the tub. The latter is simply not allowing enough air in the region to support combustion. A simple step-by-step process is as follows:

1.Be sure that any valves from the gas and oxygen supplies to the torch are open and that both gases are available.

2.Open the torch's gas and then oxygen valves for a second or two to purge any air that might be in the lines, then close them. There is a definable noise change when air has passed through the torch and gas has arrived. In addition, you can smell the gas. Neither of these aids are available with oxygen.

3.If you are using a match or lighter, first start the flame on the match (or the lighter), hold it near the tip of the torch, and then slowly open the gas (red) needle valve until you have a threeto four-inch flame.

4.If you are using a striker, slowly open the gas (red) needle valve about 1/ 4 to 1/3 of one revolution, then strike the striker several inches from the tip of the torch. Once the flame has ignited, set the size of the flame to three to four inches.

5.Slowly open the oxygen (green) needle valve until the flame is lightly hissing. If the flame is flickering and jumping around, increase the oxygen. If the flame is very thin and hissing loudly, decrease the amount of oxygen.

Note: You control the size of the flame with the gas and the character of the flame with the oxygen. The character of a flame refers to its shape of either a coolbushy flame (not enough oxygen) or hot-hissy flame (too much oxygen). If you make a major change of flame size, you will probably need to readjust the oxygen for the new flame size. Minor changes of flame size will maintain the same character.

8.1.3 How to Prevent a Premix Torch from Popping

Popping is caused when the flow rate of the gas leaving the tip is slower than the flow rate of the flame burning its way back into the tip. Once the flame enters the tip, it can ignite the gas and oxygen within (the tip) in an uncontrolled manner, resulting in an explosion and a loud "pop."

The chance of popping a torch can be decreased by making sure that you always have a "hissing" to soft flame especially when decreasing the rate of oxygen or gas. A small, light fluttering flame is more likely to pop. Although annoying, the popping is not likely to do any damage to the torch, it is possible for a major pop to blow the flexible tubing off the torch's hose connections.

If a torch pops, a flame may continue to burn inside the torch, although this situation is not likely (a light whistling noise indicates that this condition exists). To extinguish an internal flame, first turn off the gas and oxygen needle valves on the torch. Then turn the oxygen valve forcefully open for a moment and close it. Do

482 The Gas-Oxygen Torch

not maintain the oxygen on full with the gas valve open. The oxygen pressure from the compressed oxygen tank is more than likely greater than city gas pressure, and maintaining the oxygen in an open position for an extended time can force oxygen up the gas line. This can cause a major pop with dangerous potential consequences.

Finally, relight the torch and continue. Although they are not necessarily common occurrences, experience will help you avoid "pops."

Because of the propensity for the premix torch to "pop," there is a recommended procedure for shutting the torch off. Even if you are capable of controlling the gas flow so proficiently that you never pop your torch, this procedure is still recommended.

1.Open the oxygen needle valve (CCW) fast and, just as fast, close the valve (CW) to increase the flow rate of the oxygen and blow the flame out. That is, once the flame is blown out, turn the oxygen off. Do not set up a condition where you are forcing oxygen up the gas line or you will potentially set up a large pop.

2.Turn off the oxygen.

3.Turn off the gas.

4.Turn off the source of the gas.

5.Turn off the oxygen tank.

Note: It is better to blow the flame out, and then turn off the gases than to turn off the gases before the flame is out to reduce the chance of the torch "popping."

The surface mix torch does not require special shut-off procedures because the surface-mix torch cannot "pop." Therefore, it does not make a difference which gas you turn off first. There is also no reason to blow out the flame of a surfacemix torch—besides, you cannot.

8.2Using the Gas-Oxygen Torch

8.2.1Uses for the Gas-Oxygen Torch in the Lab

The most common use for the gas-oxygen torch in the laboratory is to work on glass, and as anyone who has ever played with glass has found out, glass is not very forgiving. There are, however, a few operations that can be mastered by a non-glassblower, and "tipping off' (sealing off) a sample in an NMR tube is one of them.

It is also possible for a beginner to fire-polish laboratory glassware that has chipped or cracked edges. However, be forewarned: Any actions on broken or damaged glassware should be done with an "it is lost anyway" attitude. When inexperienced technicians try their hand at glassblowing, they learn fast that it's

484 The Gas-Oxygen Torch

glass will drip when gravity on the glass is stronger than the surface tension forces that hold it together.

There are two additional forces involved when tipping off a sample: gas pressure buildup and vacuum from a vacuum system. Positive pressure is caused by inadvertently heating a sample (causing a pressure buildup), which makes it very difficult to seal off the sample. The possibility of a "glass aneurysm" or a rupture of the glass wall is likely from a positive pressure buildup (see Fig. 8.4, Samples A and B). On the other hand, if the sample is prepared on a vacuum line, and too much heating is done on one spot for too long, the tube will burst in rather than out (see Fig. 8.4, Sample C). Not only may this bursting destroy the sample, but allowing air into a vacuum line can damage or destroy parts or sections of the vacuum system.

Whether a sample tube is attached to a vacuum line or not, the easiest way to prevent a positive buildup of pressure is to freeze the sample with a slush bath or liquid nitrogen (depending on the solution's freezing temperature or what coolants you have available). By freezing the sample solution, you accomplish three objectives: One, you protect the sample by protecting it from the heat of the flame; two, the sample is not likely to evaporate and thereby cause a buildup of pressure (and cause a pressure burst as shown in Fig. 8.4, Samples A and B); and three, the freezing will make a cryogenic pump that facilitates the tipping off process by creating a vacuum. The sample must remain frozen during the entire tip- ping-off procedure, so do not remove it from the coolant until the tipping off process is complete. A side benefit of the sample being frozen is that if there is a poor tip-off and air gets into the sample tube, with the sample being frozen, it is not likely to react with the air. If this occurs, keep the sample in the coolant until you are able to transfer it to another container. If you are using liquid nitrogen, you are likely to freeze some oxygen in the container, but it will be a liquid on top of the frozen sample. This can be pored off into a beaker at any time, to let it evaporate.

Figure 8.5 shows two examples of a tipped-off tube: Sample A is what you want, with the tipped-off section concentrically and linearly straight. Sample B is not acceptable because the tipped-off area is bent off from center. Because of this lopsided condition, the tube will not spin properly in an NMR machine.

Glass is a very poor conductor of heat, and therefore it is not possible to heat one side of a tube to be tipped off and expect to be finished. The only way to properly tip-off a sample tube is to heat it uniformly in the area to be tipped—that is, one side and then 180° to that area (see Fig. 8-6). The gradual heating is done by alternately heating opposite sides in a frequent and uniform manner. You must try to heat the entire area gradually and uniformly. Glass becomes soft before it becomes juicy. If one side becomes juicy too fast, there is a greater chance for the wall to suck in or blow out.

Whenever possible, before work begins, the sample tube should have a constriction made, where the tip-off will be made. The smaller the internal diameter, the