1-XPOR P R E S S U R E O F S - I P H T H - I L E S E -ITL O K TE1IPER.ITURES
__-__
BY MART
R. A S D R E W 5
I n a n article' 11y t'lie author on the carbonization of tungsten filaments I I ~ heating them t o incandescence in napht'halene vapor it was st,ated that apparently not more than three-tenths of the molecules of naphthalene that struck the surface of t,he Elament reacted. This fraction is called "E". The earlier calculation was hasetl on vapor pressure d u e s extrapolated from pressures determined a t much higher temperatures 11y several other investigator;. I t seemed possible that such estrnA polation might he in considerable error at the lower extreme. Therefore! vapor pressure tlet'erminations of naphtmhalene a t several low .temperature: were iiiacle by two independent methods : The first was a modified I, 9 , -1;'C . 3 i 3 .q.; 4
2.4.
0
2.6
t
The second method depended on the use of a yihration gauge in n-hich the vibrating element instead of being a quartz fiber was a strill of fom. tenths J. P h y . Chem.. 27, 270 f1923'8. Ann. Physik. (4)29. 179 1909). ~
mil molybdenum about ten centiiiieters long by t\yo millimeters wide. This was ~ ~ e l d teod a strip of ten mil nickel ivhich was in turn welded t o two leads in the stein of the bulb. Such a gauge (See Fig. 2 ) , n-hile its rate of vibration is much less than that of a quartz fiber, is very much more rugged, and difficulties due to precession are eliminated. The range of pressure over which it is useful, i. e , , oyer which the decrement of vibration t o M amplitude occurs in a reasonable time can be varied hy varying the thickness of the strip. Determinations with this gauge were made on a condensation pump Y/ORAT/ON GAUGE exhaust system. A naphthalene reservoir nhich could be immersed in a hath a t the desired temperature was connected to the lower end of the gauge. A second reservoir of naphthalene at the same temperature was NAPHTHALENE connected to the tubing leading t o the FIG.2 pump. This tubing was constricted betlveen the t n o reservoirc. (See Fig. 2 ) . -4 liquid-air t r a p near the pump caught the naphthalene as it tlistilletl from the second reservoir. C‘z!ibration was made v i t h water vapor uqing &heel and Heuse’sl data.
f5f
Yapor pressure determinations made by thjq niethod were as follows:
Tenip.
Piecsii/.e 034 X IO-^ mm
- 3 6OC -3 0 -21
-14. j
- 6
+ .3
’
79
2 . 0
5.6
The logarithms of the values obtained by thece two methods as well as data published by -4llen? and by B a r k 3 and Miss Daly’s data published in the paper on carbides cf tungsten-’ have been plotted in Fig. 3 against the reciprocal of the absolut,e temperature. I t will be observed that the results obtained by the two methods are in fair agreement and t h a t the points lie cloeely along a straight line, which, if prolonged, coincides with Allen’s data except for his two lowest points. Barker’s three points and RIiss Daly’s data lie somen hat above this line, I t is probable that Miss Daly’s deterniinations, which were macle with the ionization gauge, were high because of tlecomposition of the naphthalene by the hot filament of the gauge. ‘.inn. Physik, 2 9 , 723 (1909‘1. J. Chem. POC.,77, 410 (1900). Z. physik. Chem., 71, 23j f1910). J. Phys. Chem., 27, 270 (1923).
T‘APOR PRESSURE O F SAPHTHALEXE
I499
From the plot of the data obtained by the writer, and taking into account the possible errors in the various sets of observation. the most accurate values may be represented hj- a curve whose equation i q log,, P =
- 4000 ~
T
+
12.275
n-here P is measured in inilliineters. The follon-ing table gives values of the vapor pre+ure calculated from this equation.
- 50
-40 - 30 -2 0
- IO 0 +IO 20
30
40 .i 0
60 io
Tlie above foriiiula gives a heat of evaporation of 18,280 calories per gram molecule. Recalculation of “E“ using the new w p o r pressure curve gives values greater than unity, which is impossible. This is undoubtetlly due to failure in the early experiments t o maintain rigid coding of the naphthalene reservoir. Stirring n-as intermittent and the upper layers of the chilling solution were 1:rohably considerably above the registered temperatures. A five degree error in the affective teni1;erature n.oulc1 cover the discrepancies. I t is, therefore, safe t o assume that “E” is unity. Vapor pressure. on this basis, and without temperature correction, are shown in Fig. 3 under the caption “Xssuming ‘E’ t o he unity.”
“E” is unity, hoTveyer, only when the filament, temperature is so high t h a t all the carbon tlepositetl on the surface diffuses into it8immediately. Decomposition. as shown in the earlier art,icle’ takes place much leas readily on a carbon surface. J. Phys. Chem., 27, 270 (19231.
I 500
M A R T R. AiYDREXS
I n conclusion the author wishes t o extend thanks to Dr. SaulDushnian and Dr. Irving Langniuir for their advice and helpful criticisms of this work.
Summary The vapor pressure curve of naphthalene from +?o" t o - jo"
