Bunsen burner

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A Bunsen burner is a common piece of laboratory equipment used for heating, sterilization, and combustion.

[edit] History

A common misconception is that the Bunsen burner was invented by German chemist Robert Wilhelm Bunsen and Christian Schonbein. Although it is named after him, it is actually an improvement made in 1855 by his laboratory assistant, Peter Desaga, on an earlier design by Michael Faraday. The improvement is called a Tirrill Burner. The main differences between these burners is the gas control valve on a Tirrill Burner and the improved structure of the Tirrill Burner. Most Bunsen burners are in fact Tirrill Burners.

[edit] Operation

The device safely burns a continuous stream of a flammable gas such as natural gas (which is principally methane) or a liquified petroleum gas such as propane, butane, or a mixture of both. At the time of its invention, the Bunsen burner would have mostly burnt coal gas.

The burner has a weighted base with a connector for a gas line (hose barb) and a vertical tube (barrel) rising from it. The hose barb is connected to a gas nozzle on the lab bench with rubber tubing. Most lab benches are equipped with multiple gas nozzles connected to a central gas source, as well as vacuum, nitrogen, and steam nozzles. The gas then flows up through the base through a small hole at the bottom of the barrel and is directed upward. There are open slots in the side of the tube bottom to admit air into the stream via the Venturi effect, and the gas burns at the top of the tube once ignited by a flame or spark. The most common methods of lighting the burner are using a match or a spark lighter.

The amount of air (or rather oxygen) mixed with the gas stream affects the completeness of the combustion reaction. Less air yields an incomplete and thus cooler reaction, while a gas stream well mixed with air provides oxygen in an equimolar amount and thus a complete and hotter reaction. The air flow can be controlled by opening or closing the slot openings at the base of the barrel, similar in function to a car's carburetor.

If the collar at the bottom of the tube is adjusted so more air can mix with the gas before combustion, the flame will burn hotter, appearing blue as a result. If the holes are closed, the gas will only mix with ambient air at the point of combustion, that is, only after it has exited the tube at the top. This reduced mixing produces an incomplete reaction, producing a cooler but brighter yellow which is often called the "safety flame". The yellow flame is luminous due to small soot particles in the flame which are heated to incandescence. When the burner is regulated to produce a hot, blue flame it can be nearly invisible against some backgrounds. Increasing the amount of fuel gas flow through the tube by opening the needle valve will of course increase the size of the flame. However, unless the airflow is adjusted as well, the flame temperature will decrease because an increased amount of gas is now mixed with the same amount of air, starving the flame of oxygen.

[edit] Fall into disuse

Bunsen burners have largely been supplanted by hot plates, heating mantles, and other similar electric heating elements as sources of heat in laboratories. Hot plates provide several advantages over bunsen burners. First, electric heating elements are much safer, producing no fumes and posing no risk of explosion. Second, a hot plate, especially in conjunction with a sand bath, provides much more even heating than a bunsen burner. Even heating is important for reducing both the duration and side products of a reaction. Third, a hot plate has a much larger operable heating range than a bunsen burner: a hot plate can be used to boil water or merely to keep things warm, while a bunsen burner is limited to the combustion temperature of the fuel gas. Finally, a bunsen burner gives off byproducts that could taint a reaction, whereas a hot plate provides clean heat. These emissions are negligible for a blue flame (which gives off only carbon dioxide and water), but can be quite apparent with a yellow flame (which gives off carbon monoxide and particulates, coating the bottom of glassware with black soot that is difficult to scrub). However, the bunsen burner still heats things much more quickly than a hot plate and is still useful in sterilization (especially in sterilizing the wire probes used to culture petri dishes) and in flame tests.

For safety, bunsen burners should be used with a heatproof mat.

[edit] Regulatory Standards and Guidelines

In the US, bunsen burners must be connected using listed connectors in compliance with ANSI Z21.24 Standard for Connectors for Gas Appliances.