An indispensible tool of the lab

The lab burner (sometimes referred to, and often erroneously so, as the Bunsen burner) is an indispensible tool for doing general chemistry lab work. It can be used to heat samples with high or low heat, for sterilization, to bend glass tubing or rods, or for other purposes.

Using a lab burner safely       Click here

Anatomy of a Tirrill burner

The diagram below illustrates the construction and operation of a typical Tirrill burner. It consists of:

• Base – The base stabilizes the burner (we don't want it to tip over) and provides a channel for the gas to pass from the gas line to the burner jet.


• Gas input nipple – This is a grippy, self-sealing fitting over which a rubber gas-supply hose is slipped.


• Gas jet – The gas jet is a small opening at the bottom of the barrel through which the combustible gas passes. Flow of gas through it can be controlled using a needle valve that is adjusted by rotating a wheel at the bottom of the burner inside the base.

Combustion of propane

The combustion (burning) reaction that occurs in some Tirrill burners is the combustion of propane, shown below. Which gas is available in a laboratory is mostly a function of location and supply. Different regions have different gas supplies and suppliers. Some are mostly methane, some mostly propane, and some a combination of simple hydrocarbon (containing only carbon and hydrogen) gases. The reaction is:

In this reaction each propane (C₃H₈) molecule combines with 5 oxygen (O₂) molecules to produce 3 molecules of carbon dioxide (CO₂) and four molecules of water. The water (and all other reactants/products) is in the gas phase.

Flame adjustments: Too much gas/too little air

Adjustments of the gas and air flow can lead to conditions in which there is not enough air available to efficiently burn all of the gas moving through the burner. In these cases we get a disorganized flame like the one shown. Often such a flame emits yellow/orange colors at the top. This is a sign of incomplete combustion.

This can easily be shown by holding a clean white ceramic evaporating dish or crucible over the top of the flame (in the yellow area) for a minute or so. Black soot will accumulate on the white surface, a sign that not all of the carbon in the hydrocarbon gas has been converted to CO₂.

Soot like that can be burned back off by holding the ceramic in a well-adjusted (hot) flame.

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Flame adjustments: Too much air

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A good flame

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Flame temperature zones

Not all regions of a lab-burner flame are the same. A good flame has two main regions of visible difference, the outer cone, usually violet-blue in color, and a bright, light blue inner cone.

That inner cone is the hottest part of the flame, and it's seen in many different kinds of flames, including a candle flame (though it's cooler and not blue) and a gas welding torch, the inner cone of which can be very bright and sharp.

The base of the inner cone is the coolest part of the flame, generally measuring about 300˚C. Here gases are well-mixed but incompletely combusted. The combustion process is just getting going in this flowing-gas system.

The tip of the inner cone is usually the hottest region of the flame, measuring over 1500˚C, whle the region above it, in the outer flame, is usually slightly cooler. In these regions, in a well-regulated flame, the combustion reaction is proceeding completely.

So to heat something as hot as possible, placing it at the tip of the inner cone is the trick. For less heat, move the sample above the outer flame where the heat has spread out or dissipated.

Safety

Lab burners are an open flame in the lab. You can easily burn yourself or someone else, either directly or indirectly (e.g. by touching something that has been heated), or catch items or people on fire. Here are the essential safety rules of lab burner use:

1. Inspect your lab burner before every use. Look at the burner, hose and gas tap. Check for leaks, cracks and obvious damage. Make sure that both connections are tight and secure.


2. Make sure that everyone else in the lab is aware that you'll be using a burner for a while.


3. Never leave a lit burner unattended. If you leave for any reason, shut the gas off first.


4. Remover all flammable material from the vicinity of the burner. Create a safety zone of about 0.5 m or more around the burner. Keep clear of overhead structures like shelves.