DENSITIES AKD MOLAR VOLUMES O F MOLTEN SALT MIXTURES
375
DESSITIES XSD lIOLLiR T'OLUMES OF XIOLTEK SALT 3IIXTLXES S. Iensionsof molten salt mixtures are being carried out in this laboratory in order t o investigate the constitution of these systems and to apply recent theories of the liquid state proposed by Eyring (4) and Frenkel ( 5 ) . EXPERIMENTAL
The sinker method of determining densities used by a number of investigators studying densities of molten salts requires large quantities of salt and may give values which are too lo^ when the salt is somewhat volatile, owing to condensation on the suspending wire. We therefore chose the dilatometer method (Klemm (1l)), employing dilatometers made of transparent silica. These had volumes of 3-6 cc. and necks of uniform bore of 3.8 mm. diameter. Reference lines mere made on the neck by a marking diamond, the dilatometer being rotated in a lathe. The disadvantages of the sinker method are thus avoided, but for most systems the method cannot be used above 700-750°C. because of chemical attack on the silica. The molten salt was poured into the dilatometer, using a suitably shaped B.T.H. C14 (British Thompson Houston) glass tube. The dilatometer was placed in a silver block inserted in a well-lagged electric furnace which consisted of two steel tubes welded in the shape of a cross set in an asbestos frame (cf. Bloom and Heymann (3)). The silver block had a cut down the dilatometer hole to render the meniscus visible during experiment. The top of the dilatometer neck was closed by a pear-shaped stopper made of B.T.H. glass. A shield made of the same glass n'as inserted between the dilatometer and the wall of the silver block. This prevented the silica from becoming opaque owing to diffusion of silver. The temperature, measured by means of a platinum and platinum-10 per cent 1-hodium thermocouple inserted in the block and a Leeds & Northrup potentiometer, was kept constant within 1°C. The position of the meniscus was read using a cathetometer. Corrections were applied for the shape of the meniscus, expansion of silica, and buoyancy. After the experiment, the molten salt was poured into a weighed crucible, and both crucible and dilatometer (with adhering salt) were weighed. With most systems it was not feasible to allow the salt to cool in the dilatometer, because the latter is often shattered. After cleaning, the volume of the dilatometers was checked; no significant change was ever found. The method was checked by measuring the density of silver nitrate (analytical reagent grade) previously determined by several investigators. We found
TABLE 1 PbClz-CdCIz (kO.20 per cent)
I
I
Mole per cent P b C l z . . 100 41.8 20.1 I 79.4 67.2 a . .. . . . . . . . . . . . . . . . . . 4.802 1 4.544 4.388 4.018 3.693 (4.763 L F J ) b X 103. . . . . . . . . . . . . 1.18 1.50 1.43 1.02 1.39 (1.44 L F J ) 1 Range i n "C.. . . . . . . . 516-710 I 545-6801 480-6SOi 515-7001 540-650, ~
0 3.366 (3.320 L F J ) 0.84 (0.69 L F J ) 582-725
Mole per cent CdCIz. . . 100 77.8 64.8 55.5 44.3 34.3 0 2.574 3.366 3.108 2.919 2.763 2.398 (1.675 J) b X IO3 . . . . . . . . . . . . . . . . 0.84 0.95 0.86 1.04 0.92 0.83 (0.63 J ) 582-725 580-700 540-680 570-680 500-690 580-690 Above 800
Mole per cent PbClz. . . . . . . . .
82.1
100
........................... 4.802 4.293 b X 103.. . . . . . . . . . . . . . . . . . . . . 1.50 Range in "C.. . . . . . . . . . . . . . . . . 516-700 0;!5:5
~
a.
a.
~
63.8 3.745
47.4 3.252 1.13 5~0~26SsO 490-680
0 (1.628 J) (0.60 J) Above 750
..........................
b X 103.. . . . . . . . . . . . . . . . . . . . .
I
(1.45 LFJ)
Range i n "C . . . . . . . . . . . . . . . . . .
61.1 53.4 42.6 20.3 MolepercentPbC12. 100 80.6 70.9 a , . . . . . . . . . . . . . . . . . 4.802 4.790 4.782 4.778 4.775 4.764 4.739
b X
lo3
. . . . . . . . . . . . . 1.50
1.45
1.31
1.42
1.28
1.26
1.08
Range in "C.. . . . . . . 516-710 520-~00m-680~445-67O444-680 380-7001478-660
Mole per cent A g C l . . . . . . . . . . 100 a . ........................... 4.698
77.3 4.875
59.7 5.006
34.2 5.190
. . . . . . . . . . . . . . . . . . .. I
1.08
1. I 2
1.07
420-590
420-580
b X
lO3.,
0.94
Range in "C.. . . . . . . . . . . . . . . . . 480-630
440-580 376
1
0 4.698 (4.715 LH) 0.94 (0.92 LH) 480-630
0 5.402 (5.405 LH) 1.04 (1.03 L H ) 440-600
377
DENSITIES AND MOLAR VOLUMES O F MOLTEN SALT MIXTURES
TABLE 1-Concluded AgCI-KCI (10.30per cent) ~~~~~
.I
Mole per cent A g C l . , . . . . . . . 100 a. ........................... 4.695 2, x 1 0 3 . , . . . . . . . . . . . . . . . . . . . . 0.94 Range in "C.. . . . . . . . . . . . . . . . . 480-630
........................
i
b X 10 . . . . . . . . . . . . . . . . . . . .
80.6 3.893 0.95 433-670
(5.405 LH) 1.05 (1.03 LH) 440-600
I
1 Range in "C.. . . . . . . . . . . . . . . ~
I
1
47.8 2.758
0 (1.628 J ) (0.60 J )
.
I
.
i
'
1.12
I1
1.03
I
1
~
380-600
Mole per cent CdC12.. . . . . . ' 100 I 53.1 a. .................... 3.366 3.049
....I
68.0 3.425
~
380-600
59.2 2.608
0.95
(0.50 J )
593-700
Above 750
'
40.0
0.82 460-680
I ~
21.8
0
0.72 604-750
1
j
(0.60 J ) Above750
PbCIrBaCIz
Mole per cent PbC12 . . . . . . . . . . I 100 a . ...................... 4.802 b x 103., , , , , , , , , . , , . , , , , , . , , I 1.50 Range in "C.. . . . . . . . . . . . . . . . . 516-710
.....I
i
I ~
~
86.2 4.620 1.35 565-700
~
1 i 1
S0.3 4.540 1.36 575-690
1 ~
~
1
69.4 4.394 1.27 660-710
1
' ' ~
0 (3.350 AG) (0.52AG) Above 1000
CdCIrBaCla ~
Mole per cent CdClz. . . . . . . .. / 100 a . .......................... .I 3.366 b X IO3. . . . . . . . . . . . . . . . . . . . . . 0.84
1
Range in "C.. . . . . . . . . . . . . . . . ,~ 582-725
i
' ~
82.9 3.438 0.93 597-700
~
1
~
64.0 3.480 0.93 580-700
1I I
i
45.8 3.493 i 0.96 600-690
~
I ~
~~~
0 (3.360 AG) (0.52AG) Above 1000
d = 3.8GG - O.O0108(t - 300) in the range between 240°C. and 340°C. This agrees well with the results obtained by Klemm ( l l ) ,d = 3.867 - 0.00107(t 300),using the dilatometer method. Jaeger (10) found d = 3.870 - 0.00102(t 300), using the sinker method. Mixtures containing lead bromide were not investigated above C03"C. because of decomposition (slight smell of bromine and darkening of the melt). Most salts were either obtained of analytical reagent purity or prepared from reagents of analytical reagent purity and recrystallized. The composition of each salt was checked by standard analytical methods. Lead bromide vvns made from lead nitrate and hydrobromic acid; silver bromide from silver nitrate and hydrobromic acid. Cadmium chloride and bromide were made and analyzed as described by Heymann, Martin, and Mulcahy (8) and Bloom and Heymann (3). Analysis of the mixtures was carried out as follows: CdClz-CdBrz: total
378
N. K. BO.%RDMAN, F. H. DORMAN AND E. HEYMANS
cadmium electrolytically, total halide gmvimetrically, PbC1d31C12: lend ns chromate (Vogel (17)) or sulfate and total chloride. CdCl.:-l;aCl: cadmium electrolytically, also double chloride. PbCl?-BaCl?: double chloride. CdClpBaC12: cadmium electrolytically. PbC12-XnC'1: double chloride. PbCl?TABLE 2
-
CdClp-CdBr? (700°C.); masimum eyperimental error, *O.l
Mole per cent CdCl;!, . . . . . . . . . . . . . . . . . . 100 3Iolar volume in ~ 1 1 1 (observed). .~ . . . . . . I 55.88 Molar volume in c111.3 (cnlculnted). . . . . 1 in per cent. . . . . . . . . . . . . . . . . . . . . . . . . . ~
,I
j
1
70.3 59.79 59.79 0.00
1i 1
per cent
54.4 61.88 61.92 -0.07
34.7 0 64.52 69.23 64.58 1 -0.09 ~
1
I
PbClr-PbDr? (GOOT.); masimum experimental error, =tO.15 per cent
LIole per cent P I ~ C I.?. .. . . . . . . . . . . . . . . . . 100 Molar volunic in ~ 1 1 1(ol,sei,vcd) .~ . . . . . . . 57.92 Molar volume i n cn1.3 (calcu1:itctl). . . . . . A i n per cent . . . . . . . . . . . . . . . . . . . . . . . . . . . , ~
80.3 59.90 59.99 0.00
19.7 63.32 63.32 0.00
15.4 67.04 67.00 t0.06
I
1
0 68.60
I ~
PhClr-AgCI (600'C.); maximum experimental error, AO.1 per cent
Molar volume in cin.3 (cal- 1 culated), . . . . . . . . . . . . . . . . A in per c e n t . . . . . . . . . . . . . .
52.60 0.00
~
~
I 49.94 47.28 j 45.14 42.18 36.08) -0.02, 0.00 1 .-0.09 -0.07 -0.101
.I
Mole per cent A g C l . . . . . . . . . . . . . . . . . . . 100 Molar voluine in ~ 1 1 1 . 3 (observed) . . . . . . . I 30.50 Molar volume i n 0111.3 (calculated). . . . . . A in per cent . . . . . . . . . . . . . . . . . . . . . . . . . ., ,
'i ,
77.3 31.50 31.50 0.00
69.7 32.20 32.23 -0.09
31.2 33.30 33.33 -0.09
0 34. 75
PbClr-CdCh (600'C.); masimum experimental error, f0.2 per cent
Mole per cent PbCl?. . . . . . . . . . Molar volume in cm.3 (observed) . . . . . . . . . . . . . . . . . . . . . Molar volume in ~ r n (cnlcu. ~ lated) . . . . . . . . . . . . . . . . . . . . . . A in per c e n t . , . . . . . . . . . . . . . . .
100
57.90
79.4
67.2
41.8
56.90
56.30
55.50
20*1 54.81
57.18 -0.49
56.78 -0.84
55.88 -0.68
55.15 -0.62
I
1
0 54.46
1 ~
PbBr:!: lead as sulfate. PbC12-AgC1: lead chloride dissolved with hot water. AgCI-KC1 and AgBr-IiBr : melt dissolved in concentrated ammonia and silver halide precipitated with nitric acid. AgC1-AgBr : mixtures not analyzed but carefully prepared by melting together weighed quantities of fused silver chloride and silver bromide.
MIXTURES
0.2
0.4
A-
0.6
B
(t°C)
0.8
1.0
MOLAR FRACTION o f A
MOLAR FRACTION o f A FIG. 1
FIG.2 FIG.1. Plots of molar volume z's. molar fraction of the first-nnnied coniponent in the systems AgBr-IiBr, AgCI-IICl, and AgC1-AgBr (600°C.). FIG.2 . Plots of molar volume vs. iiiolar fraction of the first-nanied component in t h e systems BaCl2-CdClg (700"C.), PbClz-ICl (700"C.), and CdClz-KC1 (600°C.). MIXTURES
A- B
(t°C>
M O L A R FRACTION of A
FIG.3. Plots of niolar volunie us. molar fraction of the first-named coniponent in the systems PbCIg-CdCIz (600°C.) and CdClp-KaC1 (700°C.). 370
380
N. K. BOARDMAN, F. H. DORMAN AND E . HEYMANN RESULTS
The variation of the density (table 1) with temperature ( t in "C.) is linear in all cases and given by the equation
d
=
u - 6 ( t - GOO)
The maximum errors (in brackets) are calculated from estimates of the accuracy of the chemical analysis, volume and weight determinations, and temperature. Values of other workers are given in brackets where available: J (Jaeger (10)); LFJ (Lorenz, Frei, and Jabs (13)); L H (Lorenz and Hochberg (14)); AG (Arndt and Gessler, (1)). The agreement with values obtained for some of the pure components by Lorenz and Hochberg (14) is good, whilst Lorenz, Frei, and Jabs (13) obtained values for volatile salts such as cadmium chloride which are about 1 per cent lower than ours; significantly they used the sinker method. The molar volumes of systems which were investigated throughout the whole range of mixtures are shown in table 2, together with the values calculated on the assumption that the molar volumes are additive. The deviation from additivity (A) is given in per cent. A number of systems, however, cannot be investigated over the whole range of mixtures because one of the components has a melting point above 750"C., where the attack on the silica vessel is too strong. Extrapolation of the density of that component to 700°C. (i.e., below its melting point) from determinations by other workers using the sinker method a t temperatures above 800°C. may not be sound, and deviations from additivity calculated by the aid of such extrapolated values would be open to doubt. We therefore deemed it preferable not to attempt a numerical calculation with such systems, but to judge qualitatively from the shape of the molar volume versus molar fraction curves (figures 1 to 3) whether positive or negative deviations from additivity occur. DISCUSSION
The obvious quantities with which to correlate the molar volumes of mixtures are the thermodynamic activities of the two components determined either froin partial vapor pressure or E.M.F. data, but such data are scarce in molten salt systems. E.M.F. data by Salstrom and Hildebrand (15) are available for some of our systems. However, the partial vapor pressure isotherms of many systems investigated by Greiner and Jellinek (6) are irregular and do not satisfy the Duhem-Margules equation, e.g., CdClz-CdBrz and PbC12-PbBr2. A scrutiny of the experimental data of these workers reveals that there are differences of about 10 per cent between duplicate experiments, and many of their results must therefore be regarded as doubtful. A useful approach is that based on Klemm's observation (11) that, other things being equal, melts consisting of covalent molecules, because of weak interniolecular forces, have larger molar volumes than ionic melts possessing a strong Coulomb field of force. A volume increase on mixing of two molten salts would thus indicate an increase of the amount of covalency, whilst a volume
DENSITIES AND MOLAR VOLUMES O F MOLTEX SALT MIXTURES
38 1
decrease on mixing would indicate an increase of the amount of electrovalency. As the electric conductivity is a sensitive means of distinguishing between molecular melts and ionic melts, a correlation between molar volume isotherms and conductivity isotherms of molten salt mixtures is to be expected. The systems under investigation fall into three groups : I. The molar volumes are additive within the limits of experimental accuracy (0.1-0.2 per cent), vix: CdC12-CdBr2, PbC12-PbBr2, PbCl2-AgC1, AgC1-AgBr, or nearly additive, vix.: AgCl-KC1, AgBr-KBr, CdC12-NaC1. In none of these systems does the phase diagram show evidence of intermediate phases in the solid state, except with CdCl2-KaC1, where the phase diagram indicates an incongruently melting compound which is, however, likely to be decomposed a t the experimental temperature. The system PbC12-AgCl is an almost ideal mixture, whilst the system PbClz-PbBrz shows slight negative deviations from Raoult’s lam according to E.M.F. data (15). The equivalent conductivity shows slight to moderate negative deviations from additivity (Bloom and Heymann (3) ; Harrap (7)). 11. The molar volumes show positive deviations from additivity, vix.: PbC12IIC1, CdC12-KC1. The phase diagrams indicate intermediate phases in the solid state. Conductivity isotherms with minima suggest that stable complex ions such as [PbC14]-- and [CdC14]--, and also complex ions which dissociate on rise of temperature, exist in the molten mixtures (Bloom and Heymann (3)). These conclusions are in qualitatiye agreement with activity determinations from measurements of the decomposition potentials (Hildebrand and Ruhle (9)), and with transference experiments (Lorenz and Fausti (12)) in the system PbC12-KCl. As the formation of complex ions of the above type may be regarded as equivalent to an increase of the amount of covalency on mixing, the positive deviation of the molar volume isotherm is in good accord with Klemni’s rule. 111. The molar volumes show negative deviations from additivity, vix. : PbC12CdC12, BaClJ-CdC12. There is no indication of intermediate phases in the solid. Unfortunately, no activity data are available. The conductivity isotherms show positive deviations from additivity with maxima (Bloom and Heymann (3) ; Harrap (7)). The causes of these maxima are not known with certainty. They may be due t o the fact that the amount of autocomplex forniation in the pure components is greater than in the mixture. At any rate, the strong positive deviation of the equivalent conductivity from additivity suggests that the amount of electrovalency of binding is greater in the mixture than with the pure components. The negative deviation from additivity of the molar volume isotherms1 is thus in accordance with Klemm’s rule. The fact that small to moderate negative deviations from additivity of conductivity-and in some cases of activity-are not reflected in the molar volume isotherms of the systems of group I is probably due to the fact that the possible hIasima it1 the conductivity isotherms and negative deviations from additivity of the molar volume isotherms were nlso found with niixtures of the clilorine-substituted acetic acids with perchloric acid (16).
382
Pi. I
