Table 6-1 Temperature Variations with Combined Organic Solvents0

" Data interpolated from a graph from the article by A.M. Phipps and D.N. Hume, "General Purpose Low Temperature Dry-Ice Baths," Journal of Chemical Education, 45, p. 664 (1968).

6.2.8 Safety with Slush Baths

Because the low- melting-point liquid in the slush bath is near its freezing temperature, there is little concern over toxic fumes as when working with some chemicals at room temperature. However, in the beginning, you must work in a fume hood because of the copious amounts of fumes released from the lowmeltingpoint liquid. Once the slush bath is made, it is safe to remove it to the lab. However, it is best to leave the slush bath in the fume hood if at all possible. If the slush bath is used in the lab, move the slush bath to the fume hood immediately after your work is completed.

Never pour a slush bath down the sink. The low temperatures can destroy plumbing! Instead, let the coolant boil off in a Dewar in the fume hood. Later, the low-melting-point liquid can be saved and reused.

Because slush baths achieve very low temperatures, protect your hands from the extreme cold. This protection presents some problems with the current tempera- ture-protecting gloves available and problems inherent with slush baths. Thermal gloves made out of Kevlar,* a plastic, can be dissolved by some organic solvents. Thermal gloves made out of fiberglass are okay, but they are very slippery. In addition, broken glass fibers from fiberglass gloves can get under the skin and itch for many days. Asbestos gloves^ are not allowed in most states. My personal choice among all these suggestions would be the Kevlar gloves and don't be sloppy.

Kevlar gloves are banana yellow with a surface like terry cloth.

f If your laboratory still has some asbestos gloves, you may need to check with your safety officer or the Department of Health in your area for proper disposal.

Incidentally, when making slush baths out of organic solvents, do not use utensils (such as stirrers) that will dissolve in the organic solvent you are using. Although a thermometer may already be in the container and ready to stir with, do not use it as your stirrer because it may break from the torsional forces of stirring the thick slush. Wooden dowels are excellent for mixing because they are strong and will not scratch the surface of a Dewar. You can always hold a thermometer and wooden dowel together and stir as a single device.

A potentially explosive situation can develop when an acetone slush bath is left sitting for an extended period. Over time, the acetone and dry ice separate and the acetone floats to the surface, whereas the dry ice settles to the bottom of the Dewar. The acetone soon warms up to near room temperature, but the dry ice remains near the slush bath temperature of -77°C. If any agitation causes the warmed acetone to cut into the dry ice slush on the bottom, a flume of boiled off CO2 can erupt. This flume will carry the acetone layer that was on the surface in a large spray all around the area. If there is a flame or spark (from a motor) in the path of the acetone, this accident could have far greater consequences. This situation can be easily avoided by constantly mixing the solution. A safe alternative to the acetone slush bath is the ethanol slush bath. The ethanol slush bath is somewhat warmer (-72°C) but does not display the same potentially dangerous capabilities.

6.2.9 Containment of Cold Materials

There are two concerns for the storage of cold materials: longevity of the material and safety to the user. For example, if you place an ice cube on a lab bench, it will melt. On the other hand, if you place an ice cube in an insulated container, it will also melt—but it will take longer. By providing insulation, you have added to the ice cube's longevity. If you hold an ice cube for an extended time, your hands will soon become so cold that eventually you will need to drop the ice cube. However, if you hold an insulated container containing an ice cube, there is no discomfort.

The reason for stating the obvious is to establish the purpose of specialized containers for containing cryogenic solutions. A properly made container protects the materials inside the container and also protects the users outside. In addition, the container should be able to reasonably deal with the expected physical abuses that may be encountered within the lab. The final selection of a cryogenic container is based on its shape, design, construction material, use, and function. Although it is possible to use a Styrofoam cup to contain cryogenic materials such as liquid nitrogen, it is a poor choice for the demands of a laboratory. On the other hand, placing tap water in a Dewar may be a waste of money. As with most decisions in the lab, common sense must be used when making equipment selections. Ultimately, the selection of the quality, shape, and design of a coolant container may be based on six criteria:

1. Cost of coolant. If the coolant is inexpensive and readily available, you don't need a highly efficient container.

312 High and Low Temperature

Table 6-3 Slush Bath Temperatures (continued)

Glass Dewars should always be wrapped with fibered tape (not masking tape) to prevent glass from flying around in the event that the Dewar is accidentally broken. Some commercial Dewars have a plastic mesh. This mesh is acceptable, but wrapping with tape provides much better support to prevent flying glass. In addition, wrapping with white tape (such as any sport tape) provides one extra level of insulation for the Dewar than if the Dewar was wrapped with black tape (see Sec. 7.4.4). Dewars come in a variety of shapes and sizes as can be seen in Fig. 6.5.

If frost appears on the outside of a Dewar, the vacuum within has deteriorated and the Dewar is no longer usable. This deterioration is typically caused by an imperfect tip-off at the base of the Dewar. It may take months or years for a Dewar to lose its vacuum; but once the vacuum is gone, the Dewar cannot effectively hold cryogenic fluids. It is possible, however, for a glass shop to open the Dewar, clean, re-silver, re-evacuate, and re-tip-off.

Foam-Insulated Containers. Foam-insulated containers are an inexpensive alternative to Dewars. They are doublewalled (hollow-walled containers that are filled with an insulating foam instead of a vacuum. These containers (which come in as many shapes and sizes as regular Dewars) are significantly less expensive but much less efficient than standard Dewars.