For want of a U-Tube

April 22, 2007 6:52 PM

My lab is running low on U-tubes...what can I use in place of a U-tube/porous cup for a salt bridge in a voltaic cell?

I'm a beginning high school Chemistry teacher who wants to build some voltaic and electrolytic cells with his students. But I've only got 2 U-tubes to 20 students. Is there something else I can use in it's place as a salt bridge? The set-up will look like this.

posted by The Coach LD to Education (10 answers total)

A piece of filter paper soaked in salt water.
After several failed attempts at making salt bridges, my chemistry class came to the conclusion that the salt solution needs to have so much salt that it doesn't all dissolve and whatever contains the salt bridge needs to be really soaked.
posted by martinX's bellbottoms at 7:01 PM on April 22, 2007

i just taught a chem lab a few weeks ago where we used a KNO₃ saturated agar gel sleeve to act as a salt bridge. the setup kind of worked like this:

one half cell was the normal beaker + electrode + electrolyte. the other half cell was a small test tube with a hole in the bottom (you can make these from a normal test tube with a burner and some kind of pointy metal thing, just get the end soft and poke a hole in it .. helps to have a touch of glassblowing experience, but should be easy).

the students dip the test tubes in a hot melty solution of agar gel + KNO₃, let it cool for half a minute then fill the tube as normal. then you just place the small test tube inside the beaker; the gel forms a sleeve over the bottom of the little tube and ion transport through the hole and gel completes the circuit. (if it's not really clear what i mean, send me an email and i'll send you some pictures.)

this page describes how to make the agar mix as well as gives instructions for making your own little U-bridges out of PET pipets if you want to go that route. (i think you could also use glass pipets and bend them if you're comfortable with that stuff .. )

anyway, good luck!
posted by sergeant sandwich at 7:27 PM on April 22, 2007

{thumps cane} In my day we would get some flint glass (not affiliated with vendor) and bend it into the U-shape. Flint glass will bend even over a reasonably ambitious spirit (alcohol) burner. If you want to do it nicely you use a fan attachment on a Bunsen burner and bend a few pieces for practice.
posted by jet_silver at 7:51 PM on April 22, 2007

In high school we used a strip of filter paper soaked in Potassium Sulfate solution, I believe. Worked fine for demonstartive purposes.
posted by cholly at 9:19 PM on April 22, 2007

I've seen it done with a brine-soaked paper towel, but filter paper and a buttload of salt, a la martinX's bellbottoms, would probably improve its success and survivability. But I've also bent flint glass tubing in a high school chem lab, with moderate success. (I learned the hard way that you have to move your tubing across the flame, because there are hot spots at the edges, or your 90° bend will come out as a double-45° instead.)

If you don't want to do it all yourself, offer your AP chem students a pittance of extra credit to come in after school and turn out a piece or two. That's putting the school back in old-school.
posted by eritain at 9:31 PM on April 22, 2007

Heh. We used to make eye droppers and u-tubes in my middle school chem class... so much glassware got broken over the year (it's -middle school-..) that they just started buying glass tubing of various sizes in bulk every time the supplier offered a sale, and we had at least one "lab" a year that was learning how to bend glass and make eyedroppers over a bunsen burner flame.
posted by SpecialK at 9:38 PM on April 22, 2007

Ditto on the diy glassware. It's not that hard once you get the hang of it, and lab glass is very thin-walled so a good bunsen will make short work of it. Plus it's a lot of fun.
posted by Skorgu at 5:48 AM on April 23, 2007

Fourthing making your own. It's surprisingly easy and something every chemist should know. You could even get the kids to make them, it's that easy.
posted by bonehead at 7:39 AM on April 23, 2007

The soaked paper works, but pulling your own tubes is fun and really dead simple. Make sure everyone knows that cold glass looks just like burn-your-flesh-off hot glass.
posted by Mr. Gunn at 7:47 AM on April 23, 2007

Response by poster: Thanks everyone. I think I'll try some U-tubes...it would impress the kids and, at the very least, be an excuse to play with fire.
posted by The Coach LD at 7:02 PM on April 23, 2007