Although it sounds like some kind of euphemism from Brave New World, a “pneumatic trough” is actually a very handy piece of classic chemistry lab kit. Besides providing a convenient means to collect samples of pure gases for various experiments, a pneumatic trough with a graduated container allows the easy volumetric measurement of reaction yields for gas-producing reactions.
If that all sounds too complicated, don’t sweat. What I’m going to show in this tutorial is simply how to build a simple piece of apparatus that allows you to collect pure gas samples over water. You can collect carbon dioxide, oxygen, hydrogen–almost any gas you can generate and direct down a hose.
It seems like a simple enough bit of equipment: all you need is an upside down container suspended in a bucket of water. Finding a convenient way to set that up, however, is tougher than it sounds. The pneumatic trough presented here, which uses a sheet metal “bridge” to secure the glass column, is by far the most painless and economical way to make it work that I have found. The basic idea is derived from illustrations in Robert Brent’s 1960 Golden Book of Chemistry Experiments (from which the title diagram is taken), but the addition of an aperture shaped to accept the threads of a glass jar is of my own devising.Materials
- Aluminum sheet metal – I used K&S Engineering’s stock #256, .032 x 4 x 10″.
- Plastic ice cube tray – I used a Rubbermaid 2862.
- Adhesive-backed printer paper – I used Avery 8465 full-sheet inkjet labels.
- Glass olive or jelly jar with lid – Unfortunately I don’t know the exact brand, but critical lid dimensions are given below.
- Computer with printer and imaging software
- Inside calipers or ruler
- Drill with 1/4″ bit
- Hand-held nibbler
Step 1: Measure the internal dimensions of the jar lid
Most glass jar closures these days use a simple lid design, like that shown in the photograph, consisting of an opening having the shape of the intersection between a circle of a particular diameter, and a square of dimension slightly smaller than that diameter. Put simply, it’s a circle that’s been flattened a bit on four sides. The jar will thread into any aperture that has an opening of this shape, so what we’re going to do is reproduce it. So you need to measure the inside dimensions of the lid opening across the circular diameter, and between the flats. The lid I used had an internal diameter of 2.6″ and a flat-to-flat “diameter” of 2.475″.
Step 2: Prepare a template
I used Adobe Illustrator to prepare my template. If you use the same materials I did, you can use the same template, which is available as a .pdf
. If you don’t have access to the same materials, you can of course make your own template, using Illustrator, other software (like FreeCAD), or manual drafting.
Step 3: Cut out and affix the template
I overlaid the sheet metal blank on the printed template and cut around it using a hobby knife. Then I peeled off the adhesive backing and carefully aligned the label with the sheet metal and smoothed it in place.
Step 4: Cut out the hole
I used a cordless electric drill with a 1/4″ bit to rough out the shape of the hole, as shown. Then I went back with the nibbler and carefully trimmed out to the finished edge. The nibbler leaves a fairly clean edge, but a few light file strokes here and there helped to smooth things up a bit. Be sure to test the jar fit as you go; better to have a messy edge than to remove too much material and ruin the fit.
Step 5: Make the bends
I just used my hands and bent the aluminum over the edge of my kitchen counter. The bends were a bit sloppier than I liked, but it ended up working out just fine. The important thing is to not let the central area with the jar opening get bent “out of flat” because this can upset the geometry of the opening and cause the jar to thread improperly. If I were to do it again, I would use a small piece of wood to hold the metal down on the counter while I made the bends.
Step 6: Remove the label
The labels I used peeled off the aluminum completely intact, leaving no residue and requiring no clean-up. Nonetheless, it may be a good idea to wipe down the finished bridge with isopropyl alcohol once the label is removed.
Notes and ideas
Don’t mind the two extra lines on the template in the photographs; I originally intended to put two more bends in the bridge but that proved unnecessary in practice. The extra lines are not included in the downloadable template.
The trough is assembled as shown. Its operating principle is straightforward: The ice cube tray is filled with water to a level slightly above the rim of the upended jar. Then, using suction through a piece of tubing, the atmosphere is evacuated from the jar, causing a column of water to rise and fill it completely. To collect gas, a tube from the running gas generator is inserted into the water below the upended jar, and the bubbles rise to displace the water with the generated gas. Part II of this tutorial will cover the process in more detail, showing how to collect pure oxygen from the catalytic decomposition of drugstore hydrogen peroxide.
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