M. R. Lorenzl and G. J. Janz Rensseloer Polytechnic Institute Troy, New York
I
I
A dynamic method for
~ a a s u r i nDissodatioion ~ Pressures
Measurements of dissociation pressurea have long been of interest, both from the fundamental and practical viewpoint. Although each material under study requires some special consideration, a host of materials dissociate into products where one of them is in the gaseous state, e.g., CaCOds)
-
CaO(s)
+ COn(g)
For such systems various techniques have been employed with the most widely used corresponding to a static vapor pressure measurement. This communication reports a simple and rapid method based on the thermogravimetric principle in a controlled atmosphere yielding equilibrium dissociation pressures.
Figure 1. Thermogravimetric system for dirrociation pressure measurement* A, balance with optical scale; B, furnace; C, sample crucible; D, furnace tube; E, flow-meters; F, thermocouple to recorder; P.G., product go,; and I.G., inert gas.
The essential equipment consists of a furnace, a balance, preferably a single pan type with an optical scale, flow-meters and a temperature monitoring device. The system arrangement is shown in Figure 1. For evaluation of the technique CaC03 was used. I n brief, the following procedure was adopted. A crucible containing reagent grade CaC03 was freely suspended inside a furnace tube from a precision analytical balFinmcial support from the Department of the Navy, Office of Naval Research, Chemistry Branch, Wsshington, D. C., is
gratefully acknowledged. 'Present address: General Electric Company, Research Laboratory, Schenectady, New York. HILL,K. J., AND WINTER,E. R. S., J . Phv8. Chern., 60, 1361 (1956). "AKER,
E. H., J . Chem. Soe. London, 1962,464.
ance featuring an optical scale (100 mg range). The tube presenting a closed system had a bottom gas inlet and a small hole on top serving for the free passage of the suspension wire and as gas outlet. A thermocouple was positioned as near as possible to the suspended sample without making actual contact. A desired partial pressure of COz was established in the tube by monitoring the flow rates of predried Nz and GO2. The total pressure of the gas stream a t all times was equal to atmospheric pressure. With the COz pressure fixed, the furnace was heated and temperature and weight changes were monitored. When the sample temperature exceeded the equilibrium temperature for the fixed COz pressure, the sample weight decreased corresponding to a loss of COa and the formation of CaO. The temperature was then decreased until the reverse reaction took place and the sample weight began to increase. By cycling the temperature several times through the temperature at which no weight change occurred, it was possible to determine the true equilibrium temperature withm 1°C for the particular fixed C o ppressure. Since the gas flow rates are readily adjustable, the equilibrium pressures can be measured up to one atmosphere. The results on CaC03 indicate a dissociation pressure of one atmosphere a t 899% which is well within the limits of 895 to 9 0 2 T reported more recently in the l i t e r a t ~ r e . ~Results .~ are shown in Figure 2 for measurements on CaCOa, Li2C03, and NaZC03. For the latter two, where the systems are molten, the rates of weight gain or loss are less than for CaC03, undoubtedly because of the greatly decreased surface areas presented by the melts. 800-
-
-,a -
600-
E E
? w
g
400-
m N
"
0
zoo-
_--, so0
Figure 2. Disaiation prersurer: -0-0-,CoCOrCaO; -A-AA-, li2COa (mp 710aC)-LizO; -U-0-, N a z C 0 8 (mp 858'ClNarO.
Volume 40, Number I 1 , November 1963
/
61 1
The present method has several advantages over other techniques, some of which warrent mention. The dynamic method is simple and rapid and requires equipment which is standard in most laboratories. Equilibrium pressures of the product gas are measured truly. The results are independent of the presence of any small impurity phase which on heating may also yield a gaseous product, e.g., HzO vapor. The technique has been used successfully for solids and liquids in this
612 / Journal of Chemicol Education
laboratory. The method is especially useful in cases where at times weeks are required before total equilibrium is reached. For instance, in the static dissociation pressure measurement of CaC03 it took three weeks to reach equilibrium a t 450°C and several hours at 800°C.Z By nature of the dynamic method the same results are attained in the order of an hour or less depending mainly on the maintenance of thermal equilihrium in the temperature cycling.
