THE THERMAL DECOMPOSITION OF ZIKC AND CADMIUM CARBONATES I N AN ATMOSPHERE O F WATER VAPOR ALEXANDER LEHRMAN AND NATHAN SPEAR
The decomposition pressures of zinc and cadmium carbonates are difficult to measure due to the slowness of reaching equilibrium. A number of attempts have been made to measure the decomposition pressure of cadmium carbonate. W. Miethke' tried to measure it by a static method but found great difficulty in reaching a state of equilibrium, and furthermore could not duplicate his measurements. He points out that the presence of combined water in the carbonate is necessary for the system to approach equilibrium at a measurable rate. Tzentnershver and Andrusov2 attempted the same measurement by static and dynamic methods and claim t o have succeeded. The literature does not contain the report of any measurement on the decomposition pressure of zinc carbonate. Even though Miethke states that the presence of combined water is necessary to enable the system cadmium carbonate-cadmium oxide-carbon dioxide to reach equilibrium, the authors decided to try the effect of having an atmosphere of water vapor present. The presence of one atmosphere of water vapor also makes possible a very simple method of determining the decomposition pressure of the carbonates. No effort was made to obtain great accuracy. The work was carried out to test the effect of the presence of an atmosphere of water vapor and t o see if the simplified method was possible. I n view of this it is unnecessary to take into account the small effect of one atmosphere of inert gas (water vapor) on the decomposition pressure. Apparatus.-The apparatus is shown diagrammatically in Fig. I . A is the bulb in which is placed the charge of carbonate and about I O ml. of water. The bulb is made by sealing part of a pyrex test tube to a short piece of tubing. This is connected to the manometer tube B by heavy wall rubber tubing C. The left arm of the manometer tube is about 2 5 cm. long and the right one is about 80 cm. long. The manometer is immersed in an open beaker (4 liters capacity) of boiling water which is kept boiling by the hot plate F. The bulb is heated by the small electric heater G. This heater was made by setting the resistance coils of a resistance furnace in a box made of asbestos board and packed with 850/0 magnesia. The sides of the heater were asbestos board in which holes were cut just large enough to admit the bulb on one side and the thermocouple or thermometer on the other. When inserted the bulb and thermocouple or thermometer bulb were in the center of the air space in the heater, the end of the couple or thermometer bulb being in contact with the 1
Dissertation, Berlin (1911). 1 1 1 (1926);111, 79 (1924);115, 273 (1925).
a Z . physik. Chem., 123,
\ \
DECOMPOSITION OF ZINC AND CADMIUM CARBONATES
2665
side of the bulb holding the carbonate. The temperature within the heating box was controlled by a hand-operated rheostat and switch. The temperatures above 300' were measured with a chromel-alumel thermocouple, and below this with a mercury thermometer. The thermometer was checked against the boiling point of water. Above 300' the temperature of the heater could be kept to within 5' of the desired temperature. Near the boiling point of water the temperature could be kept to within I O of the desired temperature.
FIQ.I
The boiling water covered the short arm of the manometer tube and the tube connected to the bulb almost to the entrance to the heater. A meter stick was mounted in back of the manometer tube and the heights of mercury and water in the tubes were measured on this using a right-angle triangle to decrease errors of parallax. Preparation of the Carbonates.-Cadmium carbonate was prepared by dissolving C. P. cadmium nitrate in water and precipitating the basic carbonate by addition of a solution of C.P. sodium carbonate. The precipitate was washed until the wash water gave no test for nitrates. It was then dissolved in concentrated ammonium hydroxide and carbon dioxide passed through until the solution was clear. The solution was then diluted with about fifty volumes of water and saturated with carbon dioxide. On standing a heavy crystalline precipitate settled out. This was washed with carbon-dioxide saturated water
2666
ALEXANDER LEHRMAN AND NATHAN SPEAR
and then with ethanol and air-dried. That the precipitate was crystalline could be seen when it was placed under a microscope. It was analyzed by heating a weighed sample in a combustion tube and drawing dry carbon dioxide-free air over it and then through weighing bottles containing soda lime and calcium chloride. I t contained water, 3.3%; carbon dioxide 24.1%; and cadmium oxide 7 ~ . 6 ~This ~ . indicates a molar ratio of I HzO : 3.0 COz : 3.1 CdO. The method did not work so well for the preparation of zinc carbonate. The precipitate formed on dilution of the solution of the basic zinc carbonate in ammonium hydroxide with carbon dioxide saturated water had 16.8% of water, 5 . 2 % of carbon dioxide, and 78.0% of zinc oxide. This indicates a molar ratio of I COZ: 8.3 HzO: 8.6 ZnO. The exact nature of the precipitates is unknown. They may be compounds, mixtures, or solid solutions. Tests, however, showed that they were free from ammonia and from nitrates, The cadmium carbonate may be the normal cadmium carbonate. Miethkel showed that the normal cadmium carbonate is very insoluble. This may explain why it did not hydrolyze to the extent that the zinc salt did. Method-2 to 3 grams of the carbonate and IO ml. of water were placed in the bulb and the bulb connected to the manometer. The manometer tube was put into the beaker of water as shown in Fig. I and the water in the beaker was brought to the boiling point and kept vigorously boiling. The heater was then brought up to the bulb and the temperature raised until the water in the bulb boiled. The steam formed passed into the manometer and swept out the air. Some of the water was condensed in the manometer tube and fell to the bottom of the U. The vapor produced after this condensed in the water and the absence of air was shown by the complete condensation of the bubbles in the water. Some of the carbonate was mechanically carried over into the manometer tube, but this could in no way affect the experiment. Warm mercury was then poured into the open end of the manometer and it sank through the water forming a continuous column with some liquid water on the top of both ends of the column. As the temperature of the bulb was raised more mercury was added. The liquid water on the top of the left-side column of mercury assured a pressure of one atmosphere of water vapor in the enclosed system, and which balanced atmospheric pressure on the right hand arm of the manometer. The difference in heights of the two columns was due to the carbon dioxide pressure of the carbonate. The pressures were read by measuring the heights of the mercury and water columns on the meter stick. The pressures due to liquid water were converted to pressures in heights of mercury by dividing by 13.6. No corrections were made for the effect of temperature on the densities of water and of mercury as the errors introduced by this are less than the experimental errors. At the close of each determination the heater was removed and the contents of the manometer rushed back into the bulb. The apparatus was then cleaned out and made ready for the next run.
2667
DECOMPOSITION O F ZINC AND CADMIUM CARBONATES
Results.-Three efforts were made to measure the decomposition pressure of cadmium carbonate in this apparatus. Equilibrium, however, could not be attained and furthermore the pressures reached under the same conditions of time and temperature in the three runs were not concordant. We can say, however, that the decomposition pressure reaches one atmosphere a t about 37501 which roughly checks previous work,l in which the pressure of water vapor was much less than one atmosphere. The presence of water vapor seems to exert no effect on the decomposition pressure of cadmium carbonate, neither in the time taken to reach equilibrium nor on the equilibrium pressure itself. When the zinc carbonate was put into the apparatus, however, the pressure built up very rapidly to a high value a t temperatures as low as I 50'. In order to work with our apparatus we had to use temperatures between IOO and 120' and a mercury thermometer was substituted for the thermocouple. Furthermore, equilibrium was rapidly reached and the reaction reversed rapidly on cooling. The results of two determinations are listed in Tables I and 11. Under these conditions the carbonate had an appreciable pressure just above 100'. Table I shows that a t 110' a pressure of 30 om is reached in 5 minutes. On heating to 120' a constant pressure of about 49 om is reached in I O minutes. Upon cooling to 110' the pressure rapidly falls to 30 cm and on heating again to 120' approximately 49 cm pressure is again obtained. The small deviations can be explained by deviations in temperature. Table I1 shows that at 105' a pressure of about 2 0 cm is reached in 2 0 minutes. The fluctuations in pressure are due to variations in the temperature which was controlled only to i IO. On heating to I IO' a pressure of 28 om is soon attained. This checks within our experimental error the value obtained in Table I. Upon cooling to 105' the reaction immediately reverses as shown by the pressure attaining its former value.
TABLE I Temp. "C
Pressure cmof Hg
I IO
I3 30 31.5
I20
I IO
Time minutes 0
Temp. "C I10
5 IO
Pressure cm of Hg
Time minutes
31.5
IO
30
I5
29.5
20
36
0
28.5
25
48
5
30
49
IO
30 40.5
49.5
I5
48.5
20
44.5 49.5
49.5
25
46
I5
36.5 37.5
0
46
25
5
I20
0
5 IO
ALEXANDER LEHRMAN AKD NATHAN SPEAR
2668
TABLE I1 Temp.
"C
105
Pressure cmof Hg I4
0
15
13.5
I10
Time minutes
Temp. "C
Pressure cmof Hg
Time minutes
I10
27.5
IO
5
28
15
IO
28
20
21
0
16
15
21.5
20
I9
5
19.5 I9 I9
25
I9
IO
30 35
20.5
15
I7
20
21
40
25
0
21
45
25
21
50
25.5
IO
20
105
I10
5
60
27
15
27.5
0
27
20
27.5
5
TABLE I11 T!mp. C I20
I75
Pressure cmofHg
C
Pressure cmofHg
Time minutes
7
0
0
3
30
10.5
4
40
I4
5
200
0
5 IO
I5
15
16.5
20
6 .j
IO
I7.5
25
7.5
20
19.5
35
24
55
25
65
13.5
30 40 50
I4
60
33 38
IO
I3
23
39.5 47 50 I03
Tynp.
2
I1
2 IO
Time minutes
220
0
IO
0
52.5
20
54
30
53 53
IO
20
30
2
0
I
IO
200
0
52.5
20
52
30
DECOMPOSITION OF ZINC AND CADMIUM CARBONATES
2669
For purposes of comparison a charge of zinc carbonate was put into a similar bulb and connected to a straight glass tube go cm long. This tube dipped into a bottle of mercury which carried a two-holed stopper. The arrangement served as a manometer. By tilting the apparatus the lower end of the tube was lifted clear of the mercuryand the system evacuated through the second hole in the rubber stopper. A water aspirator was used. When the limit of evacuation was reached the apparatus was set upright and on discontinuing the action of the pump mercury rose in the tube. By comparing with a barometer the residual pressure was found to be 1.5 cm. On heating the bulb containing the carbonate, decomposition set in and the pressure of the carbon dioxide was measured by the depression of the mercury column. The results obtained are shown in Table 111. It can be seen that the rate of thermal decomposition as well as the unattained decomposition pressure in the presence of only a slight pressure of water vapor is entirely different from that in an atmosphere of water vapor. It may be that the reacting systems are different in the two cases. I n the ordinary thermal decomposition the system may be represented by the components ZnO and COS. I n the decomposition in an atmosphere of water vapor the system may be ZnO, COZand HzO, or the action may be the hydrolysis of zinc carbonate as distinguished from its decomposition. The City CoU e, ColZege of the 8 i t y of New York.
