Soap Bubble Respirometry Ken Cummins Mount Royal College, Calgary, Canada
T h e apparatus shown in Figure 1, called a soap bubble respirometer, can be used t o measure low gas pressure and was constructed by substituting a horizontal glass tube for the nanometer arms previously used1 in a differential manometer. The soap bubble respirometer possesses enhanced sensitivity due to the fact that both flasks can he kept a t the same temperature, thus nullifying changes in pressure caused by external temperature variations. Hence, precise temperature control is not mandatory for high sensitivity, ~ r o v i d e dthat both flasks are subject t o the same temperature control. Even with large temperature variations, the resultant pressure changes would be nullified by the opposing flasks. Additional pressure in one of the flasks (e.g., vapor pressure of hexane) will cause the soap huhble to move t o a new nosition in the tube. This nosition will be reached when the pressure from the compresbedair on one side of the bubble iseaual to the total pressurearising from theexpanded air and additional presiure on the other side.
Flgure 1. Design of the soap bubble resplrometer.
One application of this soap bubble respirometer is reported in this article: the measurement of the vapor pressure of hexane over a temperature range 2.5-18.5 OC and with a temperature precision of +0.25 'C. T h e apparatus is first calibrated in order t o determine the cross-sectional area (c) of the glass tube and volume (V)of each flask.
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Callbratlon of Apparatus: Determlnatlon of Cross-Sectional Area c
T h e soap bubble will move a distance of H cm from its initial position when VgmL of air is injected. It can be shown that
The cross-sectional area can be calculated from the slope of the graph of H versus V,. Experlmental Procedure
Rinse a 1-m glass tube (16-mm i.d.) with distilled water, and drain. Insert a soap bubble into the tube as follows: rinse the tube with distilled water, and then drain. (The tube must bewet, containing a residual volume of water from the rinsing step; otherwise, the bubble will not last. With correct preparation bubbles can last for several hours. Avoid knocking the tube.) Add a few drops eoncentrated soap solution into one end of the tube, seal the end with a finger, and hold tube vertical to obtain a 0.5-em depth of soap solution. Position tube horizontally, and remove finger. Position the single bubble at one end of the horizontal tube, and assemble the apparatus n i t h clamps C and D removed. Record nurn temperature rT and ~tmospherirpressure (I' 1. Close clamps C and D,wait 5 mi". and then mark the inrtial Doairion of the huhble with a wdx pencil. Avoid touching any glass surface of the apparatus. Using a 5-mL plastic syringe fitted with a fine needle, inject through a septum at F successive 5-mL quantities of air slowly into one of the flasks, and measure the corresponding displacement (H cm)of the soap bubble. Determlnatlon of Volume k(mL)
T h e soap bubble will move a distance of H cm from its initial ~ o s i t i o n(both flasks are a t a t e m ~ e r a t u r eof To)when the temperat& of one of the flasks c'hanges to a ;due T (Kelvin). I t can be shown that
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' Cummlns, K.; Stedmann G. J. Chem. Educ. 1988. 65, 88
Volume 68
Number 7
July 1991
617
Figure 2. Effectof injected air volume on bubble displacement
Figure 4.
Plot of log (vapor pressure) vs. 10JITforhexane
ume (mL) of each flask; To= initial temperature (K) of both flasks; c = cross-sectional area (em2).
Vg (ml) Volume of Air Injected Figure 3. Effect of changing temperature of one flask on bubble displacement.
Experimental Procedure Follow the first paragraph in the previously described procedure (cross-sectionalarea). Insert flask B into water at room temnerarum and measure rhr inrreasinp:drsplacement (Hem, of the bubble for decreasing (:I 'C intervals) temperature vnlubs (2-20 TIhy the addition of mall qunntities of crushed ice. Vapor Pressure Curve of Hexnne The vapor pressure (PL)of a liquid introduced into one of the flasks can be calculated using the equation shown below:
Po = atmospheric pressure (KPa); H = displacement of bubble (cm); T = temperature (K) of both flasks; V = vol-
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Journal of Chemical Education
Experimental Procedure Follow the first paragraph describpd previously for ctoss-rertional area calibratron. After removal of spring clamps C and I), insert horh flask* in the same water bath at an initial temperature T h-5 'C) for 5 min. Measure atmospheric pressure Po. Close spring clamps C, D, and E, and then mark the initial position of the bubble with a wax pencil. Using aPasteur pipe, add 1-2 mL iee-cold hexane into the tube at F, but keep the spring clamp E closed. After adding the hexane, close spring clamp F. Remove spring clamp E to allow the hexane to enter the flask.Measure theincreasing displacement (Hcm) of the bubble for increasing temperaturevaluesup to 18.5 'C. At agiven temperature value, wait 5 min before recording the displacement (H), and add small quantities of ice to maintain a given temperature. Note: Addition of ice cold hexane prevents a pressure surge and subsequent disappearance of the soap bubble.
Results and Discussion The cross-sectional area ( c ) and volume (V),calculated from Fieures 2 and 3.were 0.279 em2 and 216 mL. The heat ofvapohzatiin of hexane (32.7 Kcal mol-I), calculated from Figure 4, is in good agreement with the literature value of 31.9. Figure 4, derived from the measurement of bubble dis~lacement( H e m ) with temoerature ( t O C ) , was obtained using the following data set (H, 11:(28.25,2.5),(30.1. 3.0), (34.2,6.0),(39.3,9.01,(16.0,12.0),(50.95,15.01,(59.0,18.51. Thisapparatus is thus ideally suited ro the undergraduate laboratory since i t is not only simple to assemble and use hut also inexpensively provides accurate results.
