1727
RADIOMETRIC DETERMINATION OF Cd SOLUBILITY IN MOLTEN CdClz
Radiometric Determination of the Solubility of Cadmium in Molten Cadmium Chloride
by J. MoBciiiski Institute of Nuclear Technique,C r w , Poland
and L. Suski Institute of Physical Chemistrg of ule Polhh Aoademy of Science, Cra~ow,Poland (ReceivedSeptember 7,1965)
A method for the determination of the solubility of cadmium in molten cadmium chloride by measurement of y-ray absorption is described. The decrease in the relative intensity of the radiation depending on the concentration of the absorbing solution of cadmium in cadmium chloride obeys the absorption equation. From the discontinuity of this relation the solubility limit can be determined with good accuracy. The data presented are in very good agreement with those given in the literature which have been obtained by other methods.
Introduction The problem of dissolving metals in molten salts and the structure of these solutions has recently been a subject of numerous studies, results of which have been One of the properties surveyed in several particularly important in investigations of this type is the solubility of the metal in molten salt and its temperature dependence. Knowing these parameters, one can determine for instance the partial thermodynamic functions of these s ~ l u t i o n s . ~The miscibility gap of the metal-molten salt systems as well as the details of the phase diagram have up to now been determined by thermal analysis and the chemical analysis of the individual phases. These methods, however, involve in some cases large experimental difficulties, resulting in considerable discrepancies in the data obtained by various workers. It seems to us of interest to examine new experimethods for studying the structure Of systems* In the present paper t3 method employing y radiation of radioisotopes for the determination of the solubility of the metal in its molten salts is presented. The use of y rays in investigations of properties of molten salts w&s first reported by Belayev.618 using a
CosOy source (1.17 and 1.33 Mev), he found that for various binary systems of molten salts there is a dependence of the absorption of y rays on the concentration of the components. Belayev measured the absorption of y radiation by solutions of Pb, Zn, Sn, Al, and Mg in their respective salts18using always the same source, ar,d found that their absorption properties differ from the respective values of the pure molten salts. The sensitivity of the radiometric method obtained in the work of Belayev was insufficient to determine the relation between the absorption and the concentration of metal in the molten salt. The aim of the present work was to achieve sufficient sensitivity that the relation between the absorption of the y radiation and the concentration of the metal could be obtained, permitting the determination of solubility limit in this system. (1) M. Bredig in “Molten Salt Chemistry,” M. Blander, Ed., Interacien~ pubbhe.m, Inca,New yo&, N. y.,1964, p 367. (2) J. D. Corbett in “Fused Salts,” B. R. Sundheim, Ed., McGrawHill Book Co., Im., New York, N. Y., 1964, P 341. (3) E. A. Ukehe and N. G. Bukun, Usp. Khim., 30, 243 (1961). (4) L. Topol, J . Phye. Chem., 69, 11 (1966). (6) A. I. Belayev, Im. Vysshikh Uchebn. Zavedmii, Tavetn. Met.,
3, 46 (1960).
(6) A. I. Bdayev, ibid., 4,40 (1961).
Volume 70, Number 6 Juw 1966
1728
J. M~CIASKI AND L. SUSKI
The investigations have been carried out on the Cd-CdCh system. The solubility in this system is rather high and has been determined over a large temperature range by many authors. The most complete measurements have been carried out by Topol and Landis,' who also compiled the results of previous workers. Experimental Section
y-Ray Source. The underlying experimental problem was the choice of the y-ray source. It was found that a 4-curie Tm-170 source can be conveniently used for this purpose. This nuclide emits B raya with the energies 884 and 868 MeV, y rays of 84.1 kev, as well as X-rays of the effective energy of 53.6 kev. As a result of absorption of the B particles by the material of the source, the latter also emits bremsstrahlung y rays with a continuous spectmm and maximum energy equal to that of the p particles. It should he pointed out here that there is as essential difference between the radiometric measurements conditions in the present work and those in the work of Belayev.' The use of a Tm-170 source instead of a Cow source increases appreciahly the sensitivity of the absorption method. The difference between the absorption coefficients for y rays with Cd and CdClz is much greater in the case of Tm source than it is for a Co source. Apparatus. A schematic drawing of the apparatus is shown in Figure 1. The solution was contained in a quartz ahsorption cell with strictly parallel ground walls, the distance between them being equal to 10 0.1 mm. The absorption cell was rigidly placed in an electrical resistance furnace. The construction of the furnace was such that (a) one could measure the y radiation from the Cd-CdClz solution in a plane perpendicular to the walls of the cell, and (b) one could swing the furnace around the axis of the y-ray beam. The absorption cell was connected by means of a ground joint with a glass device which permitted the addition of definite weighed amounts of cadmium under vacuum. The temperature in the furnace was constant within *lo. It was measured by means of a Ni-NiCr thermocouple placed in a quartz tube, which was sealed in the absorption cell wall, and a recorder. The thermocouple was calibrated at the melting points of metallic antimony (630°), potassium bromide (730°), and sodium chloride (SOO'), and the measurements were carried out only at these temperatures. The measurements of y r a y intensity waa carried out by means of a scintillation counter with a NaI(T1) crystal. Reagents. The material investigated waa analytical reagent grade CdCL which waa first fused and sub-
*
Figure 1. Schematic drawing of the apparatus: 1, electric resistance furnace; 2, steel collimators of the beam; 3, w a y source; 4, hest screen of the counter; 5, quarts absorption ceU with a Ni-NiCr thermocouple; 0, dosing device; 7, metallic cadmium portions; and 8, vacuum tap.
jected to huhhliig with gaseous hydrogen chloride and cadmium which was purified by vacuum distillation. Procedure. The absorption cell was filled with a known amount (ahout 80 g) of cadmium chloride previously dehydrated in an HCl stream. Accurately weighed cadmium portions were introduced to the dosing glass device in known sequence. The m m e s of the particular cadmium portions were 1.3-1.6 g. The absorption cell and the dosing device were then connected to the vacuum line. The absorption cell containing the cadmium chloride was heated to 150", evacuated to the pressure of the order of 10" mm, and kept in these conditions for several hours in order to remove the traces of HC1 and H20 from the cadmium chloride. The whole apparatus was then washed out with helium, evacuated, and finally filled with helium. Before use, helium was purified by passing through carbon adsorbers kept a t liquid nitrogen temperature. L. E. Topol and A. L. Landis, J . Am. C h . Sac.. 82, 6291 (1960); Report NAA-SR-6310, Atomics Internstional.Canoga Park. Calif.. July 1860. (7)
1729
RADIOMETRIC I~ETERMINATION OF Cd SOLUBILITY IN MOLTEN CdC12
After filling the apparatus with helium, tap 8 (Figure 1) was closed, and the apparatus was disconnected from the vacuum line and placed in the furnace. The furnace was heated up to the measurement temperature and then the intensity of the radiation traversing pure CdC12 was measured several times. The measurements were carried out over the whole energy spectrum. The time of each measurement was 5 min and the number of pulses was 5 X lo6, which corresponded to a standard deviation of less than 0.2%. The first portion of cadmium was then introduced to the molten salt by means of the dosing device and the whole bath was mixed by swinging the furnace. Meanwhile the radiation intensity was measured ( 5 min). After each mixing operation a decrease of the intensity was observed until the cadmium was completely dissolved. We regarded the mixing as complete when the results of three subsequent measurements did not differ by more than the standard deviation. The same procedure was then applied to the next portions of cadmium.
Results and Discussion At all three temperatures a distinct decrease in the counting rate was observed while adding the particular portions of cadmium. At 630 and 730' the counting rate decreased until a certain limiting amount of Cd was introduced into the bath and then it remained constant or slightly decreased. At 800' the decrease in the counting rate while adding cadmium was quite distinct until a certain limiting amount of the metal was introduced, but after exceeding this amount the counting rates became suddenly irregular and irreproducible. One could ascribe the decrease in the counting rate with addition of the initial portions of cadmium to the dissolution of cadmium in cadmium chloride, and the state where the counting rate remains constant to the saturation of the solution. The temperatures 630 and 730' are below the boiling temperature of cadmium which is 767'. The surplus of liquid undissolved cadmium was collected at the bottom of the absorption cell below the axis of the beam. However, at 800°, after exceeding the solubility limit, the undissolved cadmium boiled at the bottom. This fact can be responsible for the irregularities of the counting rates observed under these conditions. In order to determine the solubility limit of cadmium at various temperatures, one has to find a quantitative relationship between the intensity of radiation traversing the solution and the concentration of cadmium, as well as to determine the concentration at which a
discontinuity of this relation occurs. This point should correspond to the miscibility limit of the system. The well-known absorption law for constant thickness of the absorber, X, and varying linear absorption coefficient, p, depending on the composition of the absorber is dN - = -Xdp N where CW is the change of the counting rate due to the change dp, N is the counting rate for a y-ray beam traversing an absorbing layer of thickness X and a given p. The integration of this equation from p, (linear absorption coefficient of pure CdC12) to p, (linear absorption coefficient of a solution of Cd in CdCl2 of concentration C,) yields In
No - = X(P,N,
(2)
pC)
where N, and No are the counting rates for pure CdClz and Cd-CdCl2 solution, respectively. From the additivity law of mass absorption coefficients of y rays one can determine the dependence of p, on the composition of the solution pc = P c [ W ( F m
- Pd
+
PSI
(3)
where pc denotes the density of the solution depending on C,, W is the weight fraction of the metal in the solution, and and pm are the mass absorption coefficients of pure salt and pure metal, respectively. The dependence of the density of the solution of Cd in CdC12on the concentration of this solution has been given recently by Vetyukhov, et aL8 As, however, the results of Vetyukhov, et d.,* imply a very high positive volume effect of mixing in the Cd-CdC12 system, we employed in the present work the values of density derived on the assumption of the additivity of volumes. The experimental procedure in ref 8 has not been described in detail and it seems that the measurements were not carried out in a completely closed ~ y s t e m . ~ With this assumption and transforming (3) with the use of the following relations
(4)
c
mm
= -
vc
(5)
(8) M. M. Vetyukhov, A. Asylbayev, and Yu. W. Plotnikhov, Tr. Leningr. Politekhn. Inst., 223, 35 (1963). (9) We are now investigating the possibility of using radiometric methods to the measurement of the volume effect of mixing in metal-
molten salt systems.
Volume 70.Number 6 June 1966
J. M O ~ C I I AND ~ K IL. SUSKI
1730
v , = -mm +Pm
0
m, Ps
where C, and C, are the respective concentrations of CdClz and Cd in the solutions expressed in grams per cubic centimeter, msand mm are the masses of CdCh and Cd, respectively, V , is the additive volume of the solution of a given concentration in cubic centimeters, and pa and p, are the densities of CdClz and Cd, respectively (at the given temperature), in grams per cubic centimeter,I0 the exponential absorption law for the case considered here becomes
-4
-8 -42 n
-
-16
0,
(7) Figure 2 shows a plot of the dependence on the concentration of the logarithm of the ratio of counting rate at the concentration C, to the counting rate for pure salt, obtained in two independent measurements at the temperature of 730'. From the figure shown as an example one can draw the following conclusions. (a) The linear character of this relation confirms the validity of the absorption law (eq 7) for the source used and the applied geometry of the system, in spite of assuming such an approximation as the additivity of volumes. (b) There is a distinct discontinuity in the plot at the intersection of two linear dependences. This discontinuity can correspond in the general case of a solution of two substances, to a concentration at which a discontinuity of the density vs. concentration function occurs, i.e., when there appears a new structural species or a new phase in the solution. On the basis of the knowledge of the metal-salt systems it appears that this concentration corresponds to the solubility limit of the metal in the salt. This point is well reproduced in two independent measurements, as can be seen in Figure 2. (c) Some remarks are due concerning the slope of the second linear dependence corresponding to an increase in the apparent concentration of metal after exceeding the solubility limit. One would expect the line to be parallel to the concentration axis. On the basis of the results presented in this paper one has to take into consideration that there is a dynamic equilibrium between the phase of the liquid metal and the Cd-CdCb solution. As a result of the exchange processes occurring between these two phases (simultaneous dissolution and precipitation of the metal), a certain amount of cadmium may exist in a dispersed phase in the solution. This part of the metal, the amount of which increases on mixing and with increasing the metal-salt interface (adding new portions of cadmium) , evidently increases the absorption coefThe Journal of Physical Chemistry
X
2IZ" a,
-20
-
2 u 24
- 28 -32 -36 I
-40
I
I
I
!
I
I
I
Cm [g/m3I Figure 2. Results of measurements 3 and 4 at 730". Dependence of the logarithm of the relative counting rate on the concentration of metal in the solution.
Table I Meaaurement no.
1 2 3 4
5
Temp, OC
Solubility limit of Cd in CdClz, mole %
630 630 730 730 800
17.2 16.4 19.3 19.7 20.8
Deviation from the mean value, mole %
f O .4 zt0.2
ficient of the system. However, this influence is rather small and thus, in our opinion, has no quantitative meaning. From the plotted results of measurements performed (10) Landolt-Bornstein, "Physikalisch-Chemische Tabellen," J. Springer-Verlag,Berlin, 1923.
RADIOMETRIC DETERMINATION OF Cd SOLUBILITY IN MOLTEN CdCb
850
have compared these results with the data given in the 5 literature obtained by classical methods (thermal
Q
"i
t
1
p/
,o
1731
650
5501 12
'
14
116
48
20
22
24
mole %cd Figure 3. Comparison of present results with data concerning the solubility of Cd in CdClz given by other authors.
a t three temperatures and interpreted in the abovementioned manner, we have determined the Solubility limit of cadmium a t these temperatures. The results are shown in Table I. A scattering of the results a t 630 and 730' WM taken as a measure of the accuracy of the method. As can be seen from Table I, the mean deviation of the determined solubility limits amounted to h 0 . 3 mole %. I n order to check the correctness of our method, we
analysis and chemical analysis of the phases). Figure 3 shows the results of our measurements (at the temperatures 630 and 730' average values are related) compared with the results from ref 7 and 11. For the sake of clarity the data of the other papers which have shown considerable irregularities have not been included. We have also included recent results obtained by an indirect method from the measurement of the emf of suitable concentration cells.12 As can be seen, our results show good agreement with the reliable data given in the literature. This indicates that our method is correct and can be applied with high precision. It has to be pointed out that the highest temperature at which measurements of the solubility have been made by means of the thermal analysis was 752°.7 Above this temperature the same authors have applied only the decantation method. This illustrates the increasing experimental difficulties in the classical methods encountered at high temperatures. It appears from our results that the y-ray absorption method is particularly promising at high temperatures. The high sensitivity allows, when using a suitable source, the study of systems of much lower solubilities than those described in the present work. Acknowledgments. The authors are much indebted to Drs. A. Briickman and L. G6rski for suggestions concerning the technique of the radiometric measure ment. J. M. wishes to thank Professor L. Jurkiewicz, the Director of The Institute of Nuclear Technique, for the interest he has shown in this work. (11) A. H. W. Aten, Z . Phyaik. C h m . , 73, 578 (1910). (12) W. P. Mashovets and W. P. Poddymov, Zh. Prikl. Khim., 36, 813 (1964).
Volume YO, Number 6 June 1966
