Sciencemadness Discussion Board

Salt Bridges, A simplification?

BromicAcid - 19-10-2004 at 18:01

I was thumbing though one of my new chemistry books on Analytical Chemistry and came across a nice little passage on an easy to construct salt bridge:
Quote:
A salt bridge is easily constructed by filling a U-tube with a conducting gel prepared by heating about 5 g of agar in about 100 ml of an aqueous solution containing about 35 g of potassium chloride. When the liquid cools, it sets into a gel that is a good conductor but prevents the two solutions at the ends of the tube from mixing. If either of the ions in the potassium chloride interfere with the measurement process, ammonium nitrate may be used as the electrolyte in salt bridges.


Skoog, Douglas A.; West, Donald M.; Holler, James F.; Crouch, Stanley R. Fundamentals of Analytical Chemistry 8th ed. Thompson Learning, 2004

I like this idea, filling a flexible plastic tube with the mixture would give a good salt bridge that would not need refilling or tinkering with.

There are of course the other salt bridges, involving U-Tubes full of liquid that have cotton/glasswool plugs or fritted glass ends. But those have always seemed a hassle, so I figured I'd share this with everyone. Or you could soak an unglazed clay pot in 6 M HNO3 and use it to house one of the electrodes with the other on the outside, technically I think that would count as a salt bridge.

Regardless, may all of your solutions keep to their respective sides from now on!

chemoleo - 19-10-2004 at 18:06

Yes, it's a nice idea.
I guess one could also use normal gelatine, or anything that solidifies a solution of salts?
Agar is of course the material of choice, it being used all the time for DNA electrophoreses.
Another one is polyacrylamide - but much less useful for the amateur chemist.

At last, one idea is alginate - although I haven't tried it, it also makes a water-based solid medium. Alginate is widely available from arts supplies.

Twospoons - 19-10-2004 at 19:35

Funny, the agar idea occured to me the other day too. Though I had thought of making it into a membrane by casting the gel onto plastic mesh for support, giving shorter path and more area (= better conductivity).

Polyacrylamide can be bought from garden stores as "water storing crystals". Why wouldn't it be useful for the amatuer?

Esplosivo - 19-10-2004 at 22:39

This is the way I have always seen salt bridges set up (and set them up myself a couple of times), though I used potassium nitrate instead of potassium chloride. It works marvellously and can hold for quite a long time (although with time the 'jelly' will become softer and break up into smaller pieces which would mess up everything).

Quote:

Polyacrylamide can be bought from garden stores as "water storing crystals". Why wouldn't it be useful for the amatuer?


Are you sure about that? I mean, it could be anything from agar to its purified form agarose, but not polyacrylamide! It could also be one of those types of very absorbent polyols. Chemoleo referred to polyacrylamide as not useful to the amateur chemist since it is expensive - I can confirm that.

[Edited on 20-10-2004 by Esplosivo]

Hermes_Trismegistus - 19-10-2004 at 22:57

No. Not neccessarily; unless we all have different views on expensive...but no-one here lists "ass end of ethiopa" as their location (yet)

I've found it sold for 5 bucks a pound granular. It's also sold in blocks. Farmers are now using it in the field. Google polyacrilymide and furrow.

(although if you want it from a chemsupply....then again, look up the price for acs grade NaCl.)

Organikum - 20-10-2004 at 03:04

The salt bridges are basically cell-dividers I think. I am experimenting with fine-glasswool inbetween a supporting lattice for cell dividers now, better than claypots this is for sure!
The glasswool has to be freed from the glue which makes the yellow color by heating before use, the material of the support depends on the reaction of course, for Cl2 from NaCl/HCl PVC works ok as long the temperatur isnt to high.

I believe the glasswool could be doted too for ion/anion selectivity, but thats future.

Twospoons - 20-10-2004 at 13:52

"Polyacrylamide" is what is written on the side of the container of water storing granules I've bought. Probably cheap because its not reagent grade, but I'd have thought it perfectly OK for us bucket chemists!

Anyone know what this stuff dissolves in? The granular form isn't particularly useful...

Saerynide - 23-10-2004 at 11:55

hmmm... So, If I wanna electrolyze MgSO4 like that, it would make sense to fill the agar in the salt bridge with MgSO4 instead of KCl right? Otherwise, I would end up with a bit of Cl2 and KOH, right?

chemoleo - 23-10-2004 at 18:46

Just to point out, polyacrylamide is used in the biochemical sciences for running SDS - gels.
Basically what is done is that acrylamide (the monomer) is mixed with buffer and polymerised in a form/mould for the gel. Once it has finished reacting, it's a great gelatinous material (clear as glass), whose hardness can be determined by the percentage of acrylamide. Gels normally are in the range of 4-20%.
Anyway -the point is that the gel is made from the monomer - and I am not sure whehter dry polymerised polyacrylamide can be redissolved and gelatinated.
While monomeric acrylamide is hard to get, and supertoxic (carcinogenic).

Another possibilty for a gelatinous conductor is alginate (dental/arts supplies)