NOTES
172
A THERMOANALYTICAL STUDY OF THE BaClz RECIPROCAL SYSTEM 2KN03 2KCl Ba(NO&
+
+
BY VIRGINIAD. HOQAN AND SAULGORDON Pyrotechnics Chemical Reeearch Laboratory, Picatinny Arsenal, Dove?, New Jersev Received J u l y 87,1363
A thermoanalytical study of the binary oxidant system potassium perchlorate-barium nitrate' revealed varied and interesting thermal effects which suggested an evaluation of the high temperature phenomena characteristic of the stoichiometric potassium chloride-barium nitrate and barium chloride-potassium nitrate systems. Investigation of these systems utilizing the techniques of differential thermal analysis and thermogravimetry shows the fusion of a eutectic mixture of barium nitrate and potassium chloride a t 300" and a solid state metathetical reaction between potassium nitrate and barium chloride a t approximately 250". The potassium chloride and barium nitrate formed by the latter reaction are the stable salt pair in this reciprocal system.
J
3 Iz w
0-/
a
20-
k !
n 40
,
100
I
I
I
I
I
I
I
200
300
400
500
600
700
800
SAMPLE TEMPERATURE PC). Fi l.-DTA curves for 2 : l mole ratio of (A) KN03BaCE and (B) KCl-Ba(N08)t, obtained a t a heating rate of 15' per minute.
.
Reagents .-Barium nitrate, Analytical Reagent (Mallinckrodt Chemical Works), potassium nitrate, potassium chloride, barium chloride, analytical reagent (Fisher Scientific Company) were used. Instrumentation and Procedures.-The differential thermal analysis and thermogravimetric analysis apparatus employed have been described previously .a Three to five gram samples were taken for the differential thermal analyses and an equal volume of alumina served a8 the reference material. On the differential thermal analysis curves, the temperature difference between the sample and reference materials is plotted as a function of sample temperature. The furnace was programmed for a linear heating rate of 15' per minute. Three hundred and fifty mg. samples were used for all thermogravimetric analysis with a full-scale range for changes in weight of 200 mg. The furnace was ( 1 ) V. Hogan and S. Gordon, THISJOURNAL, 62, 1433 (1968). (2) V. Hogan, 9. Gordon and C. Campbell, A n a l . Chem., 29, 806
(19571.
VOl. 64
rogrammed for a linear heating rate of 10' per minute. '%he compositions studied were 2 : l mole ratio potassium chloride-barium nitrate (36-64% by weight) and 2: 1 mole ratio potassium nitratebarium chloride (49-51 % by weight). Ten grams of each composition was prepared and samples for individual determinations were taken from the bulk supply. Th,e samples containing barium chloride were dried a t 190 .
Results and Discussion Barium and potassium chlorides do not show any thermal reactions or weight losses up to 750". Barium nitrate melts a t 600" and decomposes endothermically at 700". Potassium nitrate undergoes endothermic crystalline transition from the rhombic to the trigonal lattice a t 128", melts at 334" and begins to decompose endothermically a t 750°ea As shown in Fig. 1 the potassium chloridebarium nitrate mixture exhibits partial melting at 370". It is completely liquid at 460" and begins to decompose a t 700". The potassium nitratebarium chloride system exhibits the endothermic crystalline transition of potassium nitrate at 128", then a well defined "sugar loaf" exothermic peak which begins at 255". This exotherm is followed by an endothermic peak of similar shape which begins at 300", as evidenced by the change in slope, before the system has recovered from the effects of the exothermic reaction. Partial melting is observed at 330" just after the peak of the endotherm. The sample is completely molten and clear at 500" and begins to bubble rapidly a t 550". Brown fumes signifying decomposition of nitrate ion are observed at 700". The thermogravimetric curves previously reported for these systems are equivalent and confirm these observations. The low temperature endotherm at 300" displayed by the potassium chloride-barium nitrate system results from the fusion of a eutectic mixture of the two salts. The reciprocal system potassium nitrate-barium chloride exhibits a pair of thermal bands, viz., an exotherm a t 250" and an endotherm a t 300" which can be explained as the result of an exothermal metathetical reaction and subsequent eutectic fusion of the products. For the reaction BaClz 2KNO8--t Ba(NO& 2KC1, A F at 255" is -1.5 kcal./mole and AH a t 255" is -8.2 kcal./ mole, showing that it is both thermodynamically spontaneous and exothermal. The reaction postulated was experimentally confirmed by heating a finely ground mixture of 2 to 1 mole ratio potassium nitrate-barium chloride in a muffle furnace for 3.5 hr. at 270". No visible melting was observed, although the sample sintered. X-Ray diffraction analysis of the previously heated sample revealed that it consisted of only barium nitrate and potassium chloride, the stable salt pair in this reciprocal system. The thermoanalytical data suggest that this reaction occurs in the solid state. Potassium nitrate crystals are trigonal and potassium chloride, barium chloride and barium nitrate crystals are cubic at 255". In addition the crystal radii of K + and Ba++ are almost identical, 1.33 and 1.35 A.,re~pectively.~These properties are indicative of a
+
+
(3) V. Hogan and 9. Gordon, THISJOURNAL, 63, 93 (1959). (4) L. Pauling, "The Nature of the Chemical Bond," Cornel1 Univ. Press, Ithaca, New York, 1944, pp. 343-350.
NOTES
Jan., 1960 simple lattice structure and spatial symmetry that should facilitate the ionic mobility necessary to effect a solid state metathetical r e a ~ t i o n . ~However, the rapid rate of the reaction as measured by the time interval between the initial point and the maximum point of the exothermal band on the DTA curve, Fig. 1, may not be consistent with the diffusion rates associated with volume diffusion of ions in solid state crystals at these temperatures. This time interval is approximately one minute for a nominal sample weight of two grams. As an alternative, the reaction mechanism may involve micromelting of the salts a t the crystalline interfaces with subsequent rapid recrystallization of the insoluble stable salt pair. This is consistent with both the gross sintering effect observed and the abrupt onset of the exothermal reaction at 255". This latter explanation may also apply to the previously reported exothermal solid state reaction between barium perchlorate and potassium nitrate.3
173 K . = NQU
(1)
When the particles are large the contribution from the moving particles is very small2and equation l reduces to the equation used by Streeta for the determination of the surface conductivity of clay particles As = (FK.
- k)(A/S)
As pointed out by Henry2 we cannot readily measure surface conductivity from suspension measurements since the particle charge calculated from mobility is dependent on surface conductivity, hence the calculation is made by a series of approximations. However, it is possible to avoid this, if vie put N = p / V , and since S / A = SW/(l- p ) = pp/(l p ) where p = volume concn. of the particles V = volume of one particle S, = surface area of solid per cc. of suspension p = surface area of solid per cc. of solid
then SURFACE CONDUCTANCE OF SUSPEKDED PARTICLES
+ k / F + A.S/FA
Ks = Q U P P
+ k/F + p W / F ( 1 -
P)
and
BY NORMAN STREET Department of Mining and Metallurgical Engineering, University of Illinois, Urbana. Illino~s Received J u l y S 4 , 1969
With suspensions it is possible to use an approach to the measurement of surface conductivity very similar to that used on porous plugs. This involves the assumption that the double layer ions can be redistributed throughout the interparticulate liquid without altering the conductance of the porous plug or suspension; in fact of course the average electric field is different at the surface and in the bulk solution. However, the equations available for the calculation of surface conductivity from measured zeta potentials also implicitly involve the same assumption. Thus for a porous plug' X. =
(A/S)AK
and so it follows that
F can be put equal to (x+p)/x(l - p ) , (Fricke4) and here x is a function of particle shape and conductivity and in fact corrects for the change of field in the interparticulate 'solution resulting from the removal of excess ions from a surface and their redistribution through the solution. Then (3)
and for spherical particles x = 2 and so when a is the particle radius
where A, = surface conductivity A / S = hydraulic radius AK = difference between the bulk conductivity of the electrolyte soln. and the apparent conductivity when present in the pores of the plug
For a suspension there is an additional conductance caused by the movement of the charged suspended particles themselves under the influence of the imposed field. We can write the suspension conductivity as K , = XQU
+ K/F
where N Q U K F
number of particles per cc. particle charge = particle mobility under unit field = apparent conductivity of interparticulate soln. = formation factor (i.e., F = K / K , when Q = 0, A, = 0 ) = =
However K = k(l
+ X.S/kA)
when k is the bulk value of the solution conductivity, and so (1) N. Streot, THISJOUI%NAL,62, 88Y (1968).
Experimental An approximate check of equation 3 was made using a Ca kaolinitea for which p = 44.6 X lo4 and the axial ratio was 8 (incorrectly reported as 12 by Street a ) ; with this axial ratio z = 0.6 for particles of zero conductivity. The ronductivity measurements were made a t 50 cycles per second, i . e , below the frequency at which the electrophoresis effect disappears.6
Results and Discussion Table I gives Xa calculated from the slopes of the curves and A, calculated from zeta potential = 30 millivolts.6 TABLE I k
5.3 x 10-6 3.15 X
Xs(measd.)
x 0.4 x 1
10-9 10-9
b(cs1od.)
0.53 x 10-9 0.41 x 10-9
Actually x depends on the conductivity (ap(2) D. C . Henry, Trans. Faraday Soc., 44, 1021 (1948). (3) N. Street, Australian J . Chsm., 9, 333 (1956). (4) H. Fricke, Phys. Reo., 24, 575 (1924). (5) Th. Svedberg and H. Anderson, Kolloid Z . . 24, 156 (1919). (6) N. Street and A. S. Buchanan, Austmlznn J. Chem., 9, 540 (1956).
