- Email: [email protected]
The system CdOH2O
J. Imm'g. N u ~ Chem.. 1961. Vol. 170pp. 96 to 10l. Pm'pnmn Prims Ltd.
THE SYSTEM CdO-H~O L. S. D~,~T Gt~SSER a n d R. RoY College of Mineral Industries, Department of Geophysics and Geochemistry, Pennsylvania State University, Pennsylvania (Received 4 April 1960; in revised f o r m 5 July 1960)
A l ~ r ~ t ~ T h ¢ system CdO-H,O has been studied at high temperatures and water p r e s s u ~ up to 3000 bars, and thep--t curve for the reaction Cd(OH)~ ~ CdO + HjO is presented. The kinetics of tlw dehydration of Cd(OH)| in an essentially water-free atmosphere were also investigated. Values of A / / r ~ t . and the activation energy of dehydration have been calculated. T~E r e a c t i o n M O + H i O ~-- M ( O H ) I , where M O has the r o c k - s a l t a n d M ( O H ) s the brucite structure, c a n be studied f o r several different cations. Systematic s t u d y o f such a series o f a n a l o g o u s r e a c t i o n s m i g h t e n a b l e c o r r e l a t i o n s to be m a d e between the t h e r m o d y n a m i c values o b t a i n e d a n d the prol~.rties o f the cations concerned, structural c o n s i d e r a t i o n s h a v i n g been eliminated. D a t a f o r the e q u i l i b r i u m p--t curves for the ions M g 2+, C a a+, Ni 2+, a n d M n ~(~-5) have p r e v i o u s l y been o b t a i n e d in this l a b o r a t o r y . T h e present w o r k deals with d a t a for a n o t h e r cation, c a d m i u m , which has n o t p r e v i o u s l y been studied. EXPERIMENTAL The starting materials for the study were as follows: [(1) Cd(OH)~. From a solution of C.P. Cadmium Nitrate, Cd(OH)s was precipitated by addition of excess NaOH solution; the precipitate was filtered and washed on a B(lchner funnel. Washing was continued until the filtrate showed no reaction to phenolphthalein. The precipitate was dried overnight at 100°C and ground in an agate mortar. (2) CdO. Cd(OH)s, prepared as described above, was heated to about 900~C, for a short time (because of the volatility of CdO). Decomposition was rapid, and the material produced was very reactive. Powder X-ray diffraction patterns for both compounds agreed well with those in the ASTM X-ray powder data file and showed no trace of impurities (such as carbonate). The card for Cd(OH)s, however, is not of high accuracy and shows a discrepancy between the values of a0 and co quoted, and those calculated from the powder data given. Accordingly, the powder pattern for Cd(OH)s was redetermined, using a Norelco high-angle diffractometer, with silicon as an internal standard. The new data are given in Table I. The hydrothermal studies were made using "test-tube" (or cold-seal) vessels at pressures up to about 3000 bars. The samples were contained in gold foil envelopes, and in every run but one both CA(OH)s and CdO were used. Temperatures were automatically controlled, and measured using a chromcl-alumel thermocouple and a Brown automatic precision indicator, and they are accurate to +5°C. Pressures were measured on a Bourdon gauge, calibrated against a secondary standard, and are accurate to better than -I-5 per cent. For the kinetic studies, the sample was contained in a small open platinum boat, which was placed in a horizontal furnace. The temperature was controlled to ~ 3°C with a Honeywell controller, and (t) D, M. Roy and R. Roy, Amer. Mineral 39, 972 (1954). ts) D. M. RoY and R. Roy, Amer. Mineral 39, 957 (1954). (s) D. M. ROY and R. Roy, Amer. J. $ci., 225, 574 (1957). (4) C. KLINOSUXOand R. Roy, Amer. Mineral 44, 819 (1959). (i) p. j. WYLIEand O. F. TUT'rl.E,J. Amer. Ceram. Soc. 42, 448 (1959).
~s
The system CdO-HIO
99
measured with a chromel-alumel thermocouple p l ~ immediately above the sample. The atmosphere in the furnace was controlled by passing a slow stream of nitrogen (purified in a train of sodalime and silica-gel tubes) through the furnace tube under a slight positive pressure. Runs were made at temperatures ranging from 86°C to 190°C. Weighingswere made by removing the sample at suitable regular intervals, cooling rapidly on a brass block, and weighing on a semimicro balance. The weighings were completed in less than a minute, and there appeared to be no trouble from CO~ contamination. TASLE 1.--X-RAY DIFFRACTION PAT'rI~.N FOR C d ( O H ~ $
ao,,,. (A)
z£,r,
~t
4'7a 3.02s 2'547 2-344 i'857 ~z 0"001 1-748
70 65 100 7 35 20 6
00.1 10.0 10.1 00.2 10.2 11"0 20-0 20-1 10.3 11-2 20.2 21-0
1-513 1-441
8
1"398
8*
1-274 1"142
6 4
a..,e. (A) 4.70 3-02. 2.54. 2.351 1.857 1-748 1.514 1-441
1-392 1.403 1.273 1-144
Unit cell a = 3"496~,
c = 4.702 A
* Doublet RESULTS T h e p - t curve for the equilibrium reaction is shown in Fig. 1. It was found that the CdO produced by rapid heating of Cd(OH)2 readily reconverted to the hydroxide when held under appropriate p--t conditions, (and indeed showed a slight tendency to hydrate on quenching from the CdO field) so that no trouble was experienced in reaching equilibrium from this direction. On the other hand, it was found that at points just inside the CdO field, Cd(OH)~ samples yielded CdO which had crystallized in extremely well-formed black octrahedra. This well crystallized "black" CdO would not rehydrat© when held in the Cd(OH)~ field, even after several days. Well crystallized Cd(OH)~ was also prepared from a run at ? bars and 222°C. It crystallized in deep hexagonal "hoppers" characteristic of phases growing with spiral steps. I t was found to be uniaxial negative, n~ = 1.802 4-0.004, n, = 1"?0. No evidence for the systematic formation of Cd(OH) 2 polytypes was found. In one isolated case a sample giving a completely inexplicable X-ray powder pattern was obtainedg but this did not correspond to a recognizable polytype; all attempts to repeat the observation were unsuccessful. The p - t curve was used to determine values for AH~.t by means of the Clapeyron r-Jationship; using the value of A H corresponding to 500 bars and 279°C, calculation ~,~ means of the integrated van't Hoff relation gave an equilibrium temperature of - 11°C at one bar. In the latter calculation corrections were applied throughout for the fugacity of water. Table 2 sets out some points in the curve together with corresponding values o f AH.
100
L.S. DEr,rr GLASSERand R. ROy
3200
2800
(/) n
o 2000
@
Cd(O.),oo o7.; OdO.,O
LLJ nU~ U) t~J OC n
,~
o
2400
1600
1200 o
•
o/
•
@
800 o 400
o
IO0
ZOO
220
o
240
o •
ZSO 200 TEMP. "O
300
3;tO
340
FIG. l . - - P - t equilibrium diagram for the system Cd-HzO. Open circles: Cd(OH)t. Filled circles: CdO. Double circles: well crystallized octahedra of CdO. TABLE2.--AH, Fog THE REACTIONCd(OH)2 ~ C,dO + HsO* Pressure (bars)
Temp. (°C)
AH (kcal)
2000 1000 500
303 289 279
7.5 4"7 4-2
* These values are not particularly accurate as they necessarily were read from slopes of smooth lines drawn as the best fit to experimental points.
For the kinetic studies, curves were plotted of per cent material dehydrated against time and the rate used in calculations was the initial rate. This varied from about 1 × 10-a per cent per hour at 86° to 11 per cent per hour at 190°. By plotting in the usual way, a value of 28.8 kcal is obtained for the activation energy of this reaction. This relatively high value partly explains why Cd(OH), does not decompose spontaneously under ambient conditions, as would be implied from the p--t results. DISCUSSION As shown in Table 3 the equilibrium temperature at 1000 bars is m u c h higher for M g and Ca (extrapolated) than for Ni, M n and Cd. This difference must therefore be related to the increase in ¢ovalency o f the b o n d rather than to ionic size. C o n sidering n o w the pairs C a - M g and M n - N i , the temperature decreases with ionic size for elements o f approximately the same degree o f polarizability. Cadmium, larger than either M n or Ni shows a lower temperature than either. One must conclude that
The system C d O - H t O TABLE 3 . ~ M P A R I S O N
101
OF EQUILIBRIUM DEHYDRATION TEMPERATURES AT I000 BABS
Cation
Goldschmidt radius
Equil. temp. (°C) at 1000 bars
A H (kcals)
Mg Ni Mn Cd Ca
0'78 0"78 0"91 1"03 1.06
650 300 380 290 > 1000*
8-84-9-79
4-7 (26.5)
* This value extrapolated by calculation: Ca(OH)t melts below this temperature (see Ref. 5).
this is because the C d - - O bond is considerably more covalent than the N i - - O and M n - - O bonds; this seems not to be unreasonable. Values for AHreaet. are not well enough known for any correlations with ionie properties to be established at the present time.
AcknowledKement--Finaneial support Science Foundation (Grant G4648).
for the experimental work was provided by the National
THE SYSTEM CdO-H~O L. S. D~,~T Gt~SSER a n d R. RoY College of Mineral Industries, Department of Geophysics and Geochemistry, Pennsylvania State University, Pennsylvania (Received 4 April 1960; in revised f o r m 5 July 1960)
A l ~ r ~ t ~ T h ¢ system CdO-H,O has been studied at high temperatures and water p r e s s u ~ up to 3000 bars, and thep--t curve for the reaction Cd(OH)~ ~ CdO + HjO is presented. The kinetics of tlw dehydration of Cd(OH)| in an essentially water-free atmosphere were also investigated. Values of A / / r ~ t . and the activation energy of dehydration have been calculated. T~E r e a c t i o n M O + H i O ~-- M ( O H ) I , where M O has the r o c k - s a l t a n d M ( O H ) s the brucite structure, c a n be studied f o r several different cations. Systematic s t u d y o f such a series o f a n a l o g o u s r e a c t i o n s m i g h t e n a b l e c o r r e l a t i o n s to be m a d e between the t h e r m o d y n a m i c values o b t a i n e d a n d the prol~.rties o f the cations concerned, structural c o n s i d e r a t i o n s h a v i n g been eliminated. D a t a f o r the e q u i l i b r i u m p--t curves for the ions M g 2+, C a a+, Ni 2+, a n d M n ~(~-5) have p r e v i o u s l y been o b t a i n e d in this l a b o r a t o r y . T h e present w o r k deals with d a t a for a n o t h e r cation, c a d m i u m , which has n o t p r e v i o u s l y been studied. EXPERIMENTAL The starting materials for the study were as follows: [(1) Cd(OH)~. From a solution of C.P. Cadmium Nitrate, Cd(OH)s was precipitated by addition of excess NaOH solution; the precipitate was filtered and washed on a B(lchner funnel. Washing was continued until the filtrate showed no reaction to phenolphthalein. The precipitate was dried overnight at 100°C and ground in an agate mortar. (2) CdO. Cd(OH)s, prepared as described above, was heated to about 900~C, for a short time (because of the volatility of CdO). Decomposition was rapid, and the material produced was very reactive. Powder X-ray diffraction patterns for both compounds agreed well with those in the ASTM X-ray powder data file and showed no trace of impurities (such as carbonate). The card for Cd(OH)s, however, is not of high accuracy and shows a discrepancy between the values of a0 and co quoted, and those calculated from the powder data given. Accordingly, the powder pattern for Cd(OH)s was redetermined, using a Norelco high-angle diffractometer, with silicon as an internal standard. The new data are given in Table I. The hydrothermal studies were made using "test-tube" (or cold-seal) vessels at pressures up to about 3000 bars. The samples were contained in gold foil envelopes, and in every run but one both CA(OH)s and CdO were used. Temperatures were automatically controlled, and measured using a chromcl-alumel thermocouple and a Brown automatic precision indicator, and they are accurate to +5°C. Pressures were measured on a Bourdon gauge, calibrated against a secondary standard, and are accurate to better than -I-5 per cent. For the kinetic studies, the sample was contained in a small open platinum boat, which was placed in a horizontal furnace. The temperature was controlled to ~ 3°C with a Honeywell controller, and (t) D, M. Roy and R. Roy, Amer. Mineral 39, 972 (1954). ts) D. M. RoY and R. Roy, Amer. Mineral 39, 957 (1954). (s) D. M. ROY and R. Roy, Amer. J. $ci., 225, 574 (1957). (4) C. KLINOSUXOand R. Roy, Amer. Mineral 44, 819 (1959). (i) p. j. WYLIEand O. F. TUT'rl.E,J. Amer. Ceram. Soc. 42, 448 (1959).
~s
The system CdO-HIO
99
measured with a chromel-alumel thermocouple p l ~ immediately above the sample. The atmosphere in the furnace was controlled by passing a slow stream of nitrogen (purified in a train of sodalime and silica-gel tubes) through the furnace tube under a slight positive pressure. Runs were made at temperatures ranging from 86°C to 190°C. Weighingswere made by removing the sample at suitable regular intervals, cooling rapidly on a brass block, and weighing on a semimicro balance. The weighings were completed in less than a minute, and there appeared to be no trouble from CO~ contamination. TASLE 1.--X-RAY DIFFRACTION PAT'rI~.N FOR C d ( O H ~ $
ao,,,. (A)
z£,r,
~t
4'7a 3.02s 2'547 2-344 i'857 ~z 0"001 1-748
70 65 100 7 35 20 6
00.1 10.0 10.1 00.2 10.2 11"0 20-0 20-1 10.3 11-2 20.2 21-0
1-513 1-441
8
1"398
8*
1-274 1"142
6 4
a..,e. (A) 4.70 3-02. 2.54. 2.351 1.857 1-748 1.514 1-441
1-392 1.403 1.273 1-144
Unit cell a = 3"496~,
c = 4.702 A
* Doublet RESULTS T h e p - t curve for the equilibrium reaction is shown in Fig. 1. It was found that the CdO produced by rapid heating of Cd(OH)2 readily reconverted to the hydroxide when held under appropriate p--t conditions, (and indeed showed a slight tendency to hydrate on quenching from the CdO field) so that no trouble was experienced in reaching equilibrium from this direction. On the other hand, it was found that at points just inside the CdO field, Cd(OH)~ samples yielded CdO which had crystallized in extremely well-formed black octrahedra. This well crystallized "black" CdO would not rehydrat© when held in the Cd(OH)~ field, even after several days. Well crystallized Cd(OH)~ was also prepared from a run at ? bars and 222°C. It crystallized in deep hexagonal "hoppers" characteristic of phases growing with spiral steps. I t was found to be uniaxial negative, n~ = 1.802 4-0.004, n, = 1"?0. No evidence for the systematic formation of Cd(OH) 2 polytypes was found. In one isolated case a sample giving a completely inexplicable X-ray powder pattern was obtainedg but this did not correspond to a recognizable polytype; all attempts to repeat the observation were unsuccessful. The p - t curve was used to determine values for AH~.t by means of the Clapeyron r-Jationship; using the value of A H corresponding to 500 bars and 279°C, calculation ~,~ means of the integrated van't Hoff relation gave an equilibrium temperature of - 11°C at one bar. In the latter calculation corrections were applied throughout for the fugacity of water. Table 2 sets out some points in the curve together with corresponding values o f AH.
100
L.S. DEr,rr GLASSERand R. ROy
3200
2800
(/) n
o 2000
@
Cd(O.),oo o7.; OdO.,O
LLJ nU~ U) t~J OC n
,~
o
2400
1600
1200 o
•
o/
•
@
800 o 400
o
IO0
ZOO
220
o
240
o •
ZSO 200 TEMP. "O
300
3;tO
340
FIG. l . - - P - t equilibrium diagram for the system Cd-HzO. Open circles: Cd(OH)t. Filled circles: CdO. Double circles: well crystallized octahedra of CdO. TABLE2.--AH, Fog THE REACTIONCd(OH)2 ~ C,dO + HsO* Pressure (bars)
Temp. (°C)
AH (kcal)
2000 1000 500
303 289 279
7.5 4"7 4-2
* These values are not particularly accurate as they necessarily were read from slopes of smooth lines drawn as the best fit to experimental points.
For the kinetic studies, curves were plotted of per cent material dehydrated against time and the rate used in calculations was the initial rate. This varied from about 1 × 10-a per cent per hour at 86° to 11 per cent per hour at 190°. By plotting in the usual way, a value of 28.8 kcal is obtained for the activation energy of this reaction. This relatively high value partly explains why Cd(OH), does not decompose spontaneously under ambient conditions, as would be implied from the p--t results. DISCUSSION As shown in Table 3 the equilibrium temperature at 1000 bars is m u c h higher for M g and Ca (extrapolated) than for Ni, M n and Cd. This difference must therefore be related to the increase in ¢ovalency o f the b o n d rather than to ionic size. C o n sidering n o w the pairs C a - M g and M n - N i , the temperature decreases with ionic size for elements o f approximately the same degree o f polarizability. Cadmium, larger than either M n or Ni shows a lower temperature than either. One must conclude that
The system C d O - H t O TABLE 3 . ~ M P A R I S O N
101
OF EQUILIBRIUM DEHYDRATION TEMPERATURES AT I000 BABS
Cation
Goldschmidt radius
Equil. temp. (°C) at 1000 bars
A H (kcals)
Mg Ni Mn Cd Ca
0'78 0"78 0"91 1"03 1.06
650 300 380 290 > 1000*
8-84-9-79
4-7 (26.5)
* This value extrapolated by calculation: Ca(OH)t melts below this temperature (see Ref. 5).
this is because the C d - - O bond is considerably more covalent than the N i - - O and M n - - O bonds; this seems not to be unreasonable. Values for AHreaet. are not well enough known for any correlations with ionie properties to be established at the present time.
AcknowledKement--Finaneial support Science Foundation (Grant G4648).
for the experimental work was provided by the National