obtain the vapor pressure of a material; one is to raise the heat, the other is to lower thepressure. By using a vacuum (see Chapter 7) to obtain the vapor pressure of a solvent, thesolvent will boil off, leaving thematerial behind. This procedure canbedone at room temperature or slightly above room temperature.

The catch to thevacuum method is that youmust have a controlled boil without which the material and/or solvent are liable to be sprayed all over your vacuum system. Although a solvent caneasily be pumped out of a vacuum system, it can cause serious problems if it remains in contact with stopcock grease, O-rings, or mechanical and/or diffusion pump oils. Any particulate material deposited within a vacuum line can only be removed from the vacuum line by disassembly and cleaning. With a glass vacuum system, such a cleaning may be difficult or impossible.

The standard approach to maintaining a controlled boil is to use boiling chips as one uses when boiling materials over a Bunsen burner. However, these chips have limitations because they may be very difficult (orimpractical) to remove at a later time.

One of the best approaches formaintaining a controlled boil is toplace a Tefloncoated stirring bar in a flask and then place the flask over a magnetic stirring device (asmentioned in Sec. 6.1.8). In a mediumto high-vacuum system (103 to 10"5 torr), this activity can be done in a static vacuum with a container sitting in liquid nitrogen (or a slush bath; see Sec. 6.2.7) near the material in question (see Fig. 6.4). The liquid nitrogen acts as a cryopump, thereby trapping all the solvent boiling off and maintaining a constant pressure and, therefore, a constant boil. If your using liquid nitrogen, donotpour it around thetrap until after thevacuum line has been evacuated to prevent trapping of oxygen, otherwise the system may later blow updue to expanding gases (see Sec. 7.4.3)

6.2Low Temperature

6.2.1TheDynamics of Cold in the Lab

Cold can be used in the laboratory to prevent an experiment from getting too warm, to slow therate of a reaction, to transfer materials in a vacuum system, to allow for the separation of materials (with fractional condensation), to decrease the vapor pressure of materials so as to trap them in vacuum systems, or as a (cryogenic) vacuum pump.

The physical mechanisms of cold transfer are the same as heat transfer and use the same physical processes of conduction, convection, and radiation/absorption (for more information on these processes seeSec. 6.1.1).

Both hot andcold express different degrees of thermal energy and are directly related to each other. Thedecision of what is hot or cold is a subjective choice. Heat (energy) always transfers to cold (lack of energy). So, whichever object is

relatively hotter will transfer some of its thermal energy to the object that is relatively colder. Thus, the water running through a condenser does not cool the condenser. Rather, the condenser loses some of its heat to the water, which in turn leaves the condenser (by way of tubing) and is replaced by water, ready to be heated. As the heated water is removed, it takes that amount of heat energy away from the condenser. Although this distinction may seem inconsequential, it is fundamental to the understanding of what is taking place in the cooling process. Cooling could also be considered a "removing heat" process.

To cool materials in the lab, you need to select cooling materials that are sufficiently cold and have the heat capacity to remove the necessary thermal energy. The following subsections describe techniques used to make things cold in the laboratory.

6.2.2 Room Temperature Tap Water (=20°C)

Water is used as a coolant in condensers and diffusion pumps. For simple exothermic reactions, you can use water as a heat sink by placing glassware (containing a reaction) into a water bath. Alternatively, in an exothermic reaction, you can also use a cold finger of running water to prevent overheating. However, all these examples can only cool to the temperature of tap water (usually room temperature). For every step below room temperature, more equipment and money are required.

6.2.3 Ice (0 C)

Many labs have freezers or ice machines to make ice.* If the ice is in cubes or blocks, it will be necessary to smash it into more usable, smaller, crushed pieces. The ice should be placed in some kind of cloth bag (or the like) during smashing, to prevent the ice pieces from flying around the room. Be sure to wear safety glasses any time you are smashing anything—and watch the fingers!

6.2.4 Ice with Salts (0 C to -96.3 C)

Various salts can cause freezing point depressions. These depressions are the results of the ions' colligative properties within solution. A significant freezing point depression is created not by any particular type of material, but rather by the number of particles you have in solution. The effects can be enhanced by achieving a supersaturation of material. For example, if you mix ice with a salt such as NaCl, you will end up with two particles within solution (Na+ + Cl") (the temperature of ice water supersaturated with sodium chloride (23% by weight) is - 20.67°C). If you place a salt such as CaCl2- H2O into ice, you will end up with three particles within the solution (Ca+ + 2C1~) [the temperature of ice water

*If you constantly lose the scoop in the ice, tie one end of a string or cord on the scoop and tie the other end on the ice machine.

important reason to never place liquid nitrogen in a noninsulated container is that someone may inadvertently try to pick up the container and severely burn his or her hands.

6.2.7 Slush Baths (+13° to -160 C)

A slush bath can be described as a low-melting-point liquid (typically a hydrocarbon solvent) that is being kept in a partially frozen state by either liquid nitrogen or dry ice. The temperature will remain constant as long as you continue to add liquid nitrogen, or dry ice, to the bath to maintain its "slushy" state. Table 6-3 is a comprehensive list of slush baths made of dry ice (CO2) and liquid nitrogen (N2). Duplicate temperatures indicate a choice of solvent or coolant.

To make a slush bath, pour the selected low-melting temperature liquid into a Dewar, then pour the coolant in while stirring briskly. A wooden dowel is wonderful for stirring because it will not scratch the Dewar's surface. There is no concern for contamination from the dowel because it is not likely to affect the performance of the slush bath.

During the mixing process, tremendous amounts of solvent fumes are likely to be given off along with condensed water from the air. Therefore, the original preparation of a slush bath should be done in a fume hood. Once the slush bath begins to reach equilibrium, the amount of vapors leaving the Dewar decreases and it is safe to remove the bath from the fume hood.

To make a liquid nitrogen slush bath, first pour the desired low-melting temperature liquid into a Dewar. Then slowly, while constantly stirring, add the liquid nitrogen until the desired consistency is achieved. Different low-melting liquids will have different viscosities. It is therefore desirable to know the potential viscosity limits of the slush bath you are using. This information is best retained by asking someone; otherwise, make a test slush bath before you set yourself up for an actual experiment or process. Knowing the viscosity of a certain slush bath is important, otherwise you may expect one slush bath to be as thick as another, and you might add an excess of liquid nitrogen. If you add too much liquid nitrogen, it will become the predominant cooling medium and the resultant temperature will be cooler than listed in Table 6.3. However, as the liquid nitrogen boils off, the temperature will settle to the listed temperature.

To make a dry ice slush bath, be sure that your low-melting-temperature liquid has a freezing point above -78°C. Follow the same procedure as with the liquid nitrogen bath above, but use crushed dry ice. When crushing dry ice, use a hammer (not a pair of pliers or a wrench) and place the dry ice in a cloth bag so the pieces do not fly around the room. Always handle dry ice with thermal gloves or tongs, and never handle dry ice with your bare hands or you might severely burn your hands and fingers. The primary advantage of using dry ice over liquid nitrogen is that it is less expensive, safer to work with, and more readily available. On the other hand, it is more difficult to work with. Similar to liquid nitrogen, if you add too much dry ice, it becomes the predominant cooling medium and the result-

ant temperature will be cooler than listed in Table 6.3. As the dry ice evaporates, the temperature will settle to the listed temperature.

By combining mixtures of organic solvents, it is possible to achieve temperatures that you may not otherwise be able to obtain because of lack of material or to avoid other materials. For example, with various combinations of orthoand meta-xylene and dry ice, you can achieve temperatures from -29°C to -72°C (see Table 6-1).

Regardless of which coolants or low-melting-point liquids are used to make a slush bath, no slush bath combination can be a perpetual (static) temperature system. Rather, a slush bath is a dynamic collection of materials that are either settling or boiling off. Thus, they require constant monitoring with (preferably) two liquid-in-glass thermometers* (or a thermocouple probe), consistent agitation, and occasional replenishing of coolant.

An alternative to the slush bath is the coolant, or cooling bath. These baths are handy when you may not have (or wish to use) the required low-melting-tempera- ture liquid for a particular temperature. However, they require more work to maintain their specific temperatures. Liquid nitrogen is recommended for cooling baths because dry ice can be difficult to introduce to the bath in sufficiently small amounts. Like the slush bath, the cooling bath should be mixed in a fume hood.

The cooling bath differs from the slush bath because less coolant is used during initial mixing. Thus, the mixture never obtains the slushy state of the slush bath. The resultant bath temperature is warmer than the coolant, and by varying the amount of coolant you can vary the temperature. The trick is to select a low-melt- ing-point liquid that is sufficiently low to provide a wide working temperature range. One good cooling bath liquid is methanol, which has a freezing temperature of -98°C, is reasonably safe to use,f and can easily be used for cooling baths. Another reasonably safe, low- melting-point liquid is petroleum ether (30°-60°) which has a freezing point of approximately -120°C, and can thereby provide a greater range of low temperatures.

The difficulty involved with cooling baths is that you control the temperature by varying the amount of coolant in the mix. This control requires constant attention and the slow but constant addition of more coolant as the bath continues to warm during use. Despite the extra labor, many people prefer cooling baths because there is a greater choice of safe solvents. In addition, fewer solvents must be stored to obtain a wide range of temperatures.

By using two liquid-in-glass thermometers, you can verify the quality of both thermometers by their agreement in temperature readings. If the temperatures do not agree, one of the thermometers may have a bubble in the stem or some other defect. Unfortunately, this trick does not let you know which is the defective one, but it provides a clue to the problem.

+ Methanol is flammable and poisonous.