tion becomes superheated before boiling begins, the test tube can act like a cannon, shooting its hot contents up to 20 feet.

4. Heat along the bottom side gradually moving the test tube up and down the length of the solution (see Fig. 6.2).

It is preferable not to heat a beaker or Erlenmeyer flask with Fisher or Bunsen burners. The reason is discussed in detail in Sec. 1.1.10, which discuses the structural shapes of flasks. In essence, the problem is that the sharp angle of the side walls may not be able to disperse the thermal strains created by the expanding heated bottom. Round-bottom flasks, on the other hand, have no side walls and therefore do not have this problem.

There are occasions where you will need to heat materials in a beaker because no other means of heating are available. In these occasions, do not let the flame come in direct contact with the glass. Rather, place the beaker on a wire screen or on a wire screen with a ceramic square.* Even though the screen will prevent a direct concentration of heat at any one point on the beaker, use a soft blue flame, not an intense "hissy" flame.

6.1.4 Steam

Steam is one of the safest and easiest techniques to provide even, uniform heat in the lab. Because steam can provide an even heat distributed uniformly across the entire bottom of a container, there are no fears of hot spots or solution superheating. In addition, the steam bathf is made up of simple components with no electrical wires or moving parts. Thus, there is nothing to wear out or to be damaged in a steam bath by accidental spills of heated materials.

Although steam can technically be any temperature greater than 100°C, under normal conditions the user can consider the temperature to be about 100° C. Superheated steam (>250°C) is possible, but special insulation and equipment is required.

Regardless of relative safety, caution with the use and handling of steam is still required. Careful monitoring of the equipment for the duration of the experiment or process is still important. From a safety aspect, there are two things to keep in mind about steam:

Some labs may still have wire screens with asbestos squares attached. These screens should be placed in plastic bags and disposed of with the proper agencies. There are replacement screens available that use a ceramic material and are not hazardous.

f As opposed to the steam source.

Condensed

water out

Nested lids

Fig. 6.3 The steam bath with nested lids.

These two facts point out one of the biggest dangers of steam: What you cannot see can hurt you. Caution is advised.

If you are fortunate enough to work in a facility that has self-generated steam, you can easily attach one end of a hose to the outlet and the other end to the steam bath or other steam delivery device. Be sure to use heavy-duty flexible tubing to withstand the high heat of the steam, and use as short a piece of tubing as possible so the steam will not condense before it gets to the steam pot.

Your delivery system is likely to be a steam pot (also called a steam bath) (see Fig. 6.3). Steam pots may be made out of copper, brass, or cast aluminum. One of the benefits of using steam for heating is that the steam bath is designed to accommodate a wide variety of flask sizes; all that is necessary is to select a nested lid that is just smaller than the size of your flask. The lid should be selected before the steam valve is opened. Otherwise the container will become too hot to handle safely, and further setup will become more difficult and dangerous. If you need to change a nested lid after it has been heated, be sure to use tweezers or tongs.

All steam pots have one tube allowing steam in and have another allowing condensed water out. Be sure that neither tube kinks or plugs up. A constricted steam tube may burst due to steam pressure. A constricted drainage tube will cause the pot to fill up with water, which will halt the progress of your work. If the tube continuously kinks, replace it with a newer and/or heavier-walled tube.

If you do not have a built-in steam line, you can create your own steam. Boiling water over an open flame, on a hot plate, or using an immersion heater are all adequate techniques.

Very elaborate steam apparatus configurations have been written up by Lane et al.,1 for superheated steam distillations. These superheated steam setups are fairly elaborate pieces of apparatus which combine steam generation and distillation in one piece. They are therefore not setups that can be pieced together in an afternoon. A more simple design of a steam generator has been proposed by Hagen and Barton.2 It is a fairly inexpensive, yet robust design.

Whichever steam generator design you select, it must be continuously monitored. If a steam generator runs out of water, the unit may be damaged, and the experiment or process that uses the steam will stop.

The only water you should use in an electrically heated steam generator is deionized water to decrease (and/or limit) the corrosion of internal parts caused by

294 High and Low Temperature

water electrolysis. In addition, as salt concentration goes up,current conduction increases, which may lead topower failure.

Steam units in general, and electrical steam generators in particular, should always be placed away from easy access to limit thechances of accidental burns or unintended electrical contact.

6.1.5 Thermal Radiation

Thermal radiation is provided byheat lamps which emit large amounts of infrared light. Besides keeping food warm at smorgasbord-type restaurants, heat lamps can be used to warm and/or dry samples.

Although heat lamps cannot provide great amounts ofheat, they have the advantage of being able towork at a distance. Because heat lamps donotrequire contact or airto heat materials, they can beused to heat materials in a vacuum.

Do notstare at the light of a heat lamp because it can burn your retina. Similarly, do notleave your hands exposed to its light for more than a fewseconds at a time because it will sunburn and damage your skin.

6.1.6 Transfer of Energy

By using thepart of theelectromagnetic spectrum between 0.1 and 10cm, more commonly known asmicrowaves, energy can betransferred from waves to matter. This is particularly effective to materials that are considered opaque to microwaves. By blocking a microwave, energy is absorbed andthe material is heated. Water and fat areboth considered opaque to microwaves. Materials that aretransparent do notabsorb anyenergy andtherefore do notheat upunder theeffects of microwaves. Ceramic and glass materials are considered transparent and do not (appreciably) heat upunder microwave bombardment. However, as inmost things in life, this is not a black and white issue because nothing is totally opaque or transparent. Both glass and ceramic materials doheat up in a microwave oven, but not to thedegree that materials that areconsidered opaque.

There have been a variety of uses of microwave ovens in thelaboratory; these include the processing of ceramics, development of various plastics, catalysis, and acid dissolution of various materials.3 One developing useof microwave ovens is the drying of samples for weighing.

The process of drying samples, which used to take some oneto twohours in a drying oven, canbebrought down to about five to tenminutes in a standard inexpensive kitchen type of microwave oven. Thetechnique is to place the weighing bottle and cover in a clean beaker. Place three glass rods that have been bent to an angle of about 45° over the edge ofthe beaker to support a watch glass. The watch glass is used to ensure that nothing falls into the beaker, and the glass rods are used to allow water vapor to escape.

The exact time required for drying of samples needs to be determined for each type of microwave oven being used due to the varying power (watts) of each type. It was demonstrated that it is possible toinadvertently melt borosilicate glass ina

microwave oven7 if it is kept on too long and if there is any inadvertent metal deposits on the glass.* Since all glassware has potential contact with metal during its production, all glass has the potential of melting in a microwave. It is, however, an uncommon occurrence even during extended microwave heating sessions. The amount of time required to sufficiently dry a sample is considerably shorter than the time required to bring the glass to potential melting.^

The standard kitchen microwave oven has low, medium, and high settings, but these do not indicate the level of intensity of the electromagnetic beam. Rather, it creates cycle times for the beam to be on or off. Because hot areas are more opaque than cold areas, hot areas get hotter than cold areas. The level settings are used to heat areas of the material being heated, then cycle off so conduction can heat colder sections. As the oven cycles on and off, a more uniform heating can be created. It is unnecessary to use anything but the highest setting when drying samples, although when drying large samples, it may be prudent to use a lower setting.

6.1.7 Hot Air Guns

Hot air guns are used to provide isolated, directed heat to individual targets. This method may be used to heat an individual sample on a vacuum line or to soften the grease on a stopcock.* Although one could use a standard hair dryer, it cannot provide the control or endurance of an industrial-strength hot air gun.

There are two types of hot air guns: those that have a fan and those that do not. The obvious advantage of those with fans is that they are self-contained. The disadvantage is that the fan motor can create sparks, which could be hazardous depending on ambient gas conditions. The disadvantages of the motorless (no fan) design: You must have access to pressurized air, and the unit must have both a power cord and air hose for operation.

Motorized air guns allow extra air to enter the side of unit through an air gate to vary the amount of extra air that can enter the air gun. The less air that can enter the side, the hotter the air exhaust. A hot air gun can produce extensive heat which can easily peel paint or crack a window pane. By concentrating the heat, from a hot air gun on a glass vacuum line, it is possible to have the glass walls be sucked in by atmospheric pressure.

* This contamination can come from tweezers that are used to hold the glass rod while they are being bent into the required shape to the tools that are used to shape the weighing bottle.

+ The reason the melting occurred in the first place is that the original ovens from which the drying times were determined were half as powerful as newer ovens that were later obtained. Unfortunately, power was not considered and the original drying times were used.

*Any small-sized stopcock that requires heat to turn should be removed, cleaned, and regreased at the first opportunity. Large stopcocks may require a hot air gun for all rotation. Because heat ages stopcock grease faster than normal, the grease should be replaced two to three times more often than the other stopcock grease.