118
E. R. VANARTSDALEN AND I. S. YAFFE
Vol. 59
ELECTRICAL CONDUCTANCE AND DENSITY OF MOLTEN SALT SYSTEMS : KC1-LiCl, KCl-NaCl AND KC1-KI1 BY E. R. VANARTSDALEN AND I. S. YAFFE chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee Received October 8 , 1064
Electrical conductance and density have been determined as functions of temperature for the fused, binary salt s stems potassium chloride-lithium chloride potassium chloride-sodium chloride and potassium chloride-potassium iodide. Jpecific conductance increases with temperature and is generally more temperature-dependent immediately above the melting point than a t higher temperatures. Molar vofumes have been computed from density data; they are very closely additive for the chloride mixtures, but deviate slightly in a positive direction for the chloride-iodide system. On the other hand, equivalent conductance isotherms for all three systems studied show quite marked negative deviations from additivity, with certain mixtures being less conducting than either pure component. Conductance has been treated as a rate process and values have been calculated for the heat of activation and the entropy of activation. In general, it was found that the heat of activation is temperature dependent, although lithium chloride appears to be an exception. Both heat and entropy of activation tend to maximum values for those mixtures which exhibit minimum conductance. ualitative explanations are presented to account for the various effects in terms of charge density, internuclear distance an short range order within the melts.
w
In recent years our lack of knowledge concerning the fundamental properties of molten salt systems has been accentuated by the greater emphasis now being placed on high temperature processes. Notwithstanding the need for specific data concerning particular fused systems for specific industrial processes] such as electrolytic production of aluminum, etc., a greater need exists for obtaining fundamental data about molten salts in general so that there may be a better understanding of the properties of the fused state. It was with these thoughts in mind that we have embarked upon a program of study of the basic physical chemistry of molten salts. Molar volume and electrical conductance are among the fundamental properties of liquid systems, and well-established experimental techniques for measurement of these physical characteristics of aqueous solutions are applicable with slight modification to use at higher temperatures. So it was decided to initiate our program with a study of these two quantities. Furthermore] a considerable amount of previous work concerning these properties of pure fused salts furnishes a firm foundation for our work on mixtures. A number of prior publications2sa have pointed out that various maxima, minima and other points of inflection can be found in molar volume and conductance isotherms for molten salt mixtures. Such points of inflection have frequently been considered to indicate the existence of complex ions in melts. It was our hope that our studies might give us some reliable information about the existence, stability and reactions of complex ions which certainly exist in numerous molten salts a t high temperatures. In order to lay a satisfactory foundation for more complicated systems, it was decided to investigate first some halide mixtures in which there appeared little likelihood of the formation of complexes. Therefore] the molten systems potassium chloridelithium chloride, potassium chloridesodium chloride and potassium chloridepotassium iodide were (1) Presented at the Symposium on High Temperature Chemical Reactions at the 126th Meeting of the American Chemioal Society, New York, N. Y . , Sept. 15, 1954. (2) (a) H.Bloom, J. W. Knaggs, J. J. Molloy and D. Welch, Trans. Faraday SOC.,49, 1458 (1953); (b) Y. V. Baimakov and S. P. Samusenko, Trans. Leningrad Industrial Inst., 3 (1938). (3) V. A. Izbekov, Ukrain. Alcad. Naulc. Institut Khim. (Proc. 1st. All-Union Conf. Non-aqueous Solutions), p . 142 (1935).
chosen for our initial investigations. Although the first system had been studied previously by Karpachev, Stromberg and P a d ~ h a i n o v a ,and ~ the second by RyschkewitschI6"and Barzokowsky, 5b we thought them worthy of reinvestigation. The possibility of complex formation in the chlorideiodide system was not overlooked. The more recent work on conductance and molar volume of molten salts is listed in references 2 through 14. These papers contain references to earlier work.
Experimental The furnace assembly used for these measurements is shown in Fig. 1. The furnace, 3.5" inner diameter by 16" long, was provided with shunts for removing or imposing thermal gradients. A platinum-rhodium crucible, which contained the melt, was inside a heavy-walled nickel tube, which, in turn, was enclosed by an outer inconel tube. The crucible was placed inside an alumina crucible in order to insulate the melt from electrical contact with the assembly. Appropriate heat baffles were inserted as illustrated. In order to provide an inert atmosphere, argon was admitted to the assembly through the hollow nickel support rod. The temperature of the furnace was controlled to 0.2" by an anticipating thermocouple which operated a zero-suppression-ty e, 6 millivolt range, Leeds and Northrup S eedomax Type ifrecorder-controller in conjunction with a lfjeeds and Northru "DAT" controller. The actual temperature within tge melt was measured with a calibrated platinum, 87% platinum-13% rhodium thermocouple by means of a Rubicon precision otentiometer. By appropriate adjustment of furnace siunts and with the described control equipment, the temperature of the molten salt, which (4) S. V. Karpachev, A. G. Stromberg and N. Padchainova, Zhur. Obshchei Khim., 8, 1517 (1935). (5) (a) E. Ryechkewitsch, 2. Efelctrochem., 39, 531 (1933); (b) B. P. Barsokowsky, Bull. Acad. Sci. U R S S , Class Chim., no. 5, 825 (1940). (6) J. 0. Edwards, C. 8. Taylor, A. S. Russell and L. F. hiaranville, J . EEectroohem. SOC.,99,527 (1952). (7) P. W. Huber, E. V. Potter and H. W. St. Clair, U. S. Bureau Mines, Report of Invest. 4858 (1952). (8) R. C. Spooner and F. E. W. Wetmore, Can. J . Chem., 29, 777 (1951). (9) J. Byrne, H. Fleming and F. E. W. Wetmore, ibid., SO, 922 (1952). (10) H.Bloom and E. Heymann, Proc. Roy. SOC.(London), 1888, 392 (1947). (11) W.J. Davis, 8. E. Rogers and A. R . Ubbelohde, ibid., 2'20A, 14 (1953). (12) A. E. van Arkel, E. A. Flood and N. F. H. Bright, Can. J. Chem., 91, 1009 (1953). (13) J. 9. Peake and h l . R. Bothwell, J. Am. Chem. SOC.,76, 2653 (1954). (14) J. O'M. Bockris, J. A. Kitchener, S. Ignatowicz and J. W. Tomlinson, Trans. Faraday SOC.,48, 75 (1952),et seq.
Feb., 1955
ELECTRICAL CONDUCTANCE AND DENSITY OF MOLTEN SALT
fluctuated less than the furnace temperature, could be maintained to better than 2~0.1' at temperatures up to 1000" with a thermal gradient of no more than 0.2" over the height of the melt (usually about 2.5").
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119
T I
i 0 In.
,-
+-
6 m m ID THERMOCOUPLE SHEATH
'/
(A)
6 m m I D ELECTRODE TUBES (81
4 in.
Fig. 2.-Capillary Fig. 1.-Furnace assembly: A, Pt crucible; B, Alundum crucible; C, Ni support tube; D, quartz thermocouple sheath; E heavy-walled Ni tube; F, inconel tube. G, furnace; external taps for shunts; I, stainless steei support rods. J, bottom ring; K, wing nuts; L, radiation shields; h, radiation shield support; N, brass top plate; 0, cell support; P, cell yoke; Q, rubber stopper; R, cooling coils. Figure 2 is a diagram of the quartz dip-type capillary conductance cell used. Cylindrical platinum electrodes rest on the shelf a t the top of the capillary tubing. Resistance measurements were made with a Leeds and Northrup precision "Jones" bridge. I n order to eliminate the mutual inductance caused by the close proximity to each other of the electrode leads, i t was found necessary to make resistance measurements from 2000 to 20,000 c.p.5. (The measured resistance varied less than 0.5y0 in this range.) A linear extrapolation to infinite frequency of measured resistance versus (frequency)-a was then made so that the inductance-free resistance could be obtained. The conductance cells were calibrated in demal aqueous potassium chloride solution a t 25.00" using platinized electrodes according to the method of Jones and Bradshaw.16 Allowance can be made for expansion of the quartz cells on raising the temperature, but such correction amounts to less than 0.05y0 and was therefore considered unnecessary. The cell constants were of the order of 300 to 500 cm.-l and were of the proper order of magnitude to agree with the conclusion that the resistance of the cell was determined almost exclusively by the salt melt within the capillary. Recalibration of the .cells following two or three days use in the molten salt mix-
d,
(15) G. Jones and B. C. Bradshaw, J . Am. Chem. Soc., 6 6 , 1780 e(1933).
conductance cell.
tures showed that the cell constants did not change by more tha? 0.3%, and, on the average, about O . l % , although the platinum black of the electrodes was converted to platinum grey by the melts. Density measurements were made by weighing submerged within the melt a calibrated platinum bob suspended by a thin (36 gage) Pt-Rh wire from an analytical balance. The expansion of the platinum bob was computed16 in order to obtain the volume of the bob a t each temperature. For both conductance and density measurements, data were taken going up and down the temperature scale; no difference was observed in the measurements upon this temperature cycling. During temperature changes, the conductance cell and the- density bob were raised out of the melt. Duplicate measurements were made a t each temperature in order to assure thermal equilibrium. Normally, the same melt was used for both types of measurements. Mixtures were made by weighing appropriate amounts of carefully purified and dried reagents" into the platinumrhodium crucible which was then placed in the furnace. With dry purified argon flowing through the assembly, the temperature was slowly raised above the melting point of the salt mixture. Such precautions were necessary in order to prevent hydrolysis of the salts by traces of moisture. At the conclusion of an experiment, samples of the melt were removed and checked for neutrality in aqueous solution. The data from a few melts which showed evidence of (16) L. Holborn and A. L. Day, Ann. Physdk, 4 , 104 (1901); cited by R. F. Vines, "The Platinum Metals and Their Alloys," The International Nickel Company, Inc., New York, N. Y., 1941, p . 17. (17) Analytical grade materials were recrystallized and premelted under an atmosphere of hydrogen or argon to dry them and then cooled to room temperature, crushed roughly and stored in a dry atmosphere.
E. R. VANARTSDALEN AND I. S. YAFFE
120
TABLEI SPECIFIC CONDUCTANCE OF KCI, LiCI, NaC1, K I Compn., mole %
100 KCl
100 LiCl
8.
mho?m.-l
778.9 782.4 787.0 790.0 799.0 800.5 804.1 823.0 846.0 850.9 851.2 865.0 877.5 889.0 892.5 905.5 906.7 925.1
2.1628 2.1816 2.1984 2.2074 2.2342 2.2407 2.2478 2.3020 2.3507 2.3609 2.3618 2.3901 2.4174 2.4476 2.4484 2.4745 2.4777 2.5156
622.8 634.4 643.9 667.8
5.6923 5.8262 5.8922 6.0083
691.1 712.5 741.0 760.3 783.3
6.1185 6.2253 6.3704 6.4626 6.5399
Compn., mole %
100 K I
3.4497 3.5804 3.5821 3.6057 3.6060 3.6431 3.6441 3.6601 3.7145 3.7662 3.8041 3.8502 3.8637 3.9154 3.9129 3.9416 3.9680 3.9701 4.0151 4.0653 4.0616 4.0719 4.1595 4.1701 4.1861
685.4 689.0 697.5 698.5 708.6 718.5
1.2440 1.2613 1.2940 1.2961 1.3234 1.3546
AND
K,
OC.
mho crn.-l
738.0 739.5 745.7 754.3 768.2 770.1 783.2 785.7 791.1 815.7 830.7 847.4 877.2 897.6 910.7
1.4041 1.4132 1.4207 1.4395 1.4639 1.4643 1.4955 1.4970 1.5022 1.5440 1.5637 1.5905 1.6232 1.6555 1.6664
706.9 707.2 726.5 760.0 765.0 765.4 804.0
2.0765 2.0773 2.1331 2.2529 2.2634 2.2644 2.3735
807.6 832.3 855.7
2.3842 2.4397 2.4975
59.55 KCI 40.45 Licl
589.0 634.2 641.6 657.5 662.6 685.8 701.2 737.5
1.8480 2.0726 2.1144 2.1783 2.2028 2.2864 2.3602 2.4734
41.20 KCI 58.80 LiCl
389.8 394.0 441.5 443.0 476.1 483.8 486.0 514.8 522.2 535.2 553.5 588.3
1.1819 1.2040 1.5162 1.5314 1.7290 1.7811 1.7964 1,9524 1.9987 2.0655 2.1638 2.3375
450.1 450.4 450.7 465.5 502.2 507.3 509.8 544.1 547.5 563.9 585.1 586.6 608.6
1.9074 1.9123 1.9239 2.0773 2.3222 2.3580 2,3722 2.5765 2.5954 2,6881 2.7969 2.8073 2.9130
521.1 529.4 529.7
3.0443 3.1075 3.1087
80.04 KCl 19.96 LiCl
(N.B. considerable corrosion of cell occurred at higher temp.) 100 NaCl 802.3 3.0887 804.6 806.4 806.5 808.0 808.2 817.8 817.9 827.4 839.8 857.8 871.4 892.7 899.4 911.5 913.3 922.4 933.3 933.8 947.3 971.5 972.0 972.3 1006.9 1010.1 1021.4
1,
29.64 KC1 70.36 LiCl
18.23 KCI 81.77 LE1
VOl. 59
MIXTURES Compn mole %'
L
06.
mho?m.-a
551.8 562.6 579.0 599.7 612.2 622.5 626.4 642.5 670.8 671.2
3.2440 3.3054 3.4006 3,5121 3,5708. 3.6268 3.6438 3.7214 3.8492 3.8500.
79.60 KCI 20.40 NaCl
718.4 718.9 730.5 730.9 766.3 789.7 815.1 839.0 853.7 868.4 889.8 Y30.1
1.8246. 1.8344 2.1446. 2.1453. 2.2466 2.31% 2.3784 2.4379, 2.4638 2.4958 2.5422 2.6109
59.00 KCI 41.00 NaCl
673.5 679.0 692.1 708.3 734.9 740.7 740.9 769.8 794.5 822.8 853.0 853.7 881.5 907.9
1.5316 1.9085 2.1766. 2.2370 2.3393 2.3639 2.3643 2.4472 2.5196 2.5848 2.6634 2,6641 2.7223 2,7725
48.77 KC1 51.23 NaCl
665.9 674.4 679.2 706.2 726.8 738.7 769.5 788.9 818.8 843.4 846.7 875.1 887.6 912.5
2.1986 2.2523 2,2789 2.3687 2.4374 2.4696 2.5633 2.6107 2.6914 2,7523 2.7569 2.8273 2.8533 2.9019
34.85 KCI 65.15 NaCl
690.7 695.4 709.1 736.7 756.8 783.6 823.6 825.0 825.5 853.4 898.4
2.3236 2.5049 2.5562 2.6556 2.7175. 2.7971 2.9210, 2,9254 2.9281 3.0066 3.1139.
ELECTRICAL CONDUCTANCE AND DENSITY OF MOLTEN SALT
Feb., 1955
121
TABLE I (Continued) Compn., mole %
27.06 KCl 72.94 NaCl
15.23 KCI 84.77 NaCl
80.22 KCI 19.78 K I
4
K
OC.
mho c'm.-1
919.7 944.2
3.1611 3.2163
708.0 717.1 735.1 750.5 773.9 788.8 819.5 841.0 857,7 876.4 899.4 927.9
2.5583 2.6866 2.7591 2.8160 2.8977 2.9472 3.0365 3.0947 3.1397 3.1914 3.2344 3.2864
758.0 759.6 762.7 767.2 773.2 789.4 795.2 811.2 816.8 817.3 835.0 852.4 875.3 895.5 909.2 927.2
2,9859 3.0827 3.1085 3.1237 3.1444 3.1938 3.2092 3.2607 3.2787 3.2803 3.3309 3.3784 3.4415 3.4885 3.5233 3.5609
711.8 726.8 741.6 745.4 761.2 767.4 790.2 797.6 814.9
1.7043 1.7530 1.7936 1.8071 1.8517 1.8712 1.9243 1.9500 1.9905
Compn., mole %
61.12KCI 38.88 K I
45.15 KC1 54.85 K I
t, OC.
mho?m.-l
817.8 845.6 865.0 875.1 883.3 903.1
1.9998 2.0559 '2.0843 2.0985 2.1130 2.1402
642.3 643.5 645.3 649.6 660.0 664.0 692.7 713.1 732.6 759.2 783.8 817.3 843.6 873.2 905.7
1.0824 1.1932 1.3418 1.3868 1.4395 1.4500 1.5350 1.5982 1.6446 1.7120 1.7596 1.8321 1.8806 1.9252 1.9725
607.8 612.6 624.2 646.1 658.4 682.1 691.6 707.3 724.6 739.7 754.7 781.5 807.5 808.7 825.1 839.5 861.0 875.8
1.2450 1.2585 1.2896 1.3500 1.3834 1.4398 1.4666 1.5004 1.5376 1.5732 1.6055 1.6654 1.7141 1.7208 1.7441 1.7715 1.8128 1.8332
Compn., mole %
ob.
mho
&I. -1
903.7
1.8815
25.67 KC1 74.33 K I
634.9 644.8 667.8 698.0 722.3 745.8 759.9 779.6 810.8 829.0 856.9 876.4 904.5
1.2578 1.2837 1,3369 1.4114 1.4683 1.5115 1.5400 1.5777 1.6347 1.6547 1.7012 1.7296 1.7631
15.30 KC1 84.70 KI
668.4 686.0 709.1 730.5 737.6 742.2 766.3 772.2 787.8 794.9 822.9 847.5 870.2 904.0
1.3069 1.3493 1.3961 1.4316 1.4439 1.4585 1.5024 1,5089 1,5429 1.5517 1.6051 1,6440 1.6772 1.7282
6.04 KCl 93.96 KI
690.5 719.8 750.4 769.8 799.1 830.4 857.0 877.4 878.4 902.1 902.7
1.2913 1.3612 1.4307 1.4701 1.5239 1.5759 1.6126 1.6388 1.6402 1.6663 1.6666
conductance, t is temperature in degrees centigrade and a, b and c are constants for any particular composition. Values of the constants for these equations have been derived by the method of least squares and are listed in Table 111, along with the applicable temperature range. These equations were used for subsequent calculations. Results and Discussion Densities of the pure salts and of their mixThe specific conductance of various compositions tures were determined as functions of temperaof the systems KCl-LiCl, KC1-NaC1 and KC1-KI was measured, in general, from a few degrees above ture. The data which are given in Table I1 are the melting point to over 900". These data are represented very precisely by linear equations, listed in Table I. Figure 3 shows our data on the p = a - bt, from a few degrees above the melting four pure salts and their comparison with other point to the highest temperature a t which measurepublished work. I n general, our results agree rea- ments were made (usually the same temperature sonably well with the more recent determinations range as for specific conductance). I n this equation by other investigators. Representative isotherms p is density in g./cc., t is temperature in degrees at 800" showing the variation of specific conduct- centigrade and a and b are constants for any given ance with composition are illustrated in Fig. 4. salt or mixture. Numerical values of the constants Except for the data within 10 or 20" of the melting for all mixtures studied were derived by the method point, the specific conductance data could be repre- of least squares and are listed in Table 111. The sented quite exactly by quadratic equations in density data for molten potassium chloride are in bt et2; where K is specific good agreement (0.301,) with the data reported by temperature, K = a hydrolysis were discarded. Samples of the melts also were analyzed gravimetrically t o determine the exact composition of the mixtures. I n the case of the chloride mixtures analysis was for total chloride by precipitation as silver chloride, while the analysis of the potassium chloride-potassium iodide mixtures was for total potassium as potassium sulfate by fuming with excess sulfuric acid and drying at 550'.
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E. R. VANARTSDALEN AND I. S. YAFFE
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