VAPORPRESSURE OF CADMIUM SELENIDE
s o v . , 1961
19.19
THE VAPOR PRESSURE OF CAD&lIUM SELENIDE BY W. J. WOSTEN Physics Laboratory of the National Defence Research Organization T.N.O., The Hague, Netherlands Received February 67, 1961
The vapour pressure of CdSe was measured between 740 and 900' by Regnaults method.' The result is log p2Cd X ps.2 (atm.3) = log Kp = - (34.400) (1/T) 20.9 for CdSe of the Wurtzite structure. The standard heat of sublimation of CdSe (25') is 39.1 kcal./mole, the standard heat of formation-37.5 kcal./mole. The entropy of CdSe (hex) at 25' is 18.6 cal./deg. mole.
+
Introduction
TABLE I
CdSe has important photo-electric properties. Measurement of the vapor pressure will be useful for the preparation of photo-conductive layers by evaporation or growing of crystals from the vapor. CdSe dissociates in the vapor
SATURATED VAPORPRESSURE OF CdSe
+
2CdSe(hex)-+ 2Cd(,) Sew (1) log K , = log p2Cdpse2(atm.3) = - AI€/2.3RT AS(w=1)/2.3R (2) and if the vapor is stoichiometric: (PCd = l,/$& log pCd =
+
- AH/6.9RT + ASp-1/6.9R
+ 0.1
(3)
We have measured the vapor pressure by an indirect method.'J h nitrogen stream is passed over the vaporizing CdSe at constant temperature and total pressure (Ptot). The gases are assumed ideal and the flowing gas is saturated with CdSe vapor of stoichiometric compositioii. Pttot)is proportioiial to the numher of gaseous molecules present (nt). For each component i in the gns the partial pressure p , is proportional to ni and by Dalton's law, the partial pressure of cadmium will be ncd PCd = ncd = ptot nt 7LNr f nCd f nSen pCdSe = pCd f pSei 1.5 pCd
(4)
(5)
Experimental (see Fig. 1) Oxygen was removed from the nitrogen stream by a copper heater. The gas was dried in a liquid air trap. The flow rate of 43 cm.3/niinute was kept constant. The saturation of the nitrogen gas with CdSe was proved by lowering the flow rate. The volume and temperature of gas in E (Fig. 1) and the atmospheric pressure (P) were measured to calculate n N z (Table I). The nitrogen pressure in E was found by subtracting the saturated vapor pressilre of water of the temperature in E from the atmospheric pressure. The furnace was rezulated to constant t.emperature and meamred with a calibrated Pt-Pt-lOyo Ith thermocouple in B (Fig. 1). The temperature differences over a length of 30 cni. in the center of the furnace were 1 2 ' . The CdSe was prepared from the elements (99.995% aure). The reaction between these elements is complicated by tile low solubility of CdSe in the molten components. A layer of CdSe is formed between the components and Cd vapor cannot esrape because the vapor pressure of selenium is higher than the Cd pressure. The vapors of the elements react violently with each other. About 100 g . of CdSe was prepared in an evacuated quartz ampoule (diam 4 cm., length 20 em.) from the elements in stoichiometric quantities. By partially heating the tube in a, horizontal furnace a t 800' high pressures are prevented. First t,he selcniuni ovaporateu to the colder parts of the ampoule. \\'hen the c:tdmiuni begins to evaporate t,hc r e x tion with selcnium proceeds rapidly. The non-reactiiig portions of the elements evaporate to the colder parts of the ampoule. Then the ampoule is reversed. This is repeated (1) Regnault, A n n . Chirn. ( P a r t s ) 1 6 , 1 (1845). (2) K. Jellinek and G. A. Rosner, Z. p h y a i k . Chern., 143, 51 (1929).
No.
T,OK.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 23
1016 1033 1037 1037 1050 1054 1060 1069 1080 1081 1083 1094 1096 1113 1116 1123 1128 1137 1151 1157 1157 1157 1160 1160 1162 1166 1168 1170
nsp,
ncd
x
mole
103, mg. at
258 253 255 250 257 266 252 251 255 251 249 255 252 253 254 252 255 258 254 250 241) 249 251 253 248 254 250 198
13.7 20 6 26 2 29 8 29 6 42 3 34 9 57 4 57 4 74 1 69 7 98 7 106 0 143 150 199 207 232 325 362 324 382 5 337 338 363 414 406 372
mg.
P'Cd
P , mm.
771.2 761.4 764.1 756.8 766.7 768.6 760.1 758.7 768.4 759.3 756.2 767. 1 766.0 767 . O 766.2 761.4 766.5 766.4 766.0 746,l 748.7 749.9 759.7 759 5 749.7 751.4 759.4 74-1.8
x
PCdSe
x
108, atm.
LOJ, atm.
5.39 8.16 10.3 11.9 11.6 16.7 13.9 22.8 22.7 29.5 27.9 39.1 42.5 57.2 59.3 79.0 81.9 90 7 139 14% 128 15% 134 133 144 161 162 184
8 08 12 2 15 4 17 8 17 4 25 1 20 8 34 2 34 0 44 2 41 9 58 6 63 7 85 9 89.0 118 123 136 193 213 192 228 201 200 216 242 243 276
until the reaction has gone to completion. The CdSe was powdered and heated to 800' in a nitrogen st'reitm to evaporate any excess of the components. Owing to the large difference in volatility the CdSe and any excess of Cd and Sc were deposited on different places in the tube C (Fig. I ) during the measurement. So the stoichiometric composition of the vapor could be ascertained visually after a measurement. It also was proved by chemical analysis of the deposit. If the nitrogen ga,s is not oxygen-free, some free seleriium will he formed by the reaction: 2CdSe 0 2 -+ 2CdO Se2. Chemical Analysis.-The CdSe in the quartz tube C wz1s tlissolved in 10 Ai' €TKOs and evaporated to dryness aftw adding cone. H*SO,. The residue was dissolved in a few drops of HC1. The Cdconcentration is measured in 1 If KH4C1-O.5 M h'H40H-0.0001410 gelatine with a polarograph a t 25'. The quantity of Cadmiuni (TU) has heeu calculated from these measurements. Calculations.-The method of least squares was used to express the measurements of Table I in equations 2 and 3 . Sve Fig. 2.
+
log
ICCdSe
= Log pzCd
x PSen --34'440 + 20.88 (740-900') T
+
(2a)
w.J. WOSTSS
1950
Vol. 65
H E = standard heat of formation of Cdde (hex) H l = H4 l/ZHs - Ht = 41.94 kcal./mole
+
Evaluation of the experimental results gives A H = -2Hs 2H4 Hg = 2H1 2Ha = (162.2 f 3) kcal. H I = (39.1 i 1.5) kcal./mole H6 = (37.5 f 1.5) kcal./mole (25') is calculaThe standard entropy of CdSe (hex), SOC~S~ ted from 2CdSeche~)43Cd(&j Sei(,) S'Cd = standard entropy of Cd vapor = 40.07 cal./deg. mole3 Sosez= standard entropy of Sez vapor = 60.22 cal./deg. mole3 8 = 2S°Cd f SoSep 2S°Cdse = (103 f 3) cal./deg. S'CdSe = (18.6& 1.5)cal./deg. mole
+
h
Fig. 1.-A = electric furnace; B = thermocouple tube; C = CdSe collecting tube; S = boat with CdSe; M = manometer; E = nitrogen collecting flask; B-C-D-S = quartz; a-b-c-d-j- are glass taps to control the gas stream.
+
+
+
-
Discussion (1) The assumption that CdSe dissociates was proved by heating CdSe powder a t 900' in K, or
selenium or cadmium atmosphere of equal pressure (-0.1 atm.) Sublimation was suppressed with selenium or cadmium, not with nitrogen. We also mixed selenium vapor with the nitrogen stream in the apparatus of Fig. 1. The CdSe did not evaporate from the quartz boat at 800°.3 The fraction of Se6molecules can be neglected in our experiment^.^ ( 2 ) The standard entropy of CdSe is not known. Specific heat data on CdSe are not available. Kireev5 calculated a value of (19.2 =t 2 ) cal./deg. mole for CdSe. This value agrees with our value (18.6 f 1.5) cal./deg. mole. The heat of formation of CdSe has been measured by Fabre6 to 24 kcal./mole. Kubaschemkilogives a value of (25 f 4) kcal./mole. (3) K o r n e e ~ a et , ~ al., have measured the vapor pressure of CdSe by effusion in the range 540-740' assuming CdSe molecules in the rapor
\
10-4
I
1 1
9.0 9.5 IO~/T(OK.). Fig. 2.-Dependence of the vapor pressure of CdSe on 1/T: I, our measurements of log pCd; 2, extrapolated and corrected values of log PCd of Kroneeva, et ai.? logpcd = - 11'480
+ 7.06 (740-900°)
(2b)
and from (2a)
Afflloo = (157.4 f 2.7) kcal. 4SlloO = (95.5 f 2.5) kcal./deg. A value of Acp = -6 cal./deg. mole was estimated for the by comparison with reaction 2CdSec,j + 2Cdtg)+ similar compounds (Z,S - CdS)."
log
P'CdSe
=
-
__ 10:57
+ 6.85 (530-730") Fig. 2
They calculated a heat of sublimation ( H 1 ) 9 0 0 0 ~= 50.1 kcal./mole. Comparing their results with our value indicates that CdSe also is dissociated in the range 540-740". McCabe8 has given a correction that can be applied to data obtained by the Knudsen effusion method. This allows the pressure of CdSez over CdSe to be calculated from Knudsen-cell data, assuming complete dissociation
+
AH258
+ 1100 Acp d2' = (162.2 f 3) kcal. = 95.5 + cP In 1'/298 = 103.2 f 3 cal./deg.
= 157.4
PCd
0.81~'~d~e
and after correcting Korneera's equation log pCd =
-
~
T
+ 6.736
(540-740')
T;17ithformu1,t 3 Hs Hi = HI. = H.c =
+
2CdSC(hpX) f- 2Cd(,j 2Se(,) st:tndard hcat of sublimation of CdSe standard heat of dissociation of CdSe in Cd and Sez vapor s h n d a r d heat of formation of CdSe vapor = 1.6 k e d . / mole3
H4
standard heat of formation of Cd vapor = 26.97 kcal./niole3 H s = Rtmdard heat of formation of Sen vapor = 33.14 kca1.l mole3 =
A H s I o = 150 kcal. ASsio = 90.8 cal./deg. mole 150 Acp(910-298) = 153.6 ked. d H0Cdse = AH'rgs 33.3 kcal ./mole
+
(3) "Selected Values of Chemical Tliermodynanlic Properties," Circular of the h*ational Bureau of Standards 500 (1952). (4) G. Preuner, a n d I. Brockmiiller, Z. ptwsik. Chenc., 81, 120 (1912). (5) B. h. Kireev, Zhur. Obshchez Khim.,16, 1569 (1946). (6) Fabre, Ann. chim. phys., 6, BO5 (1877). (7) I. V. Korneeva, V. V. Sokolov and A. V. Novoselova, Zhur. A-eorg. Khim., 5, 241 (1960) (Russ.). (8) C. L. hlcCabe, Trans. A m . Inst. Mining Met. Engrs., 200, 969 (1954).
Nov., 1961
SOLUBILITY
BND C O M P L E X
OF SILVERCHLORIDE IS MOLTESSITRATES 1951 FORMATION
910 Acp In - = 97.5 kcal. + 298 S°C& = 21.4 cal./deg. mole
NOTE ADDED 11; PRooF.-Recently Somorjai'z also published the vapor pressure of CdSe. His values do not agree with ours. We have tested our apparatus with CdS and found complete agreement with the results of P0gore1eyi.l~
(4) Drowart and Goldfingerg have measured the vapor pressure of CdSe by mass spectrometry at 480". Dissociation of CdSe is probable. Their value of 3 X lo-' atm. for p C d (480-530') is, however, too
Acknowledgment.-Thanks are due t o hliss M. G. Geers for the experimental work, t o prof. dr. G. A. 31. Diepen and prof. dr. G. AIeyer of the Technological University Delft for helpful discussions. Acknowledgment also is due to the chairman of the National Defence Research Organization T.N.O. for permission to publish this paper.
A&
= 90.8 f
(9) J. Drowart and P. Goldfinger, J . P h y s , 55, [ l o ] 721 (1958). (10) 0. Kubaschewski and E. L. Evans, "11letallurgical Tliermochemistry," Pergamon Press, London, 1956. i l l ) K. K. Kelley, Bull. U.S.Bur. &fin., no. 476 (1949).
(12) G. A. Somorjal, J Chem P h y s , 65, 1059 (1961) (13) A. D. Pogoreleyi, J . Phys Chem. U.S.S.R., 23, 731 (1948).
SOLUBILITY AND COMPLEX ION FORMLATION OF SILVER CHLORIDE I N MOLTES NITRATES BYR. A. OSTERYOUNG,~ C. KAPLSNAXD D. L. HILL Department of Chemistry, Rensselaer Polytechnic Institute, Troy, S. Y . Recezved .?larch 10, 1961
The solubility of silver chloride in the molten equimolar potassium nitrate-sodium nitrate solvent has been investigated a t 280" in the presence of a varying excess of chloride ion. The results are interpreted in terms of the formation of the species AgCl and AgClz-, and formation constants for these species are determined.
The solubility of silver halides in fused nitrates has been investigated recently by Flengas and Rideal,2 and by S e h ~ a r d . ~The former authors utilized a potentiometric titration procedure to determine the solubility product, K,, = [Ag+] [Cl-] of AgCl in an equimolar potassium nitrate-sodium nitrate solvent. Their K,, values were determined for the most part with an excess of chloride ion present, and the K,, remained constant with varying amounts of excess chloride. They obtained a K,, value of 4.89 X lopc (moles/1000 g.)2at 248' and a AH of 18.3 kcal. Seward studied the solubility of AgCl in various nitrates and mixtures of nitrates. He found that in one system, a mixture of 90 mole % potassium nitrate and 10% potassium chloride, the solubility of ,4gC1 increased relative to the solubility in the pure potassium nitrate. Earlier observations in this Laboratory had indicated that in a nitrate melt precipitated silver chloride would redissolve 011 addition of sufficient excess chloride. These observations appear to be at variance with the constant solubility product obtained by Flengas and Rideal. The present work was undertaken, therefore, to study the solubility of silver chloride in a fused potassium nitrate-sodium nitrate solvent in the presence of limited amounts of excess chloride. All measurements in this solvent were made a t 280 O. Experimental All chemicals were reagent grade or prepared from reagent grade materials. The equimolar KKO3-NaNO3 solvent was prepared by mixing the pure oven-dried salts in a bottle on a ball mill roller. Silver chloride was prepared by precipitation from silver nitrate and hydrochloric arid in aqueous solution. The furnace used was a Cenco-Cooley Crucible Furnace, powered by a variable transformer. Room tem(1j Atomics International, Canoga Park, California. (2) S. N. Flengas and K. Rideal, Proc. R o y . Soc. ( L o n d o n ) , ass, 443 11955). (3) R P Seuard .I P h y s Ciren 63,760 (1959).
perature was sufficiently stable so that the furnace temperature, on reaching equilibrium, could be held steady to within a degree of the desired value. B solubility determination was made by placing a weighed amount of solvent, about 200 g., in a 250-ml. beaker, adding approximately 1 g. of AgC1, and the desired weighed amount of potassium chloride. The beaker then was placed in the furnace which was provided with a cover having two holee for the insertion of a mercury thermometer and a motordriven glass stirrer. Equilibration was made at the selected temperature for periods of 1 to 4 hours, sometimes longer; then the stirrer was stopped and the undissolved AgCl permitted to settle to the bottom of the beaker for a half hour. Decantation of about half of the supernatant liquid \vas made by rapid pouring onto aluminum foil. The cooled solidified pieces of solvent were weighed, placed in water to leach the nitrates, and the insoluble silver chloride was determined by filtration through a previously weighed sintered glass crucible. As a check on this procedure, a determination of the solubility of AgCl in pure molten KXO3 was performed, and the results were in good agreement with those reported by Seward. Preliminary efforts to obtain reproducible data for the solubility of AgCl in the KN03-7SaX03 solvent by this procedure indicated that one hour equilibration a t a temperature of 280" was insufficient and that it was preferable to allow a t least three hours for the systems to equilibrate a t the desired temperature. This point was further checked by preparing one sample of the molten nitrate solvent containing approximately 0.008 mole of dissolved AgS03 and a second sample containing the same number of moles of KC1. The two fused samples were mixed together, the precipitated AgCl was allowed to settle and then treated as indicated above. The results from this procedure were in good agreement with those from the longer period equilibration of the melts containing the solid AgC1. One other comment regarding an experimental observation is pertinent. Certain batches of solvent prepared from one manufacturer's reagent grade potassium nitrate were observed to form a brown precipitate, probably silver oxidc, when the equilibration with the silver chloride was made. -1similar precipitate formed on addition of silver nitratr to the same batch of solvent. It was concluded that this potassium nitrate contained sufficient impurity, probably o d e or hydroxide, t o precipitate silver oxide, which ~ : t h less soluble than the silver chloride. In all experimental results cited here the melt used mas free of brownish prccipitate on addition of the silver chloride.
