1446
J. C. SCHOTTMILLER, A. J. KINGAND F. A. KANDA
Vol. 62
THE CALCIUM-STRONTIUM METAL PHASE SYSTEM' BY J. C. SCHOTTMILLER, A. J. KINGAND F. A. KANDA Contribu,tion from th.e Chemistry Department, Syracuse University, Syracuse, N . Y. Eeceived July 7,1868
The calcium-strontium phase system has been investigated in the liquid and solid state over the entire range of compositions. The liquidus and solidus curves were determined by differential thermal analysis whereas the solid state e uihbria were studied by X-ray diffraction methods. Complete miscibility in both the liquid and solid state were observe8 with a minimum in the liquidus-solidus a t 78 f 2 weight % strontium and a t 738". Calcium, strontium and all calcium-strontium alloys exist in three crystalline modifications, face-centered cubic, hexagonal close-packed and body-centered cubic. There is a minimum in the b.c.c. h.c.p. transition, but the h.c.p. Ft f.c.c. transition in all alloys occurs at temperatures intermediate t o the transition temperatures of the pure components. The variation of with atomic % a t room temperature for f.c.c. alloys is linear in conformity with Vegard's law. A t 415" the variation of both and ~0 of the h.c.p. modification is linear with atomic per cent. for the calcium rich alloys; however, positive deviation from Vegard's law occurs with strontium-rich alloys. Positive deviation from Vegard's law also occurs in the strontium-rich b.c.c. alloys (615").
Introduction Although phase equilibrium diagrams for the systems calcium-barium,2 strontium-barium,a strontium-rnagnesi~m,~calcium-magnesium6 and barium-magnesium6 have been completely or partially determined, no complete investigation of the phase relationships in the calcium-strontium system has been reported. This system is of interest because both components undergo analogous phase transitions a t elevated temperatures: f.c.c. h.c.p. e b.c.c. The more reliable values which have been reported for the melting point of calcium range from 839 t o 851°,2J-9 and for strontium from 768 to 7-71O . W Klemm and Mikaio and King,Ii using X-ray diffraction methods, found that at room temperature calcium and strontium form a continuous series of face-centered cubic solid solutions over the entire range of compositions. Kingi1 reported a linear variation of unit cell size with atomic per cent. in conformity with Vegard's l a ~ , ' ~ Jwhile 3 Klemm and Mikaio reported a slight negative deviation from Vegard's law. Both calcium and strontium have been found to exist in three allotropic modifications, face-centered cubic (f.c.c.), hexagonal close-packed (h.c.p.), and body-centered cubic (b.c.c.). A recent report by Smith, Carlson and Vest14 lists only the f.c.c. and the b.c.c. modifications for pure calcium which was especially low in magnesium content (0.01%). Sheldonlz using high temperature X-ray diffraction techniques, observed the f.c.c. e h.c.p. transition in calcium at 335 i lo", and the h.c.p. (1) Abstracted from the thesis of J. C. Sohottmiller submitted to the Chemistry Department of Syracuse University in partial fulfillment of the requirements for the Ph.D. degree, March, 1958. (2) E. A. Sheldon, Thesis, Syracuse Univeraity, 1949. 80, (3) R. J. Hirst, A. J. King and F. A. K%nda,THISJOURNAL, 302 (1956). (4) J. P. Ray, Thesia, Syracuse University, 1949. (5) R. E. Parkinson, Thesis, Syracuse University, 1933. (6) W. C. Zeek, Thesis, Syracuse University, 1956. (7) A. von Antropoff and E. Falk, 2. anorg. allgem. Cham., 187,405 (1930). (8) F. Hoffmann and A. Schulze, Phzlsik. Z.,86, 453 (1935). (9) F. Weihke and W. Bartels, Z . anorp. al2gam. Chem., S18, 241 (1934). (IO) W. Klemm and G . Mika, 2. onorg. Cham., 248, 155 (1941). (11) A. J. King, J . Am. Chsm. Soc., 64, 1226 (1942). (12) L. Vegard. Z. Phyezk, 6 , 16 (1921). (13) L. Vegard and H. Dale, Z. Rwat., 67, 148 (1928). (14) J. F. Smith, 0. N. Carlaon and R. W. Vest, J. Electrochsm. Soo.. lPS, 499 (;lPF3)*
b.c.c. transition at 610 i 10'. Sheldon, et U Z . , ~ ~ and Hirst, et C C ~ reported .,~ 213-215 10" as the transition temperature for the f.c.c. h.c.p. transformation in strontium and 605-608 f 10" for the h.c.p. b.c.c. transition. Smith, et a2.,I4 reported only a f.c.c. a b.c.c. transition for calcium at a temperature of 464". Experimental
*
Purity of the Metals and Alloys.-Research grade calcium metal used in this study was obtained from Dominion Magnesium Ltd., of Toronto, Ontario, Canado. Spectrographic and chemical analysis showed the calcium to contain less than 0.1% magnesium and nitrogen combined and 0.1% Sr with traces of Ba, Na and K. The melting point was found to be 843 f lo. The strontium metal was obtained from King Laboratories Inc., Syracuse, N. Y . The commercial grade strontium was purified by vacuum distillation at a temperature of 815' and a t a pressure of less than I p . Prior to the distillation the charge of strontium was kept for several hours a t a temperature of 500" and a t a pressure of less than 1 p . This distillation procedure was an adaptation of the procedure described by Smith, el a1.,14 for the purification of calcium. The melting point of the distilled strontium was found to be 774', an increase of 9" over that of the commercial product. Spectrographic analysis showed the distilled strontium to contain 0.1% Ca, 0.1% Ba and 0.1% Si, with traces of Mg, Na, AI,Fe and Li. Since the metals used in this investigation react rapidly with ordinary atmospheric gases, special precautions were observed in handling them. Alloy samples of nearly constant volume (9 cc.), weighing from 12 to 20 grams deuending on the composition, were prepared by weighing to within 0.001 g., appropriate amounts of the metals in an argon atmosphere. Fusion of the alloys was likewise carried out under an atmosphere of argon purified by passage over hot bsrium chips. Chemical analyses of the fused samples agreed within i1.5% of the make-up composition. X-Ray specimens for room temperature diffraction studies were prepared under a protective coating of oil. This method could not be used for the high temperature studies; therefore, a special unit was used which allowed for maintenance of an argon atmosphere during the preparation of an alloy powder and the loading and sealmg of the powder into Vycor capillary tubes. Thermal Analysis.-In general the thermal. analysis was performed with the equipment and technique previously reported by Hirst, et aE.* Instead of a watercooled electric resistance furnace of the tyue used by Hirst, a furnace consisting of four "Globar" elements arranged concentrically around the cylindrical iron crucible was used to facilitate attaining and controlling higher temperatures. Controlled heating and cooling rates of 1-4"/minute were used in the investigations. Temperatures, measured with a chromel-alumel thermocouple, were recorded by a variabIe range Brown recorder.lB (15) E.A. Sheldon and A. J. King, Acta Cryst., 6 , 100 (1963). (16) F. A. Kanda and R. C. Shaver, J . Am. Cer. SOC.,86, 101 (1953).
#
Nov., 1958
CALCIUM-STRONTIUM
The recording thermocouple was calibrated and checked frequently against melting point standards and a standard Pt-90yo Pt, 10% Rh thermocouple certified by the National Bureau of Standards. In addition, temperature differential us. time was recorded simultaneously on a separate Brown recorder. The temperature differential was obtained by immersing one junction of the differential thermocouple in the alloy charge and the other in alumina powder in another part of the furnace. X-Ray Diffraction.-Room temperature and high temperature X-ray diffraction methods were apdied to determine the equilibria in the solid state. A Debye-Scherrer type high-temperature camera, similar in design to that described by Hume-Rottery, et uL.,lTwas used in these investigations. The entire unit, including the temperature controller and recorder, was calibrated by observing, w t h the aid of a telescope, melting points of standardized salt mixtures in capillaries mounted in the camera. Temper:tures of specimens could be read with a reliability of Az2 ; however, transition temperatures determined relative to the first observable appearance or disappearance of particular diffraction lines could not be bracketed closer than rt6'. Pyrex capillaries usually were used to contain the specimens for investigations below 500°, and Vycor capillaries were used above this temperature. Although the glass capillaries were darkened somewhat a t temperatures above 575" and diffraction lines of the metal oxides began to appear after long exposure, the transition temperatures were shown to be unaffected. Samples mounted so that the metal was not in contact showed the same transition temperatures as samples which were in direct contact with the glass capillary.
Results Calcium and strontium display complete miscibility in both the liquid and solid state (Fig. 1). A minimum occurs in the liquidus-solidus at 78 i 2 weight yostrontium and at a temperature of 738 f 1 ". The melting point of calcium was found to be 843 1" and that of strontium 774 1". Points on both the liquidus and solidus curves were obtained on cooling cycles by differential thermal analysis. The liquidus points were reproducible to within i1", and the solidus points to within k 2 O . The two-phase region liquid plus solid is very narrow, never exceeding a range of 6". Calcium, strontium and all the alloys were found to exist in three crystalline modifications, f.c.c., h.c.p. and b.c.c. The room temperature modification is f.c.c. for all compositions. The h.c.p. phase appears at 344 f 6" in pure calcium and at 230 i 6" in pure strontium in good agreement with the values reported by Sheldon2and Hirst, et uZ.* I n all alloys of calcium and strontium this transition occurs at temperatures between the transition temperatures of the pure components. b.c.c. transition for pure calcium The h.c.p. (610 f 6") and strontium (621 f 6") are likewise in good agreement with the values previously reof calcium in strontium and of p ~ r t e d . ~Solution ,~ strontium in calcium lowers the h.c.p. Ft: b.c.c. transition temperature until a minimum is reached at about 55 weight yostrontium and at a temperature of 510 5". The lattice constants for pure calcium and strontium in their three allotropic modifications are shown in Table I. The variation of U O with atomic per cent. in calcium-strontium alloys a t room temperature is linear in accordance with the predictions of Ve-
METALP H A S E
(17) W. Hume-Rothery and P. W. Reynolds, Proc. Roy. Soc. (London), 816'7, 25 (1938).
I C C
L
'
0
10
I
20 30 40 50 60 70 80 90 100 Weight yo strontium. Fig. 1.-The hase diagram of the system calciumstrontium. The {quidus-solidus is lotted on an enlarged scale. The "com lex phase" was ofserved only for 100% calcium and for tge 10% calcium-90% strontium alloy.
6.100
4 6.000 CI B 5.900 'a
.3
2 5.800 *%
fj5.700 5.600
*
*
1
100
1447
SYSTEM
5.500
I
30 40 50 60 70 80 90 100 Atom yo strontium. Fig. 2.-The lattice parameter, ao, of f.c.c. calciumstrontium alloys versus atomic yo strontium. Data were obtained a t room temperature. Vegard's law is followed at all compositions. 0
10 20
4.900 4.800
d
4.700 4.600
4.500
4.400 4.300 0
20 30 40 50 60 70 80 90 100 Atomic % strontium. Fig. 3.-The lattice parameter, m, of b.c.c. calciumstrontium alloys versus atomic % ' strontium. Data were obtained a t 630'. Positive deviations from Vegard's law occur at high strontium concentrations. 10
TABLE I F.c.c. H.c.p.
B.o.c.
Calcium a0 = 5 . 6 0 1 h 0.0015 A. (25O) ao = 4.00 f:0.02 A. (415O) co = 6 . 5 0 f 0.02 A. ao = 4.488
* 0.005 A. (615')
-
Strontium
6.084 z t 0.002 A. (25') ao = 4.33 f 0.02.&. (415') cp = 7.05 f 0.02A. ao '4.87 =t0.02 A. (830')
ao
gard's lawla (Fig. 2). However plots of & for b.c.c. alloys (630") and a0 and co for h.c.3. alloys (415")
1448
J. C. SCHOTTMILLER, A. J. KINGAND F. A. KANDA
Vol. 62
However the distilled product melted at 842 f 1". A sample of 99.95% calcium, obtained through 4.400 the courtesy of J. F. Smith14 of the Institute for Atomic Research, Ames, Iowa, was found to melt 4.300 at 842 i 1". The spectrographic and chemical analysis of this particular lot of calcium indicated: , 4.200 ,p,-Mg, O . O l % , N, 0.009%; and C, 0.02%. Other 2 impurities found in the calcium were present to the / 4.100 extent of 0.001% or less. 4.000 If the true melting point of calcium is different from the value reported here, then all calcium sam3.900 ples investigated in this Laboratory must have a constant amount of some impurity originally present or introduced during the melting point deter0 10 20 30 40 50 60 70 80 90 100 mination. Atomic % strontium. Allotropy of Calcium.-Smith, et aE.,l4 reported Fig. 4.-The lattice parameter, UO, of h.c.p. calciumstrontium alloys versus atomic % strontium. Data were no h.c.p. modification of calcium for samples conobtained at 415'. Positive deviations from Vegard's law taining 0.110 and 0.010% magnesium but observed occur at high strontium concentrations. the h.c.p. phase in samples containing 0.300% magnesium. The calcium used in the present investiga7.100 I tion contained 0.1% magnesium, and a h.c.p. phase was observed. Investigations in progress in this 7.000 Laboratory on the calcium-rich end of the Ca-Mg system indicate that magnesium lowers the b.c.c. S 6.900 h.c.p. transition to a temperature below 450" 6.800 between 98.8 and 100% Ca.lg J It is possible that the rate of transformation in 6.700 high purity calcium is much slower than in the less pure product, where impurity sites may act as cen6.600 ters of nucleation in the transformation. Under such conditions the h.c.p. phase in very pure cal6.500 cium might escape detection if sufficient time were not allowed for the phase transformation to occur. 0 10 20 30 40 50 60 70 80 90 100 A "450" transition" has been reported frequently Atomic % strontium. in published calcium binary systems. This is probFig. 5.-The lattice parameter, eo, of h.c.p. calcium- ably the Ca-Mg eutectic (m.p. 456"). Thermal ' strontium. Data were evidence of this eutectic up to 98.8 wt.70 calcium strontium alloys versus atomic % obtained a t 415'. Positive deviations from Vegard's law has been observed in this Laboratory. It is interoccur a t high strontium concentrations. esting to note that the melting point of calcium reagainst atomic % show positive deviations from ported for these systems showing the "450" transilinearity in the region of high strontium concentra- tion" varies from 808 to 820". The melting point tion although the variation is linear or near-linear at found in.this Laboratory for 98.1 and 98.80/, calcium alloys is 810 and 823", respectively. Both of high calcium concentrations (Figs. 3 , 4 and 5). The "complex" phase of calcium,2~14,18 was ob- these alloys show the eutectic thermal break a t served in pure calcium from 300 to 345" and in the 456". X-Ray evidence shows no transition involving a change in structure at 456". 90% Ca-10% Sr alloy from 285 to 330". Deviations from Vegard's Law.-The positive The "d" values for this phase are in good agreement with those reported by Smith, et a1.I4 The ap- deviations from Vegard's law12 at elevated tempearance and disappearance of this phase was re- perature indicates that the addition of calcium to versible with temperature. Attempts to identify strontium has little effect on the cell dimensions of the structure of the "complex phase" have so far strontium until 15 to 20 atomic % calcium has been added. Further addition of calcium causes proved unsuccessful. the lattice to contract sharply. However, the Discussion addition of strontium to calcium produces a linear Melting Point of Calcium.-Although the melt- increase in the cell dimensions up to 50 atomic %ing point of calcium has been reported to be as Similar phenomena were observed by Klemm high as 851",' the results of this investigation indicate that 843 i 1" is the melting point of the and MikalO in their room temperature X-ray studies of the systems Ca-Ba and Sr-Ba. The failure highest purity calcium presently available. The calcium used in this study, which had a pur- of the Ba lattice to contract upon addition of Ca or ity of 99.8%, melted at 843 l". Vacuum distil- Sr was attributed to the solution of some Ca or Sr lation techniques which had resulted in the eleva- atoms interstitially. This explanation appears untion of the melting point of strontium from 765 to tenable upon consideration of the relative sizes of 774" were applied t o purify further this calcium. the atoms involved. It seems more reasonable to regard all solid solu-
1
P
a
,
*
(IS) L. Graf, Mstollwi~ldchoft,12, 649 (1938); L. Graf, Phys. Z.,
86, 661 (1934).
(19) G. L. Becker, Syracuee University, unpubliahed dats.
"
KINETICS OF NITROGEN FORMATION FROM NITROUS ACID AND AMMONIUM ION
Nov., 1958
tion in these systems as substitutional. The lattice of the smaller atoms must expand to accommodate the larger atoms whereas smaller atoms can be substituted in the lattice of the larger atoms without affecting the lattice constants appreciably. Whether or not lattice contraction occurs upon initial substitution of smaller atoms certainly must depend on interatomic forces operative in the crystal. It is significant that in the calcium-strontium system at
1449
room temperature substjtution of calcium atoms in the lattice of the larger strontium atoms is accompanied by a linear lattice contraction whereas a t higher temperatures, where interatomic attractions are partially overcome by thermal agitation, the lattice fails to contract proportionately. Acknowledgment.-The authors are grateful to the Atomic Energy Commission for the financial support of this work on Contract AT(30-1)1910.
THE ICINETICS OF NITROGEN FORMATION FROM NITROUS ACID AND AMMONIUM OR METHYLAMMONIUM IONS1 BY GORDON J. EWING AND NORMAN BAUER Chemistry Department, Utah State Uniuersity, Logan, Utah Received J u l y 81, 1968
Rate constants for the Dusenberry-Powell interpretation of the Van Slyke reaction with ammonium and with methylammonium ions are found to be several-fold smaller than reviously reported when an important side reaction of nitrous acid is taken into account. An evaluation of the secondary saft effect shows that the kinetics of Van Slyke-type reactions cannot be worked out in detail without knowing the activit coefficients for nitrous acid dissociation It also shows that the AbelTaylor rate law expression fits the available facts. Zorrections for nitrous acid decomposition and requirements for buffering indicate that neither proposed rate law can be accepted as yet. The unequivocal presence of methyl chloride in the product gas when sodium chloride is used in the aqueous solvent for methylamine supports the suggestion of Austin and others that the amine is converted to a carbonium ion intermediate in the Van Slyke reaction.
In spite of many studies2-' over the past thirty years, the kinetics of even the simplest Van Slyke reactions between nitrous acid and amino groups are still obscure. Here we wish to point out certain deficiencies in recent studies and to report new values for the rates of Nz-formation from ammonium and from methylammonium ions in aqueous phosphate solutions. Major difficulties in determining rates of the reaction RNH2 HNOz = ROH Nz H20 have been (1) the parallel decomposition of nitrous acid, which also yields a gaseous product: 3"02 = 2N0 HNOI H20; (2) a number of other side reactions2; and (3) control of pH. Abel and coworkers4 obtained a third-order rate law for the reaction of nitrous acid and ammonium ion
+
+
+ +
+
d(N,)/dt = le(NH*+)(NOz-)(HNOz)
while Taylor6 obtained a similar law for the corresponding reaction with methylammonium ion. In both studies, precautions were taken to limit the decomposition of nitrous acid but neither adequately controlled the pH. Dusenberry and Powell,s on the other hand, buffered their reaction mixtures somewhat but failed to evaluate"1properly the rather considerable effect of nitrousuacid decomposition, as we shall show below. In addition, they buffered their reactions with phosphate which may' influence the mechanism. The DusenberryPowell (D-P) rate law was second order, d(N%)/dt (1) Supported by Western Regional Project W-31, Utah Agricultural Experiment Station. (2) A. T. Austin, J . Chem. Soc., 149 (1950). (3) J. C. Earl, Research (London), 8 , 120 (1950). (4) E. Abel, H. Sohmid and J. Sohafranik, 2.physik. Chem., Bodenstein Feslband, 510 (1931). ( 5 ) T. W. J. Taylor, J . Chem. Soc., 1099 (1928). ( 6 ) J. H. Dusenberry and R. E. Powell, J . A m . Cham. SOC.,73, 3266 3269 (1951). (7) A. T.Austin. E. D.Hughes, C. K. Ingold and J. H. Ridd, ibid.. 74, 555 (1952).
= k=p(RNH3+)(HN02),for both NH4+ and CH3NH3+. Austin and eo-workers' have repeated Taylor's work and "have found it entirely accurate," but we feel the results are inconclusive without a more careful study of the pH dependence. Influence of Activity Coefficients on the p H Dependence of Van Slyke Rates.-The AT and k ~ values p can be evaluated properly and tested for constancy over a significant pH range only when the true concentration of undissociated nitrous acid, (HN02)tr,is known. Up to now the apparent values (HN02)apphave been used on the unwarranted assumption that the activity coefficients involved in nitrous acid dissociation are unity. That is, Bronsted's secondary salt effect has been neglected. We shall evaluate the effect of introducing reasonable activity coefficients; and show that their neglect leads to substantial errors in rate constants, depending on pH. Also, it turns out that when the secondary salt effect and the effect of nitrous acid decomposition are both neglected in the pH range 1 to 4, it is not possible to distinguish between the Abel-Taylor (A-T) and Dusonberry-Powell (D-P) rate laws. As seen from the following obvious expression for (HNOZ)~,, there are two ways in which activity coefficients influence an evaluation of nitrous acid concentration: (1) through the ratio of the thermodynamic to the apparent dissociation constants, (Fa = Ka/Kd) , where Fa = (YH ?NO,-) Y H N O ~contains the individual ion activity coefficients; and (2) through the discrepancy between the apparent acid concentration determined in the usual way by a , the true acid concentrapH meter, ( H + ) p ~and tion, (H+) = ( H + ) p ~ / ~ ' ~ + . +
