AN EXPERIMENT FOR MEASURING THE THERMAL CONDUCTIVITY OF GASES HARRY W. LINDE and I.B. ROGERS Massachusetts Institute of Technology, Cambridge, Massachusetts
Tm
analysis of gaseous mixtures by thermal conductance depends upon the fact that the rate a t which heat is lost from a hot wire varies with the composition of the gas in contact with the mire ( I , 2, 3, 4, 5). Industrial applications of this principle have long been known, particularly for the analysis of flue gas. Although inexpensive equipment is commercially available, the method has been included in very few courses on instrumental analysis because of the difficulties encountered in preparing and handling mixtures of gases. If gashandling equipment is used, the student must spend a large fraction of the laboratory period in becoming familiar with the methods for handling gases before getting to the portion of the experiment dealing with the measurement of thermal conductance. One way of minimizing gas handling is to take a sample of gas, such as air, and measure its thermal conductance before and after removing one component, for instance, carbon dioxide. This technique, however, suffers from the disadvantage that unknowns cannot he easily prepared. The purpose of the present paper is to show that many of these problems can he avoided by analyzing gaseous mixtures resulting from the passage of an inert "carrier" gas through a volatile liquid. Binary liquid mixtures of different compositions are easily prepared for use in making calibration curves and as unknowns. .OUTLINE OF THE EXPERIMENT
Apparatus. Figure 1 shows a simple method for setting up the apparatus, which consists of two drying
r
i
g 1.
.
Gas-train for the Andysi. of Liquid by Measuring tho Th-rmd Conductmc. of its V.OF
tubes, three 250-ml. gas-washing bottles, a thermal con~ductancebridge (ref. (5) gives a list of manufacturers) and two glass stopcocks to control the flow rates. One gas-washing bottle serves as a "vaporizer" and has glass wool in the neck to catch droplets of liquid The
_ - - _ _ . -J_ L -_ - - - _- - -_ _- _- -JjOW-MAC I~ ~
CONWCTiVlTI VNlT ~
CONTROL PANEL
Figure 2. Schematic Diaeram fmr the Conduativity B ~ i d g am d th. h o o i a t e d C o n t d and Measuring Ckcu/t. (Gors-Mas. BIT B ~ i d . 4
R~--current adjuster, 50-ohm radio type potentiometer R r r e r o adjuster, 2-ohm radio type potentiometer SWI-on-off switch. SPST toggle MA-d.-o. miiiihmmeter
other two are bubble counters for gaging the rate of flow. To minimize the absorption of vapors care was exercised in making connections involving rubber tuhing to insure glass-to-glass contact. The source of the carrier gas can be a compressed air line, a nitrogen tank, or room air drawn through the system by means of a vacuum line or an aspirator. The electrical system is shown in Figure 2. The d.-c. milliammeter and the resistor, R,,serve to control the current flowing in the circuit while the resistor, Rz, serves to balance the bridge a t zero output with the carrier gas in all cells. An ordinary 6-v. lead storage battery supplies the e. m. f . The unbalance of the bridge may be read using a potentiometet or, less accurately, a microammeter (2). Operation. After the electrical and gas-train connections are made and the current through the hot wires adjusted to the value recommended by the manufacturer, the bridge is allowed to warm up. Then, while passing the carrier gas alone through both sides at a flow rate of about 3 to 6 bubbles/sec. and a head of water of about 2 in. in the bubhle counters, the bridge is brought to an approximate balance by adjusting R2. The balance of the bridge is checked at approximately 5-min. intervals. After about 15 t o 25 min., when the changes in balance have become less than 0.5 mv./min.. one can assume, without introducing serious errorin the measurements, that the bridge has reached equilibrium.
NOVEMBER, 1951
standard solutions and the calibration curve in about 90 min., following which, unknown solutions of acetone and water prepared by the instructor may be analyzed. Because of the large volume of solution used, no difficulty has been experienced from a change in the composition of a sample during an analysis. Using the same apparatus it is possible to compare the efficienciesof different drying agents a t the same or different rates of gas flow. The unbalance produced, however, is usually small and inconclusive. One can also examine the change in unbalance due to tilting (3) and to vibration (a sharp tap has been found to cause fluctuation as large as 0.30 mv.) of the bridge. Possible Rejinements. In the first place, because the amount of liquid vaporized is a function of its temperature and because the temperature of a volatile liquid decreases noticeably during a run, a thermostat should Fi3. The Rate of Approaching a Steady V d u e upon P-in. m M l . / M i n . o f Nitrogen t h o u g h Purr Acatona (Gow-Mac. BIT B~idga) increase the reproducibility. Second, although most commercial bridges are designed to minimize the effect. Then the "vaporizer," a gas-washing bottle containing of large variations in the flow rate, one might gain prethe liquid whose vapor is to be measured, is placed in cision by substituting for the bubble counters simple one line and its vapor, together with the carrier gas, flow metem which are commercially available. allowed to flow through one side of the bridge. (The At the present time, work is being carried out along flow rates through the two sides of the h~idgemay have the lines outlined above in the hope of increasing the to be readjusted.) Readings are then taken until a sensitivity and the reproducibility of the technique so constant value is reached. A period of about 10 to 15 that it can be annlied to the analvsis of a wide variety .= min. is usually required as shown in Figure 3. of liquids. Resulk The extents of unbalance obtained with a number of pure solvents a t about 25°C. are given in the 20, table together with the effect of changing the carrier Unbalance Produced by Vapors Obtained from Pure Liquids at About 25°C. Using a Gow-Mac Thermal Conductance Unit (Model B/T) Commund Water Methyl aloohol Acetone n-butyl alcohol Benzene
Unbalance, mv.
Cawier
16.4 15.0 3.5 -60 -1.61 -57
Dried air Nitrogen Nitrogen Nitrogen Dried air Dried air
gas
gas from nitrogen to dry air. Values for unbalance are given merely as an indication of t,he order of magnitude of the unbalance; the signs, although consistent wit,hin the set, are arbitrary. The values depend on the sensitivity of the bridge (usually defined as the millivolts of unbalance produced per 1 per cent of carbon dioxide added to air) and the ambient temperature, the latter affecting chiefly the vapor plessure of the liquid. A study over the range of 10 to 30°C. showed an increase in unbalance of about 1 mv.l°C. for acetone. Ordinarily, it is more satisfactory to study the effect of a temperature change by cooling the liquid because, if it is heated more than a few degrees above room temperature, vapor will condense throughout the system. Figure 4 shows a calibration curve for mixtures of acetone and water containing from 0 to 100 volume per cent of acetone. The students can prepare both the
4. cdib..tion curve for ~ i r t ~of m~ c e t o n eand water Prrgared by Bubbling 6W ML./Min. of Nitrogen through Samples 2 Cm. Dnep (Gow-Mac, BIT Brid.4 rim
LITERATURE CITED (1) DAYNES,H. A,, "Gas Analysis by Measurement of Thermal Conductance," Cambridge University Press, London, 1933. (2) MINTER,C. C., J. CHEM.EDUC.,23, 237 (1946). C. C., AND L. M. J. BURDY,Anal. Chem., 23, 143 (3) MINTER, 1145l\
(4) PALMER, P. E., AND E. R. W-EATER,U. S. Bureau of Standards, Tech. Paper No. 249, 18, 35 (1924). E. R., "Thermal conductance," chap. in W. G . (5) WEAVER, BEEL. Editor. "Phvsical Methods of Chemical Analysis." ~cademicPress, I;., S e w Yark, 1950, Vol. 11.
.
