I L
WILLIAM H. HOWLAND and LEO
F.
EPSTEIN’
General Electric Co., Knolls Atomic Power Laboratory, Schenectady, N. Y.
Reaction Rate of Solid Sodium with Air Wherever metallic sodium is used, its reaction with air poses a problem. Here are experimental results which show the influence of temperature and impurities on reaction rate
THE
rate of reaction of sodium with air and the effect of sodium purity and temperature on this process have been measured in this laboratory. I t is generally known that chemical reaction rates are extremely sensitive to the purity of the reactants. For example, it has been observed that a freshly cut surface of triply distilled solid sodium will not oxidize nearly as rapidly as will commercial grade sodium. Distilled lithium (99.99% pure) can remain in air for 10 hours or longer before losing its metallic luster (Z), but the surface of 99.9% pure lithium becomes completely blackened in a few seconds after exposure to air. Whereas 99.95% pure zinc dissoIves completely in 10% hydrochloric acid at room temperature, in the same time under identical conditions “chemically pure” zinc (99.99%) loses 53% by weight and “spectroscopically pure” zinc (99.999% or better) loses only 0.02910. In addition, the rate of reaction of sodium with dry oxygen gas seems to be 1 Present address, Vallecitos Atomic Laboratory, General Electric Co., Pleasanton, Calif.
sensitive to the nature of the gaseous constituents present. Thus, it has been reported (4) that with dry oxygen or oxygen-helium mixtures, the reaction proceeds very slowly except for an initial rapid takeup of the gas, several days being required for completion. The presence of nitrogen gas, however, even in the parts per million range, seems to sensitize the surface of the metal so the oxidation process occurs in seconds rather than days. With dry air, then, a rapid and continuous reaction is to be expected. Other factors influencing the rate of the sodium-air reaction are the moisture content of the atmosphere and the temperature of the sample. In this experiment, temperatures were maintained at 30°, 55’, and 80’ C. f 1’ C., while no attempt was made to control the moisture content of the air which averaged about 30% relative humidity at room temperature (25’ C.); consequently, the measured reaction rate is the sum of the rate of oxidation and the rate of formation of the hydroxide, each influencing the other, with the diffusion of air through
r
I
Two distinct problems were brought into focus in this study:
b
Pumps for circulating liquid sodium as a heat transfer medium require “frozen” sodium seals to prevent air from contacting the highly reactive sodium. Engineers wanted to know how long such seals could hold up in air. b Highly purified lithium was observed to tarnish in air less rapidly than a freshly cut surface of the commercial metal; essentially the same observation had been made with distilled and commercial sodium. In reducing these apparent effects to a quantitative basis, the investigation brought forth an interesting and provocative phenomenon-the effect of impurities on reaction rate vanishes as the temperature approaches the melting point.
I
the moist oxide-hydroxide layer as the rate determining factor. This diffusion layer is made u p in general of water, sodium hydroxide, and sodium oxide, and possibly some carbonate, with the relative proportions of these constituents varying with atmospheric conditions. Thus, at room temperature where the vapor pressure of water is low, the white solid reaction pioduct appears to be moist and droplets form and eventually fall off; at higher temperatures there is no visible moisture and the oxide-hydroxide surface appears to be comparatively dry. While the relative amounts of water, hydroxide, oxide, and other constituents of the crust and the relative amounts of liquid and solid reaction products change with temperature, the variation in the composition and phases present is a continuous function of the temperature. In this experiment it is possible to observe only the over-all effect of these conditions, and in this study only the net reaction rate is reported. Experimental
Three thermostated furnaces were used, each containing sodium filled borosilicate glass U tubes of uniform cross section. Two U tubes were placed in each furnace-one containing triply distilled sodium (subscript D) and the other containing filtered sodium (subscript F ) made by melting commercial brick sodium in a purified, argon-filled dry box and filtering through fritted glass filters into the U tubes. The composition of the filtered and distilled sodium is shown in Table I. The oxygen content in each case was assumed to be the equilibrium solubility a little above the melting point ( 3 ) . The furnace consisted of a 60-cm. length of IO-cm. diameter borosilicate VOL. 49, NO. 11
NOVEMBER 1957
193 1
Res uI ts Table 1.
Composition of Sodium
% (Weight) Filtered Distilled
Impurity Be Ca
c1 Po4
Ni
0* 0008 0.0500 0.0015
0.0005 0.0013 0.0010
0.0003 0.0010
0 0062
0.0001
Heavy metals as
Pb SO& -. 0 2
?
0.0004 0.00015 0.0010 0.0015 0.0030 0.0830 Na, 76 Na, % 99.94 99.99
-THERMOSTAT
Table II. Parabolic Rate Constants for the Sodium-Air Reaction" Filtered Distilled KF(Gram/Sq. KD(Gram/Sq. T (" C.) Cm. - Hr.l:*) Cm. - Hr.l/*)
u
30
55
One of the inverted U tubes contains triply distilled sodium and the other filtered sodium
glass tubing open at both ends and heated on the inside by a lamp bulb-type heating coil. Temperatures were controlled with bimetallic thermoregulators and measured with mercury-in-glass thermometers hanging vertically in the center near the exposed sodium surface. T o avoid the danger of heating the sodium above its melting point (97.8' C.), fuse wires of 50y0by weight bismuth-indium alloy (melting point 94' C.), connected in series with the heating circuit, were placed in the controlled temperature zone. Changes in the distance between
80
0.057 0.049 0.044
0.031 0.039 0.038
The experimental results were fitted to parabolic curves (characteristic of diffusion-limited processes) of the form W gram/sq. cm. = K P 2 by the method of least squares. The X values are given in Table 11. The probable error in W calculated from the constants is equal to or less than 1 0 . 1 gram/sq. cm. At all three temperatures (30°, 55', and 80' C . ) filtered sodium has a higher rate of reaction than does the distilled sodium, and the difference between the rates decreases with increasing temperature approaching the same value at the melting point. From the constants in Table 11, it appears that at the melting point the two rates would be the same to within the rather crude precision of these measurements. If this condition is included in a least-square fit to the rate data, the following relations are obtained : loglo Kn = -0.737
a
W gramisq. om. = Kt''2.
a mark on the upper end of each sodium column and the sodium-(sodium oxide, sodium hydroxide) interface were measured with a scale. As time progressed, the interface became extremely uneven on some of the samples, and great difficulty was encountered in obtaining an average length of the column. Measurements were continued out to more than 1000 hours in all the experiments, and approximately 30 points were determined for each composition and temperature, averaging the values on both arms of the U tube for each observation.
TEMPERATURE ' C .
- 235 ( 1 / T )
loglo K F = -1.88 -/- 190 ( 1 / T )
where T i s in degrees Kelvin (rather than centigrade) and the respective activation energies are: AHD = +1.07 kcal./mole A H p = -0.870 kcal./mole
- K. - ' K. O
These results suggest that the reaction is catalyzed by the presence of small amounts of impurity and that the catalysis reaction has a high positive energy of activation. What this catalysis reaction is must be considered undetermined at the present time. Also the details of the mechanism of the over-all reaction between solid sodium and moist air and the precise nature and composition of the barrier film must await further experimental study. The large effect of impurities in the sodium on the reaction rate appears to be entirely consistent with observations made on other systems. The apparent vanishing of the effects of impurities a t the melting point lends itself to interesting and provocative speculation. References Carpenter, H., Robertson, J. M., L'Metals," Vol. I, p. 585, Oxford University Press, Oxford, 1939. Epstein, L. F ., Howland, W. H., Science 114, 443 (1951). Lyon, R. N., ed., "Liquid Metals Handbook," NAVEXOS, P-733 (Rev.) U. S. Government Printing Office. Washington. D. C.. 1952: p. 114, Sodiumo-NaK Supplement (July 1, 1955). ( 4 ) &foyer, J. W., Ruggles, W. A., J. Opt. SOC.Aner. 44, 86 (1954). RECEIVED for review September 23, 1954 ACCEPTED July 17, 1957
lOOO/I
[OK.)
Rate constants for t h e sodium-air reaction
1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
Knolls Atomic Power Laboratory is, operated, for the U. S. Atomic Energy Commission by the General Electric Co. under contract No. W-31-109 Eng 52.
