In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Easy-To-Make Cryophoruses Rubin Battino* Department of Chemistry, Wright State University, Dayton, OH 45435; *
[email protected] Trevor M. Letcher Department of Chemistry, University of KwaZulu-Natal, Durban, South Africa
William H. Wollaston (1766–1828) was an English chemist and natural philosopher. He first described in 1813 (1) the experiments of a Mr. Leslie using a cryophorus or “frost bearer” (Wollaston coined this term). This first device used a bell jar wherein a dish of water was supported above a dish of concentrated sulfuric acid. The latter is extremely hygroscopic, and in the evacuated bell jar the evaporation of water (being absorbed by the sulfuric acid) from its dish was so rapid that the water froze within a few minutes. The cryophorus is generally two bulbs connected by tubing wherein a liquid (usually water) is sealed under vacuum, so the apparatus contains only water and water vapor (Figure 1A). When the lower bulb is immersed in a freezing mixture, the evaporation of the liquid in the upper bulb cools the liquid until it freezes. The enthalpy of vaporization is sufficiently large to cool the liquid to its freezing point. The cryophorus can be considered to be a precursor of the modern heat pipe (2). It is also an example of an endothermic process (3). This Journal has published a number of articles (4–7) on the cryophorus. The most recent article by Hunter and Knoespel (4) describes both the sulfuric acid version and the glass bulb version. Balinkin (8) describes using the “Franklin” flask (an inverted 1 L Florence flask with a concave bottom to hold a
A
B
coolant) as a cryophorus, and Baker (9) describes a modified cryophorus that works in the horizontal plane. Liquids that have been used include water, Br2, benzene, and chloroform (6). In this article we describe some easily constructed cryophoruses. We have used water, cyclohexane, benzene, acetone, and toluene in our experiments but only recommend using the safer first two. We especially recommend using cyclohexane since in all of our designs, and in all three cold baths, it responded in the shortest time and in the most dramatic fashion. There are few liquids that are usable for this experiment since the liquid should have a freezing point between about ‒50 and 15 °C, a reasonably high vapor pressure at its freezing point, a low enthalpy of fusion, a high enthalpy of vaporization, be inexpensive, safe to handle, and readily available. Experiment We used five different home-made cryophoruses for these experiments. One was based on a single vertical two-bulb apparatus made for us by a glassblower (Figure 1B, design 1).1 A second design involved making the cryophorus out of highvacuum stopcocks (Figure 1C, design 2).2 These two designs
C D
E
F
Figure 1. The cryophorus is generally two bulbs connected by tubing wherein a liquid is sealed under vacuum, so the apparatus contains only the liquid and its vapor: (A) Wollaston cryophorus; (B) design 1, single vertical two-bulb apparatus made by a glassblower; (C) design 2, a cryophorus made out of high-vacuum stopcocks; (D) design 3, involving ordinary test tubes; (E) design 4, made mostly out of copper tubing; and (F) design 5, made from ordinary lab glassware.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 4 April 2008 • Journal of Chemical Education
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In the Laboratory
worked well over long periods of time since they are well-sealed. The third type is a simple design involving an ordinary test tube plus a side-arm test tube, with the two test tubes connected by a 1/4 in. piece of stopcock-greased copper tubing going through one-hole rubber stoppers (Figure 1D, design 3). The advantage of this design and the next one is that no glassblowing is involved. Our next design was made mostly out of copper tubing connecting to an Erlenmeyer flask through a stopcock-greased rubber stopper and with a copper T junction (Figure 1E, design 4). Some skill at soldering copper tubing is required for good seals. We made a larger cryophorus (Figure 1F, design 5) that can be used in moderate-sized classrooms. This was effectively made from a 500 mL round-bottom flask for the lower bulb, and a 1 L round-bottom flask for the upper bulb. Figure 1F shows the two bulbs being connected by a length of 6 mm glass tubing, but 1/4 in. copper tubing with a T can also be used. The upper bulb was charged with ca. 75 mL of cyclohexane before about 10 to 15 mL were evaporated off. The last three designs needed to be prepared fresh for each experiment. The cryophoruses were charged by evaporating off ca. one half of the liquid in the upper bulb via the side arm, which was sealed with a screw clamp on a piece of latex pressure rubber tubing. These worked well, but they all developed slow leaks since it was difficult to obtain good seals around the rubber stopper.3 Use of a 50 mL Erlenmeyer flask for the upper bulb in designs 3 and 4 rather than the test tube has the advantage of a larger surface area for the liquid than a test tube. The evaporation of one-half of the liquid in the upper bulb (or tube) was carried out using a water aspirator to attain low pressures (ca. 15–25 torr). A vacuum pump may also be used. Evaporation was enhanced by placing a beaker with hot water (ca. 60–80 °C) around the upper liquid-containing bulb. With respect to safely handling the small quantities of volatile organic solvents, the water aspirator carries the condensed vapors down the drain with large quantities of water. A cold trap with a vacuum pump could be used to trap the organic solvents. Three different cold baths were used for the immersion of the lower bulb: liquid nitrogen (‒196 °C), powdered dry ice (‒79 °C), and an ice/salt mixture (‒11 to ‒13 °C). The liquid nitrogen and dry ice work well, whereas the ice/salt mixture only works well (albeit slowly) with the permanently sealed cryophoruses (designs 1 and 2). Cyclohexane was the only liquid to work well and reproducibly and dramatically in all cold baths and with all designs.
Hazards All of the organic liquids are flammable. Benzene is a known carcinogen, and it should be avoided. Liquid nitrogen needs to be handled carefully so that it does not contact the skin; also insulated gloves need to be used to avoid inadvertent freezing on contact with a container. The dry ice was powdered by wrapping it in a cloth and smashing with a hammer. It was then poured into a 1 L beaker. Insulated gloves need to be used when handling the dry ice and the beakers. Description of the Process The freezing process in the upper, liquid-containing bulb depends on a number of physical properties. A good working liquid has (i) a large enthalpy of vaporization, ∆vapH, and a small enthalpy of fusion, ∆fusH, so that the evaporative cooling rapidly reduces the temperature in the upper bulb to the freezing point; (ii) a high vapor pressure, both at ambient temperature and at the freezing point, Pf, to ensure rapid evaporation in the upper bulb; and (iii) a freezing point temperature, Tf, close to ambient temperature to ensure that the freezing point is reached quickly and before all the liquid has evaporated, and to minimize heat transfer into the upper bulb from the environment. The cryophorus is made of a material such as glass with a low heat conductivity, again to minimize heat transfer into the upper bulb. Some relevant properties of the working fluids used in the experiments are shown in Table 1. In our experience the bottom bulb cools rapidly upon immersion in the cold baths, most probably attaining the temperature of the bath in less than 1 min, and almost instantaneously for liquid nitrogen. The rate of evaporation from the Erlenmeyer upper bulb is greater than for the test tube-style upper bulbs owing to the greater surface area of the liquid. Design 1 would be intermediate in evaporation rate owing to a surface area between that of the Erlenmeyer flask and the test tube upper bulbs. The rate of conduction of ambient heat through the glass walls of the upper bulb was a factor in only one of our experiments—for the low-melting liquids of acetone and toluene. To reduce the conduction of heat in those experiments, the upper bulb was insulated with cotton batting. The major driving force for evaporation in the upper bulb is the difference in vapor pressure between the upper and lower
Table 1. Physical Properties of the Liquidsa
Liquid
Tf/K
43.929b
Pf/torr
Water Cyclohexane
279.8
2.68
353.9
33.33
40.09
Benzene
278.6
9.866
353.3
33.9
35.99
Acetone
178.7
5.69
329.3
29.1
0.0210c
Toluene
178.1
6.62
383.8
38.06
0.000205c
562
bAll Δ
vapH
373.17
ΔvapH/(kJ mol–1)
from ref 10.
6.005
Tb/K
aData
273.15
ΔfusH/(kJ mol–1)
4.588
measured at 298.15 K. cEstimated from the Antoine equation.
Journal of Chemical Education • Vol. 85 No. 4 April 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
bulbs. The greater this difference, the greater the rate of evaporation. When the lower bulb is immersed in liquid nitrogen, the vapor pressure difference is essentially the vapor pressure at room temperature (at the beginning of the experiment) and at the liquid’s freezing point (just before freezing takes place). This is also the case for dry ice as the coolant, the vapor pressure of water being ca. 0.00005 torr and that of the other liquids significantly below 1 torr. The vapor pressure of water at the ice/salt temperature of ca. ‒12 °C is 1.6 torr and 4.6 torr at its freezing point. Design 2 was the only one in which we could get water to freeze using the ice/salt bath. At ‒12 °C the vapor pressure of cyclohexane is 13.7 torr (vs 40.09 torr at its freezing point) and for benzene it is 12.7 torr (vs 35.99 torr at its freezing point). Thus, for both cyclohexane and benzene there is a sufficiently large vapor pressure difference for these liquids to freeze using an ice/salt cold bath.4
Table 2. Results of Cryophorus Tests for Five Designs Design
Liquid
Freeze Mixture
Freeze Time/s
Comment
1
Water
Liq N2
75–180
Freezes at top
1
Water
Dry ice
(600)a
No freezing No freezing
1
Water
Ice/salt
(600)a
1
Cyclohexane
Liq N2
5–20
Solid at 13–20 s
1
Cyclohexane
Dry ice
15–50
Solid at 30–50 s
1
Cyclohexane
Ice/salt
21–36
Solid at 31–36 s
1
Benzene
Liq N2
6-40
—
1
Benzene
Dry ice
24–46
—
1
Benzene
Salt/ice
(300)a
No freezing, turgid
2
Water
Liq N2
15–120
—
2
Water
Dry ice
35
—
2
Water
Salt/ice
15–120
—
2
Cyclohexane
Liq N2
5–20
Solid at 10–30 s
2
Cyclohexane
Dry ice
9–22
Solid at 20–30 s
2
Cyclohexane
Ice/salt
50–80
Solid at 70–80 s
2
Benzene
Liq N2
11–18
Solid at 15–20 s
Results and Discussion
2
Benzene
Dry ice
120
—
The summary of our results for many repeated experiments is given in Table 2. Figure 2 is a composite showing close-ups of the frozen solvents in the upper bulbs. Water tends to freeze first at the surface and then the ice slowly thickens. Gently tapping the upper bulb can sometimes speed up the initiation of freezing. Cyclohexane and benzene tended to freeze more swiftly and dramatically in an almost explosive fashion of white crystals filling the bulb owing to the rapid evaporation. As mentioned earlier, the sealed cryophoruses work well over periods of years, while designs 3, 4, and 5 need to be prepared freshly. Some of the design 3 and 4 units worked well using liquid nitrogen up to one week later. Considering the data in Table 1, it is apparent that the more rapid freezing of cyclohexane and benzene is most probably due to their significantly higher vapor pressures at their freezing points with respect to water. (Acetone and toluene both have quite low freezing points and very low vapor pressures at those temperatures.) The enthalpies of vaporization are roughly comparable for cyclohexane, benzene, and water. Although the enthalpy of fusion is significantly lower for cyclohexane than benzene and water, the major contributing effect is apparently the vapor pressure at the freezing point.
2
Benzene
Ice/salt
(300)a
No freezing
3
Water
Liq N2
231
Solid
3
Water
Dry ice
(600)a
No freezing
3
Cyclohexane
Liq N2
5–17
Solid at 22–31 s
3
Cyclohexane
Dry ice
17
Solid at 35 s
4
Water
Liq N2
70–180
Solid at 180 s
4
Water
Dry ice
132–420
Solid at ca. 300 s
4
Cyclohexane
Liq N2
6–15
Solid at 10–20 s
4
Cyclohexane
Dry ice
8–10
Solid at 18–30 s
4
Cyclohexane
Ice/salt
97
Solid at 110 s
4
Benzene
Liq N2
30–80
—
4
Acetone
Liq N2
300
With insulation
4
Toluene
Liq N2
60
Small quantity
5
Cyclohexane
Liq N2
50–60
Easily visible
aNumber
in parentheses indicates how long we waited before ending that test run.
Summary This article describes some simple cryophoruses that can be made with basic laboratory equipment. Our guiding principle in these designs was “Would a high school chemistry laboratory have the resources required to make
Figure 2. Close-ups of the frozen solvents in the upper bulbs.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 4 April 2008 • Journal of Chemical Education
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In the Laboratory
these cryophoruses?” The article also describes the use of cyclohexane as a working liquid, and shows that it is indeed better for a cryophorus than all previously reported liquids. The cryophorus is an excellent apparatus to illustrate evaporative cooling in a dramatic fashion. Acknowledgment We thank J. D. Arehart for help in fabricating the coppertubing cryophoruses. Notes 1. This model was designed by TML from the standard, traditional model. 2. This model was glassblown by RB. 3. We tried using a silicone sealant in this area but that made re-charging the cryophorus difficult and the sealant did not help significantly. 4. From the explanation given, one can design cryophoruses with other working fluids.
Literature Cited 1. Wollaston, W. H. Phil. Trans. 1813, 71–74.
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2. Smith, B. A. Phys. Educ. 1980, 15, 310–314. 3. Letcher, T. M.; Battino, R. Education in Chemistry 2004, 41, 104–105, 109. 4. Hunter, W. W.; Knoespel, S. L. J. Chem. Educ. 1994, 71, 67–68. 5. Baker, P. S. J. Chem. Educ. 1950, 27, 617–618. 6. Stone, H. W. J. Chem. Educ. 1949, 26, 481–484. 7. Baker, R. A. J. Chem. Educ. 1948, 25, 259. 8. Balinkin, I. Amer. J. Phys. 1933, 1, 86–87. 9. Baker, R. A. Amer. J. Phys. 1939, 7, 424. 10. (a) NIST WebBook. http://webbook.nist.gov/chemistry/ (accessed Jan 2008). (b) Physical Properties of Chemical Compounds. Advances in Chemistry Series; Dreisbach, R. R. Ed.; American Chemical Society: Washington, DC. 1955 (No. 15), 1959 (No. 22), 1961 (No. 29).
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Apr/abs561.html Abstract and keywords Full text (PDF) Links to cited JCE articles
Color figures
Supplement
Two movies showing cryophoruses generation
Journal of Chemical Education • Vol. 85 No. 4 April 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
