Jan., 1961
THERMODYNAMIC STUDIES OF THE SYSTEM PLUTONIUM CHLORIDE-POTASSIUM CHLORIDE81
SOME THERMODYNAMIC PROPERTIES OF THE SYSTEM PuCla-KCI FROM ELECTROMOTIVE FORCE DATA' BY R. BENZ Los A Eamos ScientiJic Laboratory, University of California, Los A l u m , New Mezico Received June 16, lQ60
The molar free energy of formation of lutonium(II1) chloride as a function of temperature (650 to 800') and of composition in liquid PuC4-KCl solutions was itermined from e.m.f. measurements made on reversible galvanic cells of the type Pu(1iq) (PuC&-KCl(liq)l ClZ(g). The free energy of formation of pure supercooled liquid plutonium( 111)chloride at one 0.03858T(kcal./mole)(958 to 1014°K.). The standard free energy of atmosphere can be represented AFlo = -206 formation of solid :plutonium(III) chloride can be represented AFIO = -221 0.05328T(kcal./mole)( 958 to 1014OK.).
+
+
Introduction I n the development of plutonium power reactors, reprocessing of :spent fuels is a vital problem. For economic reasons, reprocessing systems containing liquid alloys or liquid salt solutions a t elevated temperatures, if; of special interest. Free energies of formation of' plutonium compounds and their activities in the solutions involved are indispensable aids in the prediction and the development of possible feasible processes. From a determination of the heat of solution of plutonium metal in aqueous hydrochloric acid and the heat capacities estimated from similar substances, Brewer, Bromley, Gilles and Lofgren2have calculated the free energy of formation of plutonium(II1) chloride. A determination of the free energy of forma,tion using an independent method such as that of e.m.f. measurements seems desirable. Some of the major experimental difficulties encountered in obtaining e.m.f. measurements suitable for reliable determination of the thermodynamic quantities of formation of the general compound MX, from galvanic cells of the type M(liq ) IMX,-NX,(liq)l
Xdg)
are (1) electronic conductivity of the electrolyte, (2) non-unique oxidation state of the ions and (3) mutual dissolution of the metal and electrolyte. It is supposed that the liquid metal and electrolyte are non-volatile and that the compound NX,, if present, is appreciably more stable than MX,. This paper presents the results of a study on the galvanic cells represented by Pu( liq) IPuC13-KCl(lis)/ C1&)
(1)
for which the cell reaction is the formation of plutonium(II1) chloride in liquid PuC13-KC1 solutions, ie., the reaction Pu(1iq)
+ 3 Clz(g) = PuClj(liq, XI)
(2)
Plutonium(II1) chloride is ionic and the first effect listed above is negligible. I n the region of low concentrations and in the presence of chlorine gas, an appreciable prioportion of the plutonium(II1) chloride is converted to a second oxidation state. Only data for the solutions in which the concentration of the second oxidation state is negligible-the region of high plutonium(III) chloride concentra(1) Taken from a dissertation submitted in partial fulfillment of the requirements for the Degree of Dootor of Philosophy in Chemistry at the Univeraity of New Mexico. (2) Brewer, L. I3romley, P. W. Gilles and N. L. Lofgren, "The Transuranic Elementr," National Nuclear Energy Series, Div. IV, Vol. 14B,McGraw-Hill Book C o s ,New York, N. Y , 1949, p. 873.
L.
tions-can be interpreted quantitatively. Although the third effect, the mutual solubility of plutonium metal and PuClrKCl solutions, is appreciable, it was found to be possible to make the rate of dissolution negligible with the use of an inert porous partition to separate the electrode and electrolyte. Indeed, for the cell (l),an electrode design incorporating a thoria partition with carefully controlled porosity was found to satisfy for periods as long as 13 hours, conditions (see Procedure) necessary for a reversible cell.
Experimental Materials.-The plutonium( 111) chloride, potassium chloride, hydrogen chloride and ar on have been described previous1y.a Chlorine gas (Makeson Co., 99.3% pure) was given no treatment. The pressure of the chlorine gas, which waa measured with an accuracy of f l mm. in the range 588 to 598 mm., was corrected to one atmosphere by assuming the gas to be ideal and adding ( - R T In p ) / 2 to the observed e.m.f. values. The chemical composition of each solution (ca. 60 g. total weight) waa based upon the composition by weight of the component salts. Apparatus.-The electrolyte of the galvanic cell proper waa contained in the elbow of a 30-cm. long 1.4-cm. bore J-shaped quartz tube. During a run, the elbow of the Jshaped tube waa immersed in a liquid metal bath, within a wire wound furnace to ensure uniform temperatures. In order to protect the cell from stray currents, the liquid metal bath in a uartz container was shielded from the furnace by a grounjed stainless steel tube. The longer arm of the J-shaped tube waa mounted in a Teflon stopper so aa to center the cell with the electrolyte about 13 cm. below the surface of the liquid metal bath. The chlorine electrode waa constructed of a 30-cm. long, 6.0 mm. diameter spectroscopic graphite (National Carbon) rod. During an experiment, the electrode waa centered in the long arm of the J-shaped cell by means of a tapered joint a t the top such that chlorine gas flowing through a 1.5mm. hole drilled along the axis of the graphite rod continually bubbled over the end of the graphite rod submerged in the electrolyte. Prior to an experiment, the graphite electrode was treated with chlorine gas a t a pressure of one atmosphere for a period of 2 to 8 hours a t 1000". The plutonium electrode located in the second arm of the cell consisted of approximately 3 B. of liquid plutonium metal contained in a 0.43-cm. inside diameter, 0.75-cm. outside diameter and 1.6-cm. long porous thoria crucible centered in the bottom of a 0.8-cm. bore quartz tube which was supported from above. The quartz tube waa constricted a t the bottom so as to support and contain the thoria crucible. The thoria crucibles were fabricated from Norton 200F thoria powder (Norton Co.). They were dry pressed and fired for 13 hours a t 1750" in a hydrogen atmosphere. The porosity of the thoria crucibles was gauged by the weight per cent. of water absorbable by the crucibles. Values ran ed from 4 to 6%. When crucibles with lower values of atsorption were used, the cell exhibited a much higher electrical resistance and e.m.f. values usually were ( 3 ) R. Benz, Rl. Kahn and J. A. Leary. THISJOURVAL,63, I983 (1959).
82
R. BENZ
erratic. When crucibles with higher values of absorption were used, t,he e.m.f. values were time dependent. In its normal position, the bottom of the thoria crucible was located 1 to 3 mm. below the surface of the electrolyte such that, electrical contact between the plutonium metal and the elect,rolyte was made after the electrolyte had passed throuzh the constriction in the end of the quartz tube and the pores of the thoria crucible. The furnace assembly described above was located in a standard type stainless steel drybox for the protection of personnel against hazards of radioactivity (a-decay of plutonium). The external circuit to the galvanic cell and the thermocouple leads which passed through the wall of the drybox were constructed, respectively, of copper and platinum-rhodium extension lead wire. The external circuit was calibrated for thermoelectric e.m.f.'s which were plotted as a function of temperature to give a small linear correction to the observed e.m.f. values for the cells. A standard external potentiometric circuit was used to balance the e.m.f. of the galvanic cell. It included an arrangement for electrodeposition and electrodissolution ( L e . , elect,rolytic deposition on and dissolution of the electrodes) within the galvanic cell using a. double throw switch for reversing the current from a storage cell. The temperature measurements were made using two Pt-lO% Rh thermocouples located in the bottom of a quartz well, submerged 13 cm. in the liquid-metal bath so as to be adjacent to the galvanic cell. One thermocouple was connected to a Honeywell recorder and served to indicate approximate temperatures. The second thermocouple used for the precis. temperature measurements and checked ( 5 0 . 4 " ) against the melting point of aluminum (cert.ified by the National Bureau of Standards) before and after the potentiometric measurements, was connected to another thermocouple at 0" and a K-2 potentiometer (Leeds and Northrup Co.) standardized against a calibrated Weston Cell. Procedure .-To begin an experiment the desired quantities of salts were well mixed by bubbling hydrogen chloride gas through the melt contained in the assembled cell. The graphite electrode with a chlorine-gas flow rate of approximately 5 cc. /min. then was introduced and centered so that the tip of t'he electrode was located a t a depth of 2 to 6 cm. below the surface of the electrolyte. The metal electrode finally was introduced and adjusted carefully to the desired depth and t,he potentiometric measurements were begun. An inert atmosphere in the region above the liquid metal bath was provided by a continuous flow of argon gas. Measurements were repeated every 15 minutes until a steady voltage was obtained. I n t>hetemperature region 700 to 800" one to two hours were required for the voltage t o reach steady and reproducible values. A single measurement usually extended over a period of ten minutes. During this period the e.m.f. reading was verified to be reproducible after applying for two seconds an elect,rodepositing current of 15 ma. at 2.5 v. and, again, after applying for two seconds an electrodissolving current. After a measurement was completed, the furnace temperature was shifted 10 to 30' increasing and decreasing at alternate measurements such that alternate equilibrium temperatures would be approached from above and below. Such a series of potentiometric measurements was carried out on each of the liquid solutions within the temperature region 646 to 741' for the plutonium(II1) chloride mole fractions of 0.800 and 0.600. The cells remained reversible over a period from 6 to 13 hours. Galvanic cells containing pure plutonium( 111)chloride did not behave reversibly.
Vol. 65
for electrolyt,ic solutions at the mole fractions XI = 0.600 and 0.800. i4fter having been corrected to that of one atmosphere of chlorine gas (corrections ranged from 0.010 to 0,011 v.) and for thermoelectric effects in the external circuit (corrections ranged from 0.005 to 0.007 v.), these data were represented (least squares fit) by linear equations of the form EX, =
ax,
+ bxlT
(3)
where axr and bx, are constants for the solution of plutonium(II.1) chloride mole fraction X1 and T denotes the absolute temperature. The results are given analytically in Table I. Included in Table I are the dat,a for pure plutonium(II1) chloride in the supercooled liquid state. These data were obtained by means of an extrapolation described below. E.M.F. DATAAS FORMATION
XI
OF
TABLE I FUNCTION OF TEMPERATURE FOR THE PUC& I N THE LIQUID BINARY SYSTEM PuCIa-KCl A
Max. dev., v.
E.m.f., v.
Temp. range, OK.
No. e.m.f. detns.
1.000 2.980 - 0.5576 10-3T 0.800 2.990 - 0.553 10-*T S 0 . 0 0 4 958 to 1014 12 0.600 3.056 - 0.5439 10-3T A0 , 004 919 to 1010 10
The calculation of the thermodynamic quantities involves, first, extrapolation of the e.m.f. data to the state of pure plutonium(II1) chloride a t 973'K. in order to obtain the thermodynamic quantities of formation of the supercooled liquid salt. From these results, the partial molar mixing quantities for liquid plutonium(II1) chloride are computed. Finally, using these and other data taken from the literature, the standard formation quantities of solid plutonium(II1) chloride are calculated. Further details of the calculations are given below. The free energy (AFl), entropy (As,) and enthalpy (AH1)of formation of plutonium(II1) chloride in the liquid PuCI3-KC1 solutions a t 973OK. and one atmosphere for XI = 0.600 and 0.800 were computed using the formulas AFi = -3Fex1
(4) (5)
AH1 = AFI
+ 973Asi
(6)
The results are listed in Table 11. The data :it the mole fraction X1 = 1.000 represent that for the formation of pure supercooled liquid plutonium(II1) chloride. These data were obtained from that for the solutions of plutonium(II1) chloride mole TABLE I1
Results bIOL4R THERMODYNAMIC QUANTITIESO F FORMATION OF LIQEID PUC1; .4T 973°K. Calculations.-The notation employed by WagAFi, Asi. AH], ner4 for the thermodynamic mixing properties is e u. kcel. XI kcal. followed. Subscript 1 denotes liquid plut,onium-38 6 -206 1 000 - 169 (111) chloride. X1 denot,es the mole fraction of -38 3 -207 0 800 - 170 plutonium(II1) chloride. The reference stat'eis that -37 6 - 212 - 175 0.600 of supercooled liquid plut,onium(III) chloride a t fractions XI = 0.800 and 0.600 by means of an 973'K. and one atmosphere. The e.m.f., denot,ed e, was determined as a extrapolation a5 a function of composition. For function of temperature in the region 646 to 741' the purpose of extrapolation, it was assumed that are constants for the functions Fl"E/X2''2 and (4) C. Wagner, "Thermodynamics of Alloys," Addison-Wesley the binary system PuCls-K2PuC15 over the region of Preas, Inc., 1952. Chap. 1.
Jan., 1961
THERMODYNAMIC STUDIES OF THE SYSTEM PLUTONIUM CHLORIDE-POTASSIUM CHLORIDE 83
KzPuCls mole fraction Xz" = 0 t o 0.333. F1'IE and SlrlEdenote the excess relative partial molar free energy and entrlopy, respectively, of mixing supercooled liquid plutonium(II1) chloride for this system. Thus, the entropy of formation of supercooled liquid plutonium(II1) chloride was taken to be the arithmetic avera,ge of the excess entropy of formation of plutonium(II1) chloride, i.e., AS,'' R In XI", a t the KPPuC16 mole fractions 0.125 and 0.333. AS1" denotes the molar entropy of formation of plutonium(II1) chloride for this system. In solutions containing high concentrations of plutonium(111)chloride, it is expected that the mixing quantities for the system K2PuClS-PuC13may approximately conform with that of regular solutions (hence, constant F 1 ' r E / X 2 1and 12 because there are no intermediate compounds. The enthalpy of formation of purl. supercooled liquid plutonium(II1) chloride W B S obtained using the relation
+
AH,
=
A F i f 973AS1
TABLE I11 PARTIAL MOLARTHERMODYNAMIC QUANTITIES OF MIXINQ LIQUIDPUClz FOR THE SYSTEM PuClrKCl AT 973°K FiM,
1.000 0.800 0.600
Hi,M
SlM,
kcal..
x 1
kcal.
e.u.
0 0
0.0 +0 3 +0.9
-1
-6
a1
00
1.00 0.59 0.040
-1
-5
The standard molar free energy, entropy and enthalpy of formation of solid plutonium(II1) chloride at) 973OK. and one atmosphere are given in row (c) of Table IV. These quantities were obTABLE IV THE CALCULATION O F THE STANDARD MOLARTHERMODYNAMIC QUANTITIES OF FORMATION OF SOLIDPuC13 AT 973" K. AND ONEATMOSPHERE AF,
AS, e.u.
AH, kcsl.
Reaction a t 973'K.
kcal.
3 a. Pu(1iq) -I- ;C12( 9) = PuC13(liq)
- 169
-38.6
-206
f
+14.7
+ 15
-53.3
-221
b. PuCla(s) = PuCla(liq) 3 c. Pu(1iq) -t $ & ( g ) = PuCll(s)
1
-170
tained by subtracting the chemical equations (a) and (b). The data for reaction (b), the melting of solid plutonium(1II) chloride a t 973OK., were computed with the aid of the approximat,ions
AF =
h:
AH
=:
A S m , ACp
A S d2"
= AS,(?",
C ~ O)
Cp(s)
- 973)
A F f 973AS = T , AS,
+
(eM),
The relative partial molar free energy entropy (SIM) and enthalpy (HIM)of mixing plutonium(II1) chloride at 973OK are listed in Table 111. The corresponding activities are given in column 5.
Fi
for the purpose of reducing the rate of mutual dissolution of the plutonium electrode and the electrolyte. Experimentally, when a liquid plutonium electrode was exposed directly to liquid PuClg KC1 solutions in the cell, the e.m.f. value dropped rapidly and continuously from that of a reversible cell. This behavior was interpreted to result from the interdiffusion of the metal and salts during the dissolution of the plutonium(II1) chloride in the metal and vice versa. Assuming this to be the major irreversible process, the galvanic cell was designed with the aim of reducing the rate of mutual dissolution-in principle, the objective is to extend the period of time during which the virtual half-cell reaction a t the metal electrode is essentially P u (pure liquid) = P u + (liq. ~ XI) 3e. To this end a thoria wall in the form of a crucible containing the liquid plutonium electrode was used, the function of which was to set up a steady transition interface between the metal and salt phase such that the net virtual cell reaction would be essentially that indicated in equation 2. The fact that the cross section of the used thoria crucibles always had a uniform green color approximately the same shade as the electrolyte-in contradistinction to the black solutions of plutonium dissolved in the electrolyteindicates that the transitional interface probably was retained near the inner surface of the crucibles. The necessary conditions for reversibility, viz., that the e.m.f. be independent of time and independent of the direction of approach to equilibrium with respect to temperature and chemical reaction ( i e . , reproducible after passing a small electrodissolving and electrodepositing current through the cell), were satisfied by all the galvanic cells. This is evidence that the thoria crucibles did function in the manner described above, as well as that the cells were reversible. Chemical analyses of the electrolytic solutions taken from the galvanic cells at the conclusion of each experiment yielded a chloride to plutonium ratio of 3.0 i 0.1. This shows that the possible concentration of a second oxidation state of plutonium m s not greater than 10% and it is concluded that effects of multiple oxidation states on the derived formation quantities of plutonium(II1) chloride is negligible. The effects on the mixing quantities which represent small differences of large quantities may be more pronounced. The potentiometric measurements for each of the electrolytic solutions were made with a precision of within 4 mv. These data were reproducible over a period of several hours during which the temperature of the galvanic cells was varied approximately 75'. The e.m.f. readings were made with an accuracy within f. 0.1 mv. Contributions to error in the e.m.f. measurements from errors in the temperature measurements were within f 1 mv. Corrections for thermoelectric effect in the external circuit were made by means of a calibration. The chlorine gas was corrected to its standard state of unit fugacity assuming the ideal gas law. The contributions to the total error in the e.m.f. measurements from these sources are less than the precision within which the measurements were made.
(7) (8) (9)
where AS, is the entropy of melting plutonium(111) chloride a t the melting point, T , = 1042OK. Reliability and. Discussion of the Data.-An important feature i n the design of the galvanic cell is the incorporation of an inert-porous-thoria partition
*
84
BENTON B. OWENAND PAULL. KRONICK
The entropies of formation of liquid plutonium (111) chloride in the various solutions are estimated to be reproducible within 8%. A source of uncertainty in the calculation of the free energy of formation of pure plutonium(II1) chloride stems from the extrapolation of the data for the solutions a t 700' to the state of pure supercooled liquid plutonium(II1) chloride. The extrapolation involves a relatively small correction, uiz., 0.5%. Discussion.-The relative partial molar free energy of mixing the plutonium(II1) chloride has large negative values which is consistent with the compound formation observed in the study of phase equilibria. The value of the standard molar free energy of formation of pure plutonium(II1)
Vol. 65
chloride a t 700' was determined to be -170 kcal. which is 4a/, more positive than that estimated by L. Brewer, L. Bromley, P. W. Gilles and If. L. Lofgren.2 Acknowledgments.-I wish to thank J. A. Learv and R. D. Baker of the Los Alamos Scientific LabGratory and Milton Kahn of the University of New Mexico for discussions and encouraging interests. I am indebted to A. N. Morgan for the plutonium, J. W. Anderson for the machined plutonium electrodes, C. F. Metz, G. R. Waterbury, C. T. Ape1 and A. Pullium for chemical analyses, and S. D. Stoddard for the thoria crucibles. This work was done under the auspices of the U. S. Atomic Energy Commission.
STANDARD PARTIAL MOLAL COMPRESSIBILITIES BY ULTRASONICS. 11. SODIUM AND POTASSIUM CHLORIDES AND BROMIDES FROM 0 TO 30°1 BY BENTON B. OWENAND PAULL. KRONICK~ Contribution No. 1614from the Sterling Chemistry Laboratory, Yale University, New Haven, Conn. Received June 83, 1060
The velocity of sound in pure water and in dilute aqueoue solutions of sodium and potmsium chlorides and bromides is reported at 5' intervals from 0 to 30°, and at a frequency of 5 megcy./sec. The isothermal partial molal compressibilities of the salts at infinite dilution, are calculated, and found to increase (become more positive) with increasing temperature. the course of the measurements, so that the variation of the Introduction velocitx of sound with concentration waa obtained "isotherThe first paper of this series3 contained a de- mally. The absolute value of the temperature was, howscription of the movable-reflector acoustic inter- ever, not known to much better than 0.01" because a disferometer and the experimental technique by which cordance of 0.004" developed between the calibrations of two of our three platinum resistance thermometers while the sound velocities were measured, and the results measurements were in progress. of these measurements on solutions of sodium and Since our apparatus was designed for precise mertsurement potassium chlorides were used to illustrate the of changes in velocity rather than absolute magnitudes, the evaluation of R20 for these salts a t 25'. The instrumental constant (a function of the angles formed by the screw and the guide rods) was evaluated from the known present paper reports the results of more extensive velocity of sound in pure water at 30". For this standard of measurements on solutions of these salts, as well reference, u~(30'), we used 1509.55 m./sec., the mean of as sodium and potassium bromides, a t 5" intervals 1509.44 reported by Greenspan and Tschiegp from the National Bureau of Standards, and 1509.66 reported by Wilfrom 0 to 30". soneb from the United States Naval Ordnance Laboratory. Experimental The instrumental constant at lower temperatures was calIn the preparation of the sodium and potassium chlorides, Analyzed C.P. grade salts were used without further purification, but the bromides were recrystallized once from distilled water. The concentration of each solution used in a velocity measurement waa determined by withdrawing a portion from the interferometer and performing a differential potentiometric titration' against a 0.07 N solution of silver nitrate. This silver nitrate solution w m standardized periodically against a thoroughly dried sample of sodium chloride purified by the method of Meites.6 The operation of the interferometer and associated electronic equipment followed closely the procedure previously outlined,a and need not be described again. All velocity measurements were made at 5 megacycles ( f 5 cycles) per second. Temperature wrts controlled to f0.002" during (1) This communication contains material from a thesis presented by Paul L. Kronick to the Graduate School of Yale University in partial fulfillment of the requirement. for the degree of Doctor of Philosophy, June, 1957. (2) The Franklin Institute, Philadelphia 3, Pa. (3) B. B. Owen and H. L. Simona, THIEJOURNAL, 61, 479 (1957). (4) N. F. Hall, M. A. Jenaen and 9. A. Baeckstrom, J . Am. Chem. SOC.,50, 2317 (1928). (5) L. Meites, J . Chsm. Ed., 29, 74 (1952).
culated from the value a t 30" and the coefficient of linear expansion of stainless steel (10-6 deg.-l).
Results of the Velocity Measurements For each salt a t a given temperature, the velocity of sound was determined in from seven to fourteen solutions whose composition ranged from 0 to 0.07 normal. The results of these measurements could be represented within the limits of their reproducibility ( f0.02 m./sec.) by the equation u = u~
+ A,c + B,c'/r
(1)
The coefficient A , is the quantit which contributes directly to the desired value of or $KO, through the equation
to,
+KO
= lOOOAa
+ 6o+v0 + (2$v0 - Mz/& - 2000
Au/Uo)Pos
(2)
in which &, is the adiabatic compressibility of (6) (a) M. Greenspan a d C. E. Teohiegg, J . Research NaU. b u r . Standards, 59, 249 (1957): (b) W. D. Wilson, J . Acouat. SOC.A m . . 81, 10G7 (1959).
