On-Demand Benchtop Gas Generator

By Sean Michael Ragan
Category: Workshop, Science
Time Required: 3 hours over two days
Difficulty: Moderate

If you should find yourself in need of small volumes of gas at about atmospheric pressure for a reaction or project, generating it on the bench can be a convenient and inexpensive alternative to buying or renting a gas cylinder. It turns out there are a number of useful gas-generating reactions between solid- and liquid-phase reagents, such as:

Hydrogen: Zn + HCl —> ZnCl + H2
Acetylene: NaC2H + H2O —> NaOH + C2H2
Carbon dioxide: CaCO3 + HCl —> CaCl + CO2
Hydrogen sulfide: FeS + H2SO4 —> FeSO4 + H2S

Note that any gas-generating reaction, even one that produces a relatively inert gas like carbon dioxide, is potentially dangerous unless performed with adequate ventilation. Depending on the reaction in question, there may be other hazards, and, as always in the laboratory and in life, no procedure should be undertaken unless you understand and are properly prepared for the risks it involves.

An all-glass reactor for the benchtop production of gases was invented in the 19th century by Petrus Jacobus Kipp, who is known today primarily for this achievement. Kipp’s design incorporates the clever feature that stopping the flow of gas separates the liquid and solid reagents inside the instrument and thereby stops the reaction. Thus the generator only produces gas when you need it, and may remain in a stable equilibrium state on the bench for days at a time, ready to resume operation as soon as you open the valve.

Being made of glass, however, a proper Kipp generator is an expensive piece of apparatus, with new models costing upwards of $250US as of this writing. However, as the useful gas-generating reactions are usually aqueous, rather than organic, an all-plastic Kipp generator is almost as useful as a glass version. PVC pipe is inexpensive, durable, ubiquitous, and easily and securely joined using cement made for that purpose. Demountable PVC fittings are available in a wide variety of shapes and sizes and can be used to provide the necessary “dismantlability” for loading solid reagent into the device. Presented here is my design for such a low cost Kipp generator, with instructions for its construction.

Steps

Step #1: Understand how it works

Essentially, the generator consists of a closed chamber, loaded with solid reagent, which communicates with a liquid reagent reservoir at one end, and from which gas escapes at the other. The liquid reservoir is open to the atmosphere, and thus the pressure of gas generated is limited by the hydrostatic pressure of the reservoir column. When the gas valve is closed, gas pressure backs up in the chamber and pushes the liquid reagent back out of the chamber, where the solid phase is retained, thus halting the reaction. Excess gas bubbles out into the atmosphere through the liquid reservoir.
My design features a radial construction which, albeit slightly more complex than the "u-tube" design, offers some advantages over it. Foremost of these is that the radial design makes it possible to remove the chamber full of solid reagent without removing the liquid reagent first. So, if necessary, the chamber can be removed and reloaded without the hassle of pouring off the liquid phase.

Conclusion

Besides any reaction-specific cautions that may be advisable, please be aware of the following potential hazards when using this device:

Asphyxiation - The generation or release of gases other than oxygen in an enclosed space creates a real danger of asphyxiation. Be sure to have adequate ventilation.

Spattering - Back pressure in the reaction chamber, especially when the valve is first closed, will cause excess gas to bubble up through the liquid reservoir. This bubbling can be vigorous, and may spatter the liquid reagent if the reservoir is filled too high.

Tipping - Although the generator is designed to be free-standing, a clamp, brace, or other support is highly recommended. An unsupported generator will spill the liquid reagent if it is accidentally tipped over.