The DETERMINATION of the THER-
MAL CONDUCTIVITY of GASES CHARLES M. MASON AND ROGER M. DOE University of New Hampshire, Durham, New Hampshire
Directions are given for the construction and use of a simfile thermal conductivity cell. Some results are given which were obtained with the afiparatus.
A modijkation of a familiar method is outlined which is suitable for use i n the elementary fihysical chemistry laboratory.
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A
NDREWS ( I ) appears to have been among the first to measure the thermal conductivity of gases. He utilized the fact that a platinum wire stretched along the axis of a glass tube, when heated by an electric current of constant strength, assumes a temperature which depends upon the nature of the gas surrounding the wire. This procedure has since been employed in modified form by many investigators (2). Weaver and Palmer (3) have used the method outlined above as a basis for the development of an accurate means for the analysis of gaseous mixtures. An apparatus using this principle and suitable for student use has been described by Mack and France (4). I t has been found in our laboratory that the cells described by these authors are difficult to make and not easily kept in working order when used by students. For these reasons a new cell has been devised which is easily manufactured and not readily damaged by rough use. It gives good results in the hands of students in the physical chemistry laboratory. THEORY INVOLVED
tion loss can be minimized by keeping the wire temperature below 400°C. (3). The variation in current with different gases to keep the temperature of the platinum wire constant will be a measure of the thermal conductivities of the gases used. The rate of transfer of heat by conduction through any medium is expressed by the equation
where "Q" is the heat in calories transferred in "1" seconds from a surface of "A" sq. cm. across a distance " P a . Ta- TI is the absolute temperature drop. k is the thermal conductivity of the medium in calorie cm. per sq. cm. per degree centigrade per second [ d . cm.-2sec.-I (OC., cm.-')-I]. I t has been found convenient to gather all the heat lost by convection, radiation, and cell conduction together as a constant and add this to that lost by conduction through the gas. This loss will be a constant for any given cell. This fives us for the heat carried from the-wire by conducti& kA (T,
- TJt
+
CA (T, - T,)t
The resistance of platinum wire varies with the ternQ= d (2) perature. At a definite temperature a platinum wire will have a definite resistance. Conversely, if the The first term of this equation represents the thermal resistance is fixed, the temperature of the wire will be conduction of the gas and the second the heat lost by fixed. Dierent gases in contact with the wire will other means. "C" is a constant for any cell and will conduct heat away at different rates. To keep the be designated by "Cell Constant." We then have the wire a t the same temperature in several gases, different equation amounts of electrical energy will have to be supplied. (k C) A (Tz - T,)t Q= d (3) All other factors being equal, the amount of electrical energy will depend upon the nature of the gas in contact for the heat lost from the hot wire. The heat in with the wire. When equilibrium is reached between the heated d o r i e s supplied to the wire is readiiy calculated from wire and the shell of the cell by conduction of heat the expression through the gas, the heat supplied to the wire by the Q 0.23912Rf (4) electrical energy must be equal to the heat lost to the thermostat by conduction, convection, and radiation. where "I"is the current in amperes flowing through the The conduction includes the heat carried away by the platinum wire of resistance R ohms in t seconds. gas and through the metal parts of the cell. For any Equating the heat loss to the heat gain we have the given cell the latter loss will be a constant. The radia- expression 182
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which is independent of the time. For two gases alternately in the same cell at the same temperature we have
conductivity. Rl and R2remain constant. The current is adjusted with R until the bridge is again balanced. The current necessary to balance the bridge is noted.
where the subscripts A and B refer to the first and second gas, respectively. When solved for the cell constant we obtain
If the thermal conductivities are known and the corresponding currents have been determined for both gases, the cell constant can then be evaluated. EXPERIMENTAL DETAILS
The type of cell found to be suitable is shown in Figure 1. This cell was made from a discarded fivepound ether can by soldering on the top a brass collar to take a No. 11 rubber stopper. In Figure 1 "a" represents the electrical leads of B&S No. 18copper wire thrust through the rubber stopper indicated by "b." "c" designates the platinum wire, B&S No. 42, silver soldered onto the copper leads a t "d". "e" designates the glass inlet and outlet tubes for the gases. The conductivity measurements were made in a Wheatstone Bridge circuit essentially the same as that employed by Mack and France (4). This circuit is shown in Figure 2. In the use of this cell the following procedure is recommended. Clean dry air is passed through the cell, in the thermostat, until it has been thoroughly swept out. The inlet and outlet tubes are then closed off. Sufficient time is allowed for the air to come to temperature. Side view, thru center The resistances R1 and R2 FIGURE 1.-CONDUCare then set so that R, R, TIVITY CELL is eaual to about one thousand oh&. This is to prevent burning out the resistance boxes by excess current. The switch is closed and R adjusted until about 0.15 ampere flows through the cell. The key K is then depressed and Rl and R2 adjusted, keeping R1 Ra equal to 1000 ohms, until the bridge is balanced. That is, the galvanometer shows no deflection. The air is then displaced with another gas of known
A B C G
AXMETERAS SENSITNE AS ISOBTAINABLE BATTERY THERMAL CONDUCTNITY CELL GALVANOXETER. LAND N. No. 2310
d€tr these conditions the platinum wire has been kept a t the same temperature-throughout the measurement and equation (6) may be used. The cell constant "C" is calculated from these values by equation (7). The thermal conductivity of any gas is then obtained by noting the current necessary to balance the bridge with that gas in the cell. RESULTS OBTAINED
It may be of interest to note the results obtained by a student using the method in this laboratory. The data obtained are given in Table 1.
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air
599
502
0.100
CG H1
599 599
502 502
0.085 0.239
for air k = 0.571 X lo-' (5) for COS k = 0.367 X lo-&
From these data the cell constant was found to be 1.637 X Using this and the thermal conductivity for air the thermal conductivity of hydrogen was (cal. cm.? sec.-I OC.-' calculated to be 4.04 X
an.). This compares quite favorably with the value of 4.05 X lo-' given in the tables (5). DISCUSSION
The conductivity cell described herein has several advantages. I t is inexpensive, easy to construct, and readily repaired. Readings can he made with the gas stationary, which is a decided advantage in the lahora-
tory. With slight modification the thermal conductivity of vapors could easily he measured. The corrections indicated in the calculations would apply equally well to any type of thermal conductivity cell used. The authors wish to express thanks to Professors Edward Mack, Jr., and W. G. France for permission to use Figure 2 and to Mr. Homer Priest for preparing the drawings.
LITERATURE CITED
(1YZ8). York City, 1934, p. 59. (3) (a) WEAmn, PALMER, FRANTZ. LEDIG. AND PICKERING. (5) "Intemationd Critical Tables." McGraw-Hill Baak Co. Ind. Eng. Chem., 12, 359 (1920); ( b ) W E A ~ AND E ~ PALNew York City, volumeV. 1926, p. 214.
