If you have pressurized air in your lab, you can use a motorless hot air gun. These units aresmaller than standard hotairguns andaretypically about thesize of small home hair dryers. Motorless hot air guns only require heating filaments because they donothave fans. To use one, a flexible tube is attached to an airsupply andthe hot air gun. If your labdoes nothave plumbed-in compressed air, or the location of your outlet is toofar away from where you want to usethehot air gun, another option is touse a compressed gas tank of air. Do not use compressed oxygen or anyflammable gasfor the air supply. You could usean inert gas, such as nitrogen foryour airsupply, butit would becostly.

Adjust the outgoing air pressure to between five to ten pounds of pressure. A needle valve on theregulator is required to regulate theamount of airpassing the heating elements. Just like a built-in fanheater, theless aircoming outof the regulator, the hotter the airout of the hot air gun, and vice versa. If the airsupply runs out, yourun therisk of burning theheating element. Always turn off the heating element before turning off the air supply. Otherwise, yourun therisk of burning the heating element.

Probably the most common piece of worn or damaged equipment used in the laboratory is a hotairgun's electrical cord. These cords arefrequently frayed due to their heavy use. Electrical cords should always bereplaced as soon as there are any signs of fraying, and hot airguns deserve noexception.

6.1.8 Electrical Resistance Heating

One of the most versatile and robust heating techniques is the use of electrical resistance heaters. They are used in as varied heating approaches as hot plates, mantles, heat strips, immersion heaters, and even blankets.

Although some electrical heating units areself-contained with their own electrical controllers (such as hotplates), many require separate controllers. If a heating unit hasno apparent built-in mechanism with which to vary temperature, theunit will require a separate controller.*

Some electrical heating units that require controllers will have standard electrical plugs to connect into to controllers. Despite thefact that you can, do notplug these units directly into wall outlets. Your heat demands are likely to be far less than full electrical power, and you can only receive full power from a wall socket. Currently, more and more electrical heating units that require electrical controllers have special plugs that can only gointo similarlyequipped controllers. Before purchasing a heating unit, verify with thesupplier or manufacturer what type offitting the unit is equipped with and whether it will becompatible with your existing equipment.

The controller is nothing more than anelectrical rheostat that can vary theamount ofelectrical current the wires receive. This rheostat in turn also varies the amount of heat the wires can deliver. Some controllers have built-in (or external) thermocouples that enable thecontroller to maintain a specific temperature.

Some heating units such as mantles and hot plates can use magnetic stirring bars which are magnetized rods covered in Teflon. Stirring bars can provide two functions. They can stir solutions to mix different materials together and assist the uniform and efficient heating of solutions. In addition, a stirring bar creates a vortex within a solution which provides a sharp point from which boiling can occur. Thus, using a stirring bar eliminates the need for boiling chips.

Magnetic stirring bars can be stored together, but they should be placed in a clean, dry, and smooth container. The clean and dry requirements are obvious, but smooth is equally important. A rough container (or dirt in the container) is likely to scratch the Teflon. Any scratches on the Teflon may trap dirt or chemicals which may affect future work. If you think that you will be unable to otherwise maintain a dirt-free container, consider keeping the tubes that the magnetic stirring bars are shipped in for storage.

Never rotate a spinning bar too fast because it is likely to cause splashing. Likewise, place the center of a flask over the center of the controller. Otherwise the spinning bar is likely to flip around like a wounded fish, causing a lot of splashing.

Electrical Heating Tapes. These units always require separate controllers. They come in various widths and lengths and are always flexible. Depending on the insulating materials, they have upper heating temperature limits from 260°C to 760°C. Although the insulation may be resistant to chemical spills, the wires within may not. Sloppiness is therefore likely to be hazardous. Some of the insulations available should not be used against metal surfaces, so if you need to heat a stainless steel container, select suitable insulation.

Electric Heating Mantles. These mantles are like a pair of cupped hands and are designed to envelop individual round-bottom flasks. Generally, each mantle size is designed to heat one size of flask, although there are a few mantle designs that can hold a variety of flask sizes and other glassware styles. Because the mantle securely supports the flask, it cannot tip or slide out of the heater. Like steam heating, the mantle provides heat equally along the entire lower surface of a flask. Some mantle designs provide insulation that completely covers an entire flask. This provides a uniform temperature throughout the flask preventing heat gradients within the solution and decreasing heat loss. Although the costs for separate mantles for each size flask can add up, mantles provide the best (and most uniform) heating for long periods of monitored, or unmonitored, usage.

Mantles, like electric hot plates, may have built-in controllers, some of which have thermostatic devices to set and control specific temperatures. In addition, some mantles have magnetic stirring mechanisms.

Similar to heating tapes, the insulation on mantles may be resistant to chemical spills but the wires within may not. Sloppiness is therefore potentially hazardous. If you have a mantle with a drainage hole on the bottom, be sure to provide some sort of collection basin underneath. Otherwise, chemicals may spill onto your lab bench.

Immersion Heaters. These devices are lowered directly into a solution for heating. Because the heaters come into direct contact with their solutions, the major

cause the bottom of the container to break by thermal shock. One way to reduce the amount of thermal shock when transferring a container on or off the hot plate is to place it on something with less heat capacity, such as a wire screen.

Hot plates are also used to heat one type of material so that a second may be heated. These secondary heaters may include water baths, oil baths, sand traps, or aluminum plates. Water and oil baths are typically used to heat a drying flask on vacuum evaporators. Oil baths are more messy and require special clean up, but the oil (typically a silicone oil) will not evaporate during drying processes that can take up to several hours. Stronski came up with a very simple and easy solution to limit water evaporation. Stronski8 recommended that the user float Styrofoam popcorn or chips up to three inches deep on water. This suggestion not only limited evaporation, but extended the maximum controlled temperature range.

The advent of various brands of microware laboratory equipment (originally developed by Mayo, Pike, and Butcher of Bowdoin College ) created a problem of how to heat these new, smaller flasks. Two techniques have developed. The first involves placing the microflasks into sand. The advantage of sand is that it can easily fill into all the external nooks and crannies of any container and provide excellent, evenly distributed heat. The disadvantage of sand is that it is a poor heat conductor and can thus require a long time to be heated to a satisfactory temperature. In addition, once you remove a flask, it is not easy to reinsert a new flask into the "filled-in" hole.

An alternative approach is a solid aluminum block. This idea was developed jointly by Siegfried Lodwig of Centralla College and Steve Ware of Chemglass, Inc. Aluminum conducts heat excellently and can be easily machined to accommodate different sized flasks. The success of aluminum heat blocks caused Lodwig to use them also for a variety of other heating techniques that had nothing to do with microware.

Because a variety of individual holes may be drilled into a single aluminum block, it is possible to easily rotate different-sized flasks on a random basis with ease and have separate holes for thermometers as well. The thermometer allows you to easily monitor temperatures for various thermostat settings. By graphing these temperatures settings, one can easily set a temperature on a Variac as easily as setting a temperature on a stove.

Drying Ovens. These units are used to facilitate the drying and storage of glassware that needs to be dry and ready at a moment's notice. Drying ovens are also used to dry samples for weighing.

Glassware should never be wiped dry because the wiping process could easily contaminate the surfaces. There is no reason to dry glassware that will be getting wet immediately after washing. Similarly, there is no reason to place items in a drying oven that could otherwise drip dry. Glass typically has a certain amount of adsorbed water at all times. The only way to reduce this water is with heat and/or vacuum. The drying oven provides limited assistance for the former and will not only help to remove some of this water, but if the container is left in the oven, will help keep the water from returning as long as the glassware remains in the oven.