Thermal Conductivity of Some Organic Liquids 0. B. CECIL ARD R. €I. llIUNCH Organic Cheniical Division, Wonsanto Chernirul Co., S t . Louis 4. 410.
F O R \-ears the literature relative t o thermal conductivity of
PROCEDURE
liquids has been characterized by both scarcity arid discrepancies. Of late, however, there is promise of eliminating this scarcity of data and correcting discrepancies in the literature. A brief summary of the past history of thermal Conductivity measurements of liquids has appeared ( 1 2 ) . T h e techniques employed have been limited primarily t o two basic methods: (A’] those employing cj.liritli,ical surfaces; and ( B ) those employing plane parallel surfaces. Fundamentally, there is little to choose between them. ;2 special case of 1Iethod .1 is the hot-wire method, in n-hich the inner surface is a fine wire supplied n-ith heat by passing current through it and measuring its temperature by measuring its resistance. A highly refined modification of the hot-wire method was used in the work described in this paper. T h e hot-wire technique first used by Goldschniidt ( 6 )was more recently revised Ly Hutchinson ( 7 ) , nho utilized a single, noncompensating tube containing a coiled hot-wire filament. T h i j fact, in conjunction with the lack of correction for end effects, n i d e absolute measurement impossible and it \vas necessary to calibrate the cell against known standards. Discrepancies and wcertainties existing in the 1jter:tture make the choice of suit:ll)le calibration liquids difficult. Schmidt and Spurlock ( I 1 ; used a dual-tube modification of the hot-wire technique t o compensate for erid effrcts arid p u t their measurements 011 an :tbsolute basis. In the present study, a cell has been designed which utilizes a wniewhat different approach, to place the results on a n absolute Imsis and enable accurate thermal conductivity measurements in 20 t o 30 minutee on small samples of liqiiid (ea. 20 nil.).
The cell is first filled with t h e liquid to be measured. .Llthoiigll the height of the liquid in the cell was not critical so long as the filament was covered, the cell was usually filled until the liquid reached a height of 1 to 2 cm. in the side arm, then placed in a t,hermostated bath, and allowed to reach thermal equilibrium. This required approximately 15 minutes and m-as determined by measuring the “zero current” reEistuiice of the filament. (AIctually, this resistance m a determined by energizing the bridge rircuit by means of a tapping key for about 1 second a t a time until the bridge was balanced. The magnitude of the energizing current was approximately 3 m a . ) K h e n a constant value Tvas obtained, the sample was considered to be in thermal equilibrium with the bath. 1-arious values of current, furnished
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The apparatus, slionm in Figure 1, consists basically of :i straight platilium wire filament, sealed into a lead glass tube so that, the filament lies axially down the tube. The fine platiiiiini filament (5.8 X m i . in diameter) is spot-m-elded to heavier platinum leads (0.031 cm. in diameter). A short spring constructed of 0.020-cm platinum is inserted between one of the leads aiid the fihmerit t o keep the filament taut. A short distance from each Fnd of the filament, potential leads are attached. .Isshown in the figure, the potential leads are brought i n through leads of the same diameter as those used for the filament. Platinum wire of the same diameter as t h e filament is spot-welded to these leads. T h e potential leads and filament are joined by hooking the leads around the filament, painting the joint with a minute amount of Du Pont silver 4922 (Electrochrmicals Department, E.I. du Pont de Xemours & Co., Inc., Wilniington, Del.), and heating t o fuse the silver. This construction gives a four-lead arrangement analogous to a four-lead platinum resistance thermometer. T h e effective length of t h e filament was 8.18 cm. This length was chosen t o give a resistance easily measured with a standard Mueller bridge designed for use with platinum resistance thermometers and could undoubtedly lie varied somen-hat without affecting the operation of the cell. The tube was 1.26 cm. in outside diameter and 1.07 cm. i l l inside diameter. A side :&mienabled filling of the apparatus.
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CM DIAMETER P t ( 2 3 hi L )
d
Figure 1. Cell for measuring thermal conductivity of liquids
by dry cells, were alloffed t o flow continuously through the filament until a steady state was reached a t each current value (current measured with a TT‘eston Model 280, 0 t o 100 DC niilliammeter ). T h e steady-state value was determined b3- following the resistance value of the calibrated filament until a steady value was obtained. -4 3Iueller bridge (Leeds and Sort,hrup LIueller temperature bridge KO. 8067) equipped with a mercury contact commutator (Leeds & Northrup mercury commutator KO.8068) was used t o measure this resistance value. T h e cell utilized a four-lead arrangement t o eliminate end effects. With the coniniutntor in the “normal” position the effective filament resistance plus one l e d resistance is included in one arn?of the bridge, the other lead resistance being included
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the variable resistance arm.
Vol. 48, No. 3
Thus the measui,ed resistance
value is actually the filament resistance plus one lead resistance
minus the second lead resistance. K i t h the commutator in the "reverse" position, the filament is in the same arni of the bridge as before, but the lead resishnces are reversed. Thus on taking the averiige of the resistance readings for the two positions of thc commutator, the resistances of the leads are canceled out, giving onlv the resistance of the effective length of the filameiit. To
Conil~iuirigl
