May, 1963
ELECTRICAL CONDUCTIVITIES
O F SOLVTIOKS OF hfETALS I N T H E I R J I O L T E R ' HALIDES
helium and then slow cooled. From the resulting solid, clear crystals were hand picked for use. The capsules were filled and weighed in an inert atmosphere box containing an analytical balance. In each case the weight of material was approximately 10 g. The capsules were then taken in a closed chamber to another inert atmosphere box equipped with welding electrodes and welded shut under one atmosphere of helium purified by passing it through a liquid N 2 cooled charcoal trap. The loss of weight during the sealing process was negligible. The capsules were then tested for leaks by checking the capsule weight a t intervals. If constant weight was not reached within three hours, a leak was assumed and the capsule was resealed. Since the values desired are the differences between heat contents, i t is imperative that the measured heat contents for any set of mixtures be compared with those calculated from pure component heat contents which have been measured a t the same temperature. The controls on the furnace are manual and were adjusted such that the equilibrium temperature with zero drift was slightly above the temperature desired. When the capsule was pulled into the center nickel cylinder, the latter was cooled below the desired temperature. The system was then allowed t,o re-equilibrate and a t the moment the temperature drifted through a value preset on the potentiometer, the capsule was dropped into the calorimeter and the heat effect measured by the usual p r ~ c e d u r e . ~At drop time the temperature drift never exceeded 0.03" per minute. At least two nieasurements were made on each capsule and additional drops were made if these heat contents differed by more than 0.170. With such an operating procedure only minor deviations from a hxed temperature should result unless a gross error in the temperature measurement occurred. In these experiments an error did indeed occur after the heat contents of the blank, pure components and mixtures a t approximately 0.2, 0.4, 0.6, 0.75 mole fraction LiF had been measured. This was due to the inadvertent brushing of the hot couple against some rubber insulating material, contaminating the couple. After the contaminated section was removed, the couple was reinserted in new protection tubing and a standardization a t 875" repeated. The heat contents of the blank, pure components, and the rest of the mixtures were then measured a t a slightly different setting on the potentiometer. The two temperatures differed by less than 0.5" and it is evident from the consistency of the two sets of data t h a t any error due to possible temperature dependency of the heat of mixing is well within the over-all experimental uncertainty.
1145
Results The experinieiital results are presented in Table I and the molar heat of mixing AHM and the deviation function AHM/X(l - X) are plotted as a function of the mole fraction LiF in Fig. 1. The values of A H M are, as indicated previously, obtained from the average of a t least two measurements. The maximum deviation of a measured AHM value from the average mas 2.5% at 0 . 1 9 3 X ~ ~ Most ~. deviations, however, were lyGor below. The maximum heat of mixing (- 1210 cal.) confirins the value estimated by A ~ k r u s t . ~The plot of AHM/ X ( l - X ) clearly shows the energetic asymmetry of the system. Since it was obvious that the data could not be represented by a straight line, an expression of the form AH" = x(1 - X)[a bX -l-c x ( 1 - x)]
+
was used. The va,lues of the constants were calculated by the method of least squares to give
AHM = X(l - X)[-3878 - lO5lX - 152lX(l - X)] From this, it is possible by standard methods to calculate the following equations for the relative partial molal heat contents of the two components
EKF --X2~1p(1306-/- 8 1 8 6 x ~ 1-~ 4 5 6 3 X ' ~ ~ ~ ) IL,F = -X2Krr(4459 + 3 9 8 2 X ~-~ 4563X'K~) A more colr'iplete analysis of these data will be deferred until completion of measurements on additional alkali fluoride systems now under study. Acknowledgments.-The author is grateful t o Dr. Milton Blander of Atomics International for suggesting this approach to the determination of heats of mixing and for encouragement and discussions during the measurements.
THE ELECTRICAL CONDUCTIVITY OF SOLUTIONS OF METALS IN THEIR MOLTEN HALIDES. VI. LANTHL4NURiI,CERIUM, I'RASEODYJIIUM, AND SEODYMIUM I N THEIR MOLTEX IODIDES1 B Y A. 8.D W O R K I K R., ~A.~ S A L L A C H , 2 b H. R. B R O N S T E I AI, N , ~A.~ B R E D I GAND , ~ ~J. D. CORBETT'~ Chemistry Division, Oak Ridge iYationa1 Laboratory, Oak Ridge, Tennessee, and Department of Chemisfry and Institute for Bto~nicResearch, Iowa State LJniversity, Arnes, Iowa Received December 20, 1966 The electrical conductivities of solutions of the series La, Ce, Pr, and Nd in their molten triiodides are similar to those of the corresponding chloride systems, and thus likewise reflect the gradual increase in stability of the dipositive ion M2+. For example, in solutions containing 15 mole 70metal, the specific electronic contribution gsoln - K , , , ~ decreases from 22 and 25 ohm-' cm.-l for La and Ce, respectively, to 4 for Pr and -0 for Nd. The positive temperature dependence of the conductivity in the concentration range where electronic contribution is sizable indicates that there is thermal excitation of electrons from the solute. The phase equilibrium diagrams near the MI3 and MI2 compositions are discussed in terms of the solute species. The MI3 liquidus data are compatible with the postulated M2+ solute or its dissociation into M 3 +and anion-like electrons. The metallic character of the molten diiodides, which are metal-like (M3+(I-j2e-j in the solid state, appears t o decrease in the melt in the order La > Ce > Pr.
Introduction Rather svsbematic variations in the electrical conductivities Of Of La' lJr' and Nd (1) Work performed for the U. 9. Atomic Enerny Commission at the Oak
Ridge National Laboratory, operated by the Union Carbide Corporation, Oak Ridae, Tennrssee. ( 2 ) (a) Oak Ridge, Tennessee: (b) Arnes, Iowa.
in their respective trichlorides3m4 can be correlated with a regular trend observed in the phase diagrams of (3) (a) €1. R. Bronstein, A. S. Dworkin, and M. A. Bredig, J . Phys. Chhem., 6 6 , 44 (1962): (b) A . S.Dworkin, H. R. Branstein, a n d 31.A. Bredig, Dzscusszon6 Faraday Soc., 32, 188 (1961). (4) A. 8 . Dworkin, H. R . Branstein, and R1. A. Bredig, J . Phys. Chem., 6 6 ,
1201 (1962).
X.S.DWORKIN, R. -4.SALLACH, H. R. BRONSTEIX, l f . A. BREDIG, ASD J. D. CoLmwr
1146
22
20
c
Experimental The molybdemm parallel electrode assembly used to measure the conductivity of the metal iodide solutions, the sapphire capillary cell used with the pure salts, and tae experimental procedure have been described in detail previ~usly.3 Measurements were largely limited to conductivities up to 20 ohm-' because of the rapidly increasing contribution of the electrode resistance to the total resistance. A capillary cell of single crystal MgO was found to be noticeably attacked by tho molten triiodides. The triiodideb were prepared from the elements by the method of Druding and Corbetta and arere vacuum sublimed before use t2 remove traces of oxide contaminants.
-
18-
-. 'E -v
16
-
(4
-
T-01. 67
(2-
'E .c ;IO-
8-
6-
4-
2-
Nd-Nd13 (820°C)
Results and Discussion Table I contains the results of the specific conductivity measurements of the pure triiodides. As was the case iiz the chloride systems, these values form a regular series of specific conductivity K us. t curves where K decreases with increasing atomic number or decreasing size of the cation. The curve for Kd13 lies somewhat lorn-er than expected for a completely regular trend in the series, a fact that may be connected with the relatively high melting point of p-Nd13. The temperature dependence in all four systems correspoiicls to an activation energy of approximately 5.5 kcal. mole-'.
+-++.0
4
Fig. 1.-Specific
8
12 16 20 24 MOLE PER CENT METAL IN MI3.
28
32
conductivity of rare earth metal-rare earth metal triiodide systems.
these systems. Specifically, these differences have been interpreted as resulting from a gradual increase in the stability of the dipositive cation (La2+ Ce2+< Pr2+ < Kd2+), as indicated by an increase in solubility of metal and ultimately formation of reduced chlorides of Pr and I n contrast to the chlorides, the diiodides of all these metals are found to exist in the solid state. I-Iorvever, there is a striking dissimilarity between the neodymium dihalides and the diiodides of lanthanum, cerium, and praseodymium, The former compounds are typically salt-like in that they are electrical insulators, while all of the latter group are electrical conductors, i e . , M3+(I-)2e-.7 Xagnetic measurements confirm that the neodymium ion is truly Nd2+, but that the lanthanum ion, and presumably also cerium and praseodymium ions, are present as M3+ in their diiodides.8 The corresponding chloride and bromide systems of lanthanum and cerium exhibit only dissolution of metal without the formation of intermediate solid phases, while praseodymium forms only PrC12.3 8-io and PrBr2.4.S Because of the unusual behavior of these four iodide systems with regard to formation of solid phases and because a greater range of metal coilcentrations can be studied in the melts, the electrical conductivities of these MI3 11 solutions have been measured. Tlzese results and some further discussion of tlie phase relationships are contained herein.
-
+
(5) L. F Druding, J. D. Corbett, and B. N. Ramsey, t o be published. L. F. Druding a n d J. D. Corbett, J . Am. Chem. Soc., 83,2462 (1961).
(a)
(7) J. D. Corbett, L. F. Druding, W. J. Burkhard, and C. B. Lindahl, Dzscusszons Faraday Soc., 31, 79 (1961). ( 8 ) R. A. Sallach a n d J. D. Corbett, t o be published. ('I) F. J. Keneshea and D. Cubicclotti, J. Chem. Eng. Data 6, 507 (1961). (lo) G. W. Mellors and S. Senderoff, J . Phys. Chem., 63, 1110 (1959).
TABLE I SPECIFIC CONDUCTIVITY, K , OF RAREEARTH METAL TRIIODIDES t , OC.
K,
ohm-' em.-'
LaIa (m.p. 77&779')7
796 822 847 871
0 456 ,493 .524 ,555
CeIa (m.p. 760-781')'
796 814 836 860
0 448 ,470 499 ,323
t , OC.
K,
ohm-] cm.-l
PrIa (m.p. 738O)j
763 786 809
0.399 ,426 .452
NdIa (m.p. 787'15
799 818 842
0 396 416 ,440
The results of the specific conductivity measurements for the metal-in-salt solutions are given in Table I1 and illustrated in Fig. 1. The conductivity behavior of these systems in general resembles that of the corresponding chloride systems3 to a remarkable degree. The rapid increase in conductivity with metal concentration in the La and Ce solutions (an approximately exponeiitial dependence) indicates considerable electronic contribution to the total conductance. The S d solutions can be described as saltlike mTith little if any electronic conductance, while the Pr solutions are again intermediate as predicted.' The conductivity increases more rapidly on addition of metal to the iodides than to the chlorides, a phenomenon that was also found with solutions of the alkali metals in their halides where it was attributed to the influence of halide ion polarizability on electron transfer.ll The maximum iii the conductivity of the Sd13 K d system a t or near 50 mole % NdIz (16.7 mole % Kd metal) may be indicative of the possible electron exchange process between two adjacent, oxidation states (Kd2+ and Sd3+) postulated previously3 for the chloride system; this aspect is under further investigation. (11) H. R. Bronsteln a n d AI. A. Bred~g,zbzd., 66, 1220 (1961).
+
May, 1963
ELECTRICAL CO;\IDUCTIVITIES O F SOLVTIOKS O F L I E T A L S IS THEIR 3 I O L T E K
HALIDES
1147
Ktotal is about the same as above, 90%. The solutio11 of 31.2 mole % S d in Nd13 ( I i d 1 ~ ~exhibits ) an --CeIa + Ce a t 820°-apparent activation eiiergy approximately equal to --La18 + La a t 840°--Mole % Mole 5% K, that of Nd13, as would be expected from the magnitude inetal ohm-1 cin - 1 metal ohm-’ cm -1 of K , which indicates ionic conductance, and from the 0 515 0 0 480 0 stability of the Nd(I1) state. The implications of the 0 874 1 96 1 00 1 81 temperature dependence as a function of conceiitratioii 2 39 3 74 I 92 5 1 and of the nature of the system are being investigated 5 54 5 08 2 74 8 5 further. 12 2 7 25 -1 75 11 9 Analysis of the published’ freezing point depressions 20 9 9 9 8 88 14 6 12 5 15 5 of the four triiodides by added metal using recently ~ 6 0 19 4 determined heats of fusionI3 gives results quite com->iously connected Rith the a-8 transformation a t a little mole than 200’ below the rneltinR point (ief. 5 ) A. S. Dnorkin and &I. A. iodide ions that are presumably the principal carriers Bredig J . Phys. Chem., 6 1 , 697 (1963). in La13. Of course, a t higher metal coneenti-atioiis, (11) Crloscopic numbers f o r L a and Ce in their chlorides and mdidcs actually appear closer t o 2 4-2.6 than 3 on the b a a s of the published phace the temperature coefficient must decrease and ulti-. diagrams ETen more striking is the obseriation of numbers of 1.6-1.7 mately become metallic (negative) in character. The a n d 2 0 n i t h the bromides of Ida and Ce (ref. 8). Since it is not likely t h a t all more coiiceiitrated solution of l5,4 mole yo I’r in €3-13 th? phase dia::iaiiis a o u l d be in error this much, these differences can be better interpieted in teriiis of a more fundamental deficiency, the foiinatlon shows a diminished though still positive temperature of significant solid solution, in L a x 3 and CeXa This seems slightly mole dependence! one-third of that of Pr13, although K,/ reasonable than in\ okins admixtures \%ith M M a or intermediate solutrs
TABLE I1 SPECIFICCO~DCCTIVITY, K , OF MI,-pII S Y ~ T E U S K
+
+
(12) H. R Bionstein a n d 111 A. Bredig, J . Am. Chem. S o c
(1958).
,
80, 2007
because of the unusual and irregular trend observed for n nithin the halides of either element.
+
depression of tlic actlvity mid liericc tlic melting poiiit comes a bout c4'cctivcly oi ily through "dilution" of the electroiis in tlie nictallic conduction band. Uiifortuiiately, the quantitative cryoscopic treatment in this case is iiot obvious. Consequently the inter\.ciiiiig "AHf" values obtaiiicd for Cc12 aiid I'rIn call incrcly be takcii as indicative of the intermediate character of tlicsc iiiclts, with a grcatcr teiidciicy toward localizatioii of tlic clcctroii i i i liquid l'r12. Ttic ability of competiiig cloctroiiic states to coexist iii solutions, of course, allows inorc subtlc trends to tic obscrvcd than is possit)lc iii p u i ~solids. I n coinparison t o tlic wsults i n lig. 1, the apparelit lirat obtaiiicd a h v c for Cc12 sceiiis somewhat low,
since the coiiductivity in tlic iiiorc dilute CcIJ Cc solutioiis is actually slightly greater tliaii with laiitlianurn. This inay be the result of only a different coiicentratioii dcpeiideiice of the conductivity in the two cases iii the iiit~crvciiirig regioii. .Uternatively, an apparelit hcat up to about 40 kcal. i i i ~ l c - could ~ obtaiiicd by a more subjective recoiistruction of tho liquidus curve tlirough a greater thaii usual scatter of cxperinieiital poiiits so as to give ail iiiitially sinallcr rate of frecziiig point dcpressioii. conductivity data for niorc coiiccntratcd solutioiis as well as for the solids at liiglier tciiiperaturcs will be useful here as well as i i i asscssiiig inore clearly tlie character of liquid 15-12.
Introduction Tho heat of foriiiatioii of hydrogcii fiuoridc plays a role iii fluoriiic: t)oiiib aiid flaiiic caloriiiictry aiialogous to thc rola played hy the hcat of foriliatioil of water iii oxygeii t)oiiih aiid flanic caloriiwtry. As iiriiistroiigl has stated, "111 tho valiit: for tho [hcat of fornutioil of I-Ilc(g)I, coinbiiictd with tho heat of solutioii of gaseous hydrogeii fluoridc iii water, lies thc koy to much of thc cxistciit litcraturc of the thcriiiochciiiistry of fluorine coiiipouiids, dcpciidciit as it has t)ccii upoii the calorimetric study of rcactioiis of aquctous solutioiis of hydrogcii fluoridc." It is cwxit'ial for progress i i i fhoriiic therinochciiiistry that the staiidard hcat of foriliatioil of hydrogen fluoridt: gas should lxt dctcriiiiiicd ivith good That t,hc goal prccisioii arid hy ~ i i o r ct,hnii 0 1 ~ iiicthod. : has iiot yet t)cclii achiovcd is cvidciit iii 'l'ablc I , ivhioh of the cluaiitit'y.2-"1 contaiiis thc iiiodorii d(?t(!riiiiti~tioiis lis iiidicatctl by thc last t:iif.ry i i i thc tablc, a coiisidcrahlc part' of tho variatioii of t8hcrcportcd rcsu1t.s arises froin t'hc \.cry uiicctitaiii corroetioii for thc iiiipcrfcotioii of hydrogcii iluoiidc gas u t rooiii tciiipcmturc. I n 1932
thc coiiipilcis of Circular 500" sclcctrd a value of JIZj''2se (-G4,2 k e d . iiiok-') \\ hlch rcplcsclitcd the bcst cstiniatc availablt. a t that tiiiic. More rcccntly .Irinstroiig' cited evidence that the r(>coiiiinendcd value should be replaced by a inore iicgativc one. It is not the purpose of this paper to discuss the rclativc iiierits of the results of previous cxpcriiiieiital incthods. Rather, the purpose is to poiiit out that ivith the advent of high prccisioii fluoriiic bomb calorinictryl2-I7 it is iiow possible to reverse thc usual procedure of coinputiiig the heat of forinatioii of ccrtaiii fluorides from that of hydrogcii fluoride, and, illstead, to obtain the hcat of forinatioii of hydrogen fluoride by coinbination of the nicasui*cd heats of ccrtaiii ~volldcfiiicd i.eactioiis. l'hc following arc cxaiiiplcs of such schcnics.
(1) C;. 'r. A r i ~ ~ s t r mI Ii I~",~ ' ~ s i l ~ ~ T i l i I ~ ! : i 't i~i i I i ~ ~ r ~ ~ ~ ~ ~ cV Iu l~. e11, i i11. i i ,I. ~ t ~ ~ , " Skinner, Editor, l n t ( ~ r s c i c w eViibIisl~[~rb, I n o . , Ncn. Y u r k . N.Y.,l!)tiY, 11. 1.1 1. ( 2 ) 11. 1.m \Vartclibcrg and 0 . l.'itziieir %. U , I O I ' L I .n i l y e i n . C h e n . , 161, 313 (10263). (:
