1.4D: Bunsen Burners

Bunsen burners are generally used to rapidly heat high-boiling liquids with low flammability (such as water). Safety note: It is important to know that they can reach temperatures of approximately \(1500^\text{o} \text{C}\),\(^5\) and can easily ignite most organic compounds. If an apparatus is improperly set up, or if there is a small gap that allows organic vapors to escape from an apparatus, these vapors can ignite with a burner. Therefore, it is generally recommended to use other heat sources to warm flammable organic liquids (for example in distillation or reflux). Bunsen burners should never be used with highly flammable solvents such as diethyl ether.

However, burners do have their place in the organic lab. Burners are often used in steam distillation (Figure 1.44a) as the vapors are generally not flammable. In this context, a wire mesh set atop a ring clamp is often used under the flask to dissipate the heat and avoid overheating one area. Burners are also used in the Beilsten test for halogens (Figure 1.44b), with Thiele tubes in melting and boiling point determinations (Figure 1.44c), and for softening pipettes to create capillary TLC spotters (Figure 1.44d). They may also be used in sublimations.

Figure 1.44: Uses of Bunsen burners in: a) Steam distillation, b) Beilsten test, c) Thiele tube, d) Softening pipettes.

Burners come in several different forms. The common Bunsen burner is six inches tall and has two models differing in how the gas and air are adjusted (a Bunsen burner is in Figure 1.45a, and a Tirrill burner is in Figure 1.45b). Small burners (microburners, Figure 1.45c) and large burners (Meker burners, Figure 1.45d) are also sometimes used.

Figure 1.45: a) Bunsen burner, b) Tirrill burner, c) Microburner, d) Meker burner.

Figure 1.46: Air valves and closed/partially opened positions on two styles of Bunsen burner: a+b) Tirrill burner, c+d) Bunsen burner.

To light a burner

1. Connect the rubber tubing on the burner to the gas line on the benchtop.
2. Open the gas valve on the burner one "turn" from closed, by either turning the gold arm on a Tirrill burner (Figure 1.46a) or notched dial near the bottom of a Bunsen burner (Figure 1.46c).
3. Open the air valve slightly so that a small opening is observed in the slats or on the screw portion of the burner (Figure 1.46 b+d).
4. Open the gas valve on the benchtop until a faint hiss of gas can be heard, then use a striker to create a spark and light the burner. If matches are instead used, first light the match and then turn on the gas. If the burner fails to light, there is either too much or too little of either gas or air. Try adjusting both and observe the effect.
5. Once the burner is lit, adjust the gas and air until a blue triangular flame appears (a "blue cone", Figure 1.47d). The flame should be 1-2 inches high and accompanied with an audible hissing of the flame. An orange flame (Figure 1.47b) forms when there is incomplete combustion of the fuel, is cooler than a blue flame, and if used to heat glassware will deposit black charcoal onto the glass. To convert an orange flame into a blue conic flame, allow more air into the burner. The tip of the blue cone is the hottest part of the flame.

Figure 1.47: a) Lighting a burner with a striker, b)Too orange of a flame, c) No blue cone, d) Properly lit burner.

\(^5\)As reported in the Fischer Scientific catalog.