A GLASS MANOMETER for LABORATORY USE by STUDENTS in PHYSICAL CHEMISTRY T. E. PHIPPS, M. L. SPEALMAN,
AND
T. G. COOKE
University of Illinois, Urbana, Illinois
Detailed instructions are offered for the construction.of an all-glass m a m e t e r of th.e type referred to herein as the "sickle" type. Attention is called to fissible uses of such a gage in experimental work.
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T
HE NEED for an all-glass manometer relatively simple in construction and operation has doubtless been apparent to many teachers of laboratory courses in physical chemistry. The manometer described herein is not original with the authors, having been used by several investigators. However, no detailed account of a method of construction has appeared in the literature, so far as the authprs are aware. The success with which the device has been used in a laboratory course in physical chemistry a t the University of Illinois has encouraged them to think that constructional details of the manometer would be of interest to others. A brief description of various all-glass gages is given by F. Daniels (I). The gage attributed to C. G. Jackson (2) and G. E. Gibson (3) is similar to the "sickle" gage used by the present authors. In modern published work it is found that gages of this type have been employed by several photochemists. For example, R. G. W. Norrish (4) used it in a photochemical study of the union of hydrogen and chlorine; E. Ponsaerts (5) in studying the radiochemical synthesis of ammonia; and H. P. Smith, W. A. Noyes, Jr., and E. J. Hart (6) in their photochemical studies. The latter refer to i t as a "pyrex gage of the Bodenstein type." I t appears, however, that the sickle gage should be credited to Gibson, since Bodenstein's original
b
c
b
c FIG^ I.-INITIAL ~ P IN FABRICATING S THE GLASS GAGE
F I G ~2.-INTEWEDIE ATE STEPS IN FABRICATING TH-E GLASSGAGE
gage (7) was of a coil type. Bodenstein and also F. M. G . Johnson (8) have credited the idea of the spiral gage to E. Ladenberg and E. Lehmann (9). Details of the method of construction of the gages used in this laboratory follow. A pyrex tube about 6 to 8 mm. in outside diameter and of standard wall thickness is cleaned with wet plugs of cotton, rinsed, and dried. A piece about 30 cm. long is fire-polished a t one end and provided with a ligbtweight rubber blowing tube which is held in the mouth of the operator during the next step. The free end is
rotated forward and backward in a quiet oxygen flame, the end is drawn off and discarded, and the tube end is then melted until it has the appearance shown in Figure l a . When the ball of glass is uniformly heated and is very soft, the operator sticks a small heated rod end, as shown in Figure lb, into the molten mass, removes it from the flame, and immediately begins a series of quick short puffs of increasing intensity which blow a thin-walled bulb of about the shape and capacity shown half size in Figure 16. As the bulb increases in size the elongation apparent in the figure is produced by drawing the molten mass slightly with the rod handle. The rod is now sealed off a t e before proceeding to the next step, and the rubber blow tube is removed. A very soft oxygen flame is now directed intermittently (cautiously) against one side of the thin bulb. The flame size is reduced as the bulb is carried through the stages a' b' and in Figure 2; the final forminp " of the inner curve of the sickle is with an extremely small soft flame. This caution is necessary in order to avoid any slight distortion of the outer surface of the sickle by radiation. Special care must be taken not to allow the flame to lick past the inner edge and produce a thickened, distorted edge, as illustrated in 3b. A ~ r o ~ e rmade l v sickle will exhibit an almost flat, ribbon-likeinn& surface and a sharp edge between this and the outside convex surface (which is the original undisturbed surface of the bulb). Figures 26 and 3a exhibit the finished sickle respectively from the side and the front. The rod r is 3 to 4 mm. in diameter and serves as the base for the stationary pointer. The sensitivity of the gage depends largely upon two constructional factors: (1) the thinness, near the end, of the original bulb, and (2) the degree of flatness of the inner surface. The movable pointer may be made conveniently as follows. A pyrex rod is drawn by hand before the flame to about 1.5 mm. diameter. A section of this rod is sealed temporarily to'a larger handle and suspended vertically. Heat is then applied near the lower end either with a micro oxy-hydrogen flame or, better, with an electrically heated loop of platinum wire. The end will drop, drawing out a fine fiber. With practice sudden tapers from 1.5 mm. to about 0.01 mm. can be produced. For very fine pointers the heating and drawing are best carried out in stages, the heated sec tion having a progressively smaller diameter. The stationary pointer is made by a similar method except that the taper is from a 3 or 4 mm. diameter down to approximately 0.01 mm. Figure 4 shows the method of assembly and alignment. The movable pointer m is sealed a t a to the sickle. The stationary pointer s, of the form shown in the figure, is sealed temporarily with wax a t b to the movable table, t, of a miaoscope stage, and the miaoscope is shifted until the two pointers are in the field of vision, v. The length of s must have been adjusted previously so as to leave a slight gap a t c when the pointers are in coincidence. With the adjusting screws of the movable stage the two
pointers are then brought very close together and exactly opposite. The clamp d permits small up-anddown adjustments of m. The gap a t c is closed by allowing a drop of molten glass from the end of a rod to melt down upon the two ends. The melting-in is completed with a small oxy-hydrogen flame. The sealing wax is now softened and the microscope table removed. After the sealing off of the extension for the wax seal a t f, the gage is ready to be sealed into the apparatus as shown in Figure 1 of the preceding article.
FIOLTE3.-CORRECT AND INCORRECT AFTE~RANCE OR THE COX-
-
p-r a-r-~ n" S. ~.r.. ara" .
LIY
FlounE
ME THE
AND ALIGNMENI.
OE
POINTERS
It is characteristic of these gages tbat they will withstand a greater difference of pressure operating in the direction to open up the sickle than in the direction to close it. Extremely sensitive gages were occasionally made which would kthstand a vacuum outside and one atmosphere inside the sickle but which would be destroyed if the connectionswere reversed. When a gage was allowed to stand for several weeks a t room temperature with a p:essure difference of about 200 mm. between the two sidesnf the membrane, a zero shift of the order of 0.05 mm. was observed. However, hysteresis during a period of a few hours was never observed. The effect of high temperatures on the membrane, and any possible change in sensitivity with temperature are two factors which have not as yet been investigated. . Extreme sensitivity was not sought in the gages used for class work. If the sensitivity be defined by the expression dx/dAp, where x is the displacement in mm. of the movable pointer and Ap the diierence in pressure expressed in mm, of Hg, it was found that the sensitivities of the gages lay between 0.1 and 0.01. The error in aligning the pointers was of the order of magnitude of the diameters of the pointers themselves; that is, about 0.01 mm. This introduced an error into the measurement of P of not more than 1nun. One example of the use of this gage is given in the preceding article, and it seems tbat this gage could be used profitably in other laboratory experiments in an elementary course in physical chemistry. Further
TABLE 3 PORYDLAS FOR CIILCDLAIION OP PBRCBKTAOB EBBOBS IN m AND K s o. DAra O r T A e L B 2 PC
dm 1W;
THB
PBBC&NTA(~B Esms IN n AND
dK
lwy
TABLB 4 K.As-o IN P
1 YY. EXPEE-NT*L ERBOB
PC
100,dP
100;da
35.9 47.9
0.128 0.113 0.102
0.611 0.417 0.316
35.9
215%
+ loo!! P
47.9
2339
+ ~ wP d _ ~59.1
59.1
do 280;
+ 1WpdP
a
IN
dK 1 0 0 ~
1.44 1.08 0.92
LITERATURE CITED
(I) VaraoaK, F. H. AND DANIELS, F.,"Thc dissociation constants of nirrogen telroxidr and of nitrogen rrioxidc." I. Am. C h m . Sac.. 53.1250 (1931). (2) BODENSTEIN; M., " ~ h d u n gund Zersetzuog der h6heren Stickoxyde," Z. physik. Chcm., 100,68 (1922).
(3) DASIEL~.F.,. M ~ n w w s J. . H.,AND WILLIAMS, J. H.. "Experimental physical chemistry," hlcGraw-Hill nook Co., Inc.. New York Cirv. 1929. D. I12 (or second edition. 1934. D. 122).
