compartments. This means that reliable standard eflf values for cr-Al,03 cannot be obtained without a correction term taking into account this difference in composition. In Li,AlF, melts saturated with A1F3 and cc-Al,O, it seems possible to obtain reliable rnd data for the formation of r-AI,OJ. In this case the melt reacts probably very little with the Al-elcctrodc. Ackl,awlrdg~nlent-We wish @an Council for Industrial financial support.
to thank the and Scientific
Royal NorweResearch for
REFERENCES
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