40
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978
(19) As one reviewer correctly points out, the introduction of the Qi2 parameter, or equivalently a variation of the Xi2 parameter with T , is only an empirical way of dealing with orientational order within the framework of the present Prigogine-Flory theory. This theory uses mixing rules (one-fluid theory) based on a random mixture, but the
A. G. Keenan and T.
R. Willlamson
major import of the present work is that the liquids are orientally ordered. A more fundamental reworking of the theory is no doubt desirable both from the point of view of thermodynamics and also the equation of state. Perhaps the analogyi5 between n-alkanes and isotropic liquid crystals may indicate a direction for future work.
The PlatinumlPlatinum OxideICarbonatelCarbon Dioxide Electrode at 350 O C in Fused Potassium and Sodium Nitrates' A. G. Keenan" and Thomas R. Williamson Department of Chemistry, University of Miami, Coral Gables, Florida 33 124 (Received November 8, 1976) Publication costs assisted by the Unlversity of Miami
A preoxidized platinum electrode has been found to respond reversibly to the electroactive species C02 and C032-in pure sodium and potassium nitrate melts at 350 "C. The electrode reaction was established to be P t + C032- = PtO + C 0 2 + 2e-. Standard potentials are reported for this system with respect to a silverlsilver nitrate reference electrode in the pure solvents. For studies involving the solute C032-,the system may be exposed to the ambient atmosphere which provides the necessary equilibrium concentration of C02.
Introduction Studies of the carbonate ion in molten nitrate solvents have produced considerable controversy. Early researchers* believed that the carbonate ion decomposed to form the supposedly stable oxide ion in the melt by the reaction CO,'--
CO, t 0 ' -
This view has persisted3 in spite of considerable evidence4 that the oxide ion is unstable and the carbonate remains undissociated up to 350 "C in nitrate meltsa5 Due to the stability of carbonate ion, the interpretation of an 02102-couple at a platinum electrode in nitrate melts containing this species must be revised. In the present work the electroactive species, the Nernst equation for the electrode reaction, and the standard potential have been determined for a platinumlplatinum oxide electrode in nitrate melts where carbonate has been added.
Experimental Section Reagent grade chemicals were used throughout. The KNOBand NaN03 were dried in vacuo at 130 "C for 24 h, then stored at 110 "C. K2C03 and Na2C03were dried at 110 "C for 24 h prior to use. AgN03 was kept in a desiccator containing silica gel. The gases C02 (Coleman grade), O2 (zero grade), and N2 (extra dry grade) were dried by passing through silica gel towers. A platinum wire with a platinum sheet (32 gauge) spot-welded to it served as the indicating electrode. The surface of this electrode was between 1.0 and 1.5 cm2 in area. A silver wire (99.9%; 18 gauge) served as the reference electrode material. A 100-mL three-necked Kimax flask containing 120 g of either K N 0 3 or NaN03 as the solvent formed the electrochemical cell. The electrodes were positioned in the side necks of the flask. This flask was then placed in a constant temperature bath6 using "Hitec" (a salt mixture) as the heat transfer medium. The solvent, upon fusion, was purged with dry N2for several hours to ensure dryness. The bath was maintained at 350 "C and its temperature was regulated by a Bayley Model 253 controller to fO.l "C. The temperature of the bath was measured with an 0022-3654/78/2082-0046$01 .OO/O
iron-constantan thermocouple which was calibrated against a platinum resistance thermometer. The platinumlplatinum oxide indicating electrode was prepared by immersion in approximately 50 g of fused KNOBat 400 "C for 1 h. This method produces about 15 layers of platinum oxide on the electrode surfaces7 The reference electrode consisted of a silver wire dipping into a 0.1 m solution of AgN08 in the proper solvent. The reference electrode was contained in a 10-mm glass sealing tube whose end was a Pyrex frit of fine porosity. The frit separated the two electrode compartments. It was partially sealed by flaming to reduce diffusion. The emf of the cell was measured to within 0.1 mV by the use of a Leeds and Northrup K-4 potentiometer with a Keithley 601 electrometer as the null detector. This model has an input impedence of 1014ohms on the millivolt range. After waiting several hours for the solute to dissolve, the emf was measured until a series of readings showed random fluctuation of about 1mV and no net drift. The last three to four readings were then averaged. Micropolarization tests, by unbalancing the potentiometer and reading the emf and null meter deflection, were peformed along with shorting the electrodes momentarily and monitoring their return to the equilibrium value. These tests confirmed the reversibility of the electrodes. The melts were saturated with mixtures of N2, 02,and C 0 2 of known composition. The flow rates of the gases could be individually varied. The mixtures were prepared by passing the gases from their individual cylinders through silica gel drying towers and calibrated flow meters into a mixing chamber and then through the melt a t atmospheric pressure. The exiting gas formed an atmosphere over the surface of the melt. Pyrex wool packed into the necks of the electrode vessel prevented back-diffusion of laboratory air. After several hours of saturation, a slow stream of the gas mixture was continued over the top of the melt throughout the emf readings. In some experiments, water vapor was passed through the melt. Runs were also made in which the melt was allowed to equilibrate with the ambient laboratory atmosphere after being purged with dry nitrogen. 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82,No. 1, 1978
PtlPt0lCO,2-ICO2 Electrode in Fused Nitrate Melts
1 ; I
220
170
200
30
I -3.0
I
I
I
-2.0
I
0.0
dioxide and oxygen. Sodium nitrate solvent.
Results The stability of the carbonate ion in molten NaN03 and KNOBa t 350 "C has been shown in a previous papern5A systematic study of the emf dependence on the concentrations of the electroactive species was comprised of carbonate variations in a fixed atmosphere of carbon dioxide and changes in the partial pressure of COz a t constant carbonate concentration. The ranges of concentration were 0.05 to 0.65 atm for C02 and 1.2 X to 2.4 X loy2 m for COZ-. Typical graphs are shown in Figures 1 and 2 plotted according to the Nernst equation. The PtlPtO electrode was anodic with respect to the silver. The data points were fitted to a least-squares line producing an average value of n in the Nernst slope 2.3RTJnF of 1.93 f 0.02 mV when C 0 2 was varied and 1.86 f 0.06 mV when carbonate was changed. A similar study in which the partial pressure of oxygen was varied at constant carbonate concentration and fixed C 0 2 partial pressure exhibited no change in emf within experimental error, thus eliminating this species from consideration in the electrode reaction. It is clear that the data fit a Nernst equation of the form
where E"' is taken vs. a silver ion concentration of 0.1 m in the reference electrode compartment. This standard potential also contains the Henry's law constant for the solubility of C02. As is commonplace in this field, partial pressures in atmospheres and molalities have been used in place of activities. The equation for the electrode reaction is therefore
+ CO,z-= PtO + C O , + 2e-
I
I
I
-0.5
-1.0
-1.5
l o g Pc0
Flgure 1. Carbonate additions in an atmosphere containing carbon
(2)
The standard potential of this reaction was determined from a series of data points (some 30 in all), in which the concentrations of C 0 2 and CO2- were both independently varied over their experimentally feasible range. The range
I
i
80
-1.0
log ( C 0 , 2 3
Pt
47
2
Flgure 2. Variation of carbon dioxide at fixed carbonate concentration. Sodium nitrate solvent.
m and for C02 to 2.0 X for carbonate was 0.5 X from 0.16 to 0.84 atm. Using eq 1 standard potentials of -469 mV in potassium nitrate and of -231 mV in sodium nitrate were found at 350 "C. The difference in potentials between sodium and potassium nitrate solvents (238 mV) may be confirmed from thermodynamic data. Extensive tables of free energy values are currently available8 and the standard potential may be determined from the free energies of formation at 350 "C of the components of the electrode reaction after correcting for the concentration units of molalities used for carbonate. The correction applied to the free energies of formation is as follows -
A G = AG," t R T l n X
I
where X is the mole fraction corresponding to the molal concentration of the solute. For reaction 2 referenced to a AglAg' (0.1 rn) electrode, a potential of -515 mV results in KNOBand -267 mV in NaN03 for a difference of 248 mV. While these potentials are quite close to the values found experimentally, only the difference in potentials is significant since the free energy of PtO has been estimated. The Pt(02electrode has now been experimentally determined to respond to the C 0 2 impurity rather than the oxygen gas as previously ~ u r m i s e d . ~Figure ? ~ 3 shows the response of the electrode in an atmosphere of 02.Recent results in nitrogen may also be explained on the same basid. The standard potentials for carbonate variations in oxygen and nitrogen atmospheres are shown in Table I. Also listed are the impurities in these gases found by Fernandezg by a BaC03 precipitation technique. Response of the electrode was found to be erratic in pure C 0 2 producing a slope of between 1 and 2 electrons. The PtlPtO electrode may also be used in carbonate containing nitrate melts without the elaboration of preparing an accurate COz partial pressure, by exposing the melt to the ambient atmosphere. Keenan and FernandezlO atm of C02in recirculated have found a value of 1.33 X
48
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978
440
420
A. G. Keenan and T.
R. Williamson
r '1 1 Goo
1
P
580
I
I
I
-2.0
-3.0
I
i I
-1.0
460
I
'
-2.0
l o g (C0,23
Flgure 3. Carbonate additions in an atmosphere of pure oxygen. Sodium nitrate solvent.
TABLE I: Comparison of the Standard Potentials for Carbonate Additions in Air, Oxygen, and Nitrogena Obsd std CO, impurity, potential, Std potential Gas atm mV using eq 6 Potassium Nitrate E" = -469 mV Air 1.3 x 10-3 - 642 - 464 Oxygen 2.7 x 10-5 - 767 - 485 Nitrogen 1.8 x -775 - 482 Air Oxygen Nitrogen Argon
Sodium 1.3 x 2.7 x 1.8 x
Nitrate E" = - 231 mV 10-3 -409 - 231 10-5 - 500 - 218 10-5 - 556 - 262 -557
a Calculation of expected standard potentials based on
carbon dioxide impurity in these gases.
laboratory air and this would lead to a standard potential of -647 mV in potassium nitrate and -409 mV in sodium nitrate. Experimental values of -642 and -409 mV were obtained. Figure 4 shows a typical run with the melt open to the ambient atmosphere. \
Discussion The PtJOzelectrode, known as the oxygen electrode, has found a great deal of use in recent years in molten nitrate solvent^.^*^ This electrode has brought about considerable controversy in regard to its mechanism of operation. The electrode has been thought to respond due to dissolved oxygen embedded in the surface of the electrode by some workers.ll However a recent trend in the literature has been toward the presence of oxide layers on the electrode ~ u r f a c e . ~With the newer experimental techniques available today, a large body of evidence for the presence of these surface oxides has arisen. Anson and Lingane7b and Every and G r i m ~ l e yhave ~ ~ chemically stripped the oxide layers from platinum and analyzed them spectrophotometrically, while Shibata12 has provided direct evidence for their existence from studies with an electron
I
I
-3.0 log
I
2e-
I -1.0
(co,z-)
Flgure 4. Carbonate additions in air. Potassium nitrate
solvent.
microscope. Arvia and co-workers13 have studied the mechanism of formation of the oxide and determined that it is due to the oxidizing nature of the nitrate melt, involving the nitrate radical. Because of the lack of response of the electrode used in the current study to oxygen, the electrode must provide the necessary source of oxide for the electrode reaction. Janz and co-workers14have studied the oxidation of the platinum electrode in carbonate melts at higher temperatures than in this study (600-800 'C) and found that oxygen was evolved at the platinum surface. They suggested that the mechanism involved first the formation of platinum oxide on the anodic surface which was later evolved by the reaction MO t O * - = M
+ 0 , t 2e-
The present results show that the PtlPtO electrode is a stable reproducible system responsive to Cot- and C 0 2 in fused KN03 and NaN03 solvents. If Cot- is the only electroactive species of interest, the cell may be used open to the atmosphere using an E"' value appropriate to the particular ambient atmosphere present. Variations in atmospheric humidity have no effect. The electrode reaction for both solvents is given by eq 2 with the corresponding Nernst expression eq 1. The erratic behavior in pure C02 may well be due to some complexing of the PtO with the COz gas in the melt in a manner similar to that observed by Fernandez9 for the Cu(Cu0 electrode, However it is more likely that a different electrode reaction is competing with the one proposed. Zambonin15 has observed the reaction C 0 3 2 -+ NO,-= CO,
+ NO3- + 2e-
taking place in nitrate melts at a platinum rotating disk electrode when the partial pressure of C 0 2 is greater than about 50%. Since this reaction has a higher standard potential than reaction 2, its competition would lead to a higher slope than expected, On the basis of potential differences between the two melts, this reaction cannot be the one taking place at lower COz partial pressures, since
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978 49
Thermodynamic Study of the Gaseous Molecule CuGe2
References and Notes
the difference involved is 339 mV or nearly 100 mV higher than should be observed if this were to be the reaction observed. The only previous work available for comparison in pure solvents is that of Shams El Din.16 He has reported a standard potential for carbonate additions at a PtlOz electrode in molten potassium nitrate of 796 mV. From Table I, the standard potential in this solvent in an atmosphere of oxygen is 767 mV vs. a silver reference electrode containing 0.1 m AgNO,. Shams El Din used a reference concentration of 0.1226 m which should cause his results to be 11 mV higher than those in this work. When corrected for this difference, his potential becomes 785 mV which is within 2.4% of the value found in this study. It is evident that both electrodes are responding to the COz impurity in the oxygen gas and that previous works involving the use of the oxygen electrode must now be revised to account for this newly discovered response of the platinum electrode. Similar results found by Schlegel17 in nitrogen and confirmed in this paper show that the electrode is responding to the COz impurity in this gas also. In a more recent paper, Desimoni, Sabbatini, and Z a m b ~ n i n working ,~~ in an equimolar NaN03/KN03 mixture, have reported results which are totally in accordance with the above conclusions.
(1) This work comprises part of the Ph.D. Dissertation of TRW and was supported in part by the Office of Naval Research, Material Sciences Division, Power Program. (2) R. N. Kust, Inorg. Chem., 3, 1035 (1964). (3) (a) M. Fredericks and R. B. Temple, Aust. J. Chem., 25, 2319 (1972); M. Fredericks and R. B. Temple, Inorg. Chem., 11, 968 (1972). (4) (a) J. Jordan, J . Electroanal. Chem., 29, 127 (1971); (b) P. G. Zambonin, ibid., 45, 451 (1973); (c) E. Desimoni, L. Sabatini, and P. G. Zambonin, ibid., 71, 73 (1976). (5) A. G. Keenan, C. G. Fernandez, and T. R. Williamson, J. Electrochem. Soc., 121, 885 (1974). (6) A. G. Keenan, K. Notz, and F. L. Wilcox, J. Phys. Chem., 72, 1085 (1968). (7) (a) F. C. Anson, Anal. Chem., 33, 934 (1961); F. C. Anson and J. J. Lingane, J. Am. Chem. Soc.,79, 4901 (1957); R. L. Every and R. L. Grimsley, J. Electroanal. Chem., 9, 165 (1965). (8) "JANAF Thermochemical Tables", The Dow Chemical Company, Midland, Mich., 1966. (9) C. G. Fernandez, Ph.D. Dissertation, The University of Miami, 1974. (10) A. G. Keenan and C. G. Fernandez, J. phys. Chem., 78, 2670 (1974). (1 1) J. P. Hoare, "The Electrochemistry of Oxygen", Interscience, New York, N.Y., 1968. (12) S. Shibata, Electrochim. Acta, 17, 393 (1972). (13) M. G. Sustersic, W. E. Triaca, and A. J. Arvia, Electrochim. Acta, 19, 1 (1974). (14) (a) G. J. Janz and F. Saegusa, Nectrochim. Acfa, 7, 393 (1962); (b) G. J. Janz and F. Saegusa, J. Nectrochem. Soc., 108, 663 (1961). (15) P. G. Zambonin, Anal. Chem., 44, 763 (1972). (16) A. M. Shams El Din and A. A. El Hosary, J . Electroanal. Chem., 9, 349 (1965). (17) J. M. Schlegel and R. Bauer, J . Chem. Soc. D , 483 (1971).
Thermodynamic Study of the Gaseous Molecule CuGeP J. E. Kingcade, K. A. Gingerich," and U. V. Choudaryt Department of Chemisfry, Texas A&M University, College Station, Texas 77843 (Received June 8, 1977) Publication costs assisted by the National Science Foundation
The high-temperature Knudsen effusion mass spectrometric technique has been used to measure the third law enthalpies of the reactions CuGe2(g)+ Cu(g) = 2CuGe(g)and CuGez(g)= Cu(g) + BGe(g),assuming different structures for the CuGezmolecule. The resulting reaction enthalpies for the preferred bent structure, together with appropriate ancillary literature data, yielded an atomization energy, AHaoo, of 506.0 f 25 kJ mol-' and a standard heat of formation, i v I f o 2 9 8 , of 580.8 f 28 kJ mol-' for gaseous CuGez.
In keeping with our current investigational interests' of polyatomic group 4 aurides, the examination by the Knudsen effusion mass spectrometric technique of the Ge-Au-(Cu, 0.02 mole fraction) system was undertakena2 Along with the gaseous diatomic molecule CuGe, which has been reported previously in literature,,14,the identification of the previously unreported triatomic molecule CuGez(g) was accomplished as part of the above study. The discussion to follow is a report of its stability under equilibrium conditions.
The species were characterized by their mass to charge ratio, normal shutter profile, and isotopic distribution. Germanium species, Ge through Ge4, and gaseous polyatomic germanium-gold molecules containing up to five atoms were also observed and identified.2 Information concerning the bond energy of the new molecule CuGe2(g) was derived from the measured = -RT In K , third-law enthalpy using the relation, Moo - TA[(GoT- H o o ) / T ]for , the pressure independent reaction
Results and Discussion
CuGe, (9) + Cu(g) = 2CuGe(g)
(1)
The Knudsen-cell mass spectrometer and the experimental procedure used have been discussed e l ~ e w h e r e . ~ , ~ and for the pressure dependent reaction A knife-edge orificed tantalum Knudsen cell, charged with CuGe,(g) = Cu(g) + 2Ge(g) (2) semiconductor grade germanium, gold, and copper within The free-energy functions necessary for the evaluation a graphite liner in a 0.49, 0.49, and 0.02 atom fraction was of enthalpy values for the molecules CuGe(g) and CuGez(g) employed in this study. The observed gaseous species used were calculated using reported and/or estimated molecular in the following evaluations are Cu+, CuGe+, Ge+, and parameters as described below. The free-energy and CuGez+. Measured ion currents are reported in Table I. enthalpy functions for Ge(g) and Cu(g) were taken from Hultgren et al.7 Those for CuGe(g) were calculated using Present address: Department of Materials, University of Wisconsin-Milwaukee, Milwaukee, Wisc. 53201. the same molecular parameters (a= 314 cm-l, re = 2.39 0022-3654/78/2082-0049$01 .OO/O
0 1978 American Chemical Society
