DIY Reference Electrodes

An inexpensive RE can be made from a piece of silver wire coated with silver chloride and suspended in a chloride containing solution. The operating premise is that the voltage of the electrode is determined by the energy difference between the metal and the metal chloride salt. If an electron is removed from the electrode, one atom of silver metal is converted to Ag+ and a chloride ion is picked up from solution to give AgCl. If an electron is added to the electrode, it goes on a Ag+ ion in the AgCl layer to give another atom of silver metal and a chloride ion is lost to the solution. In this way, the chemical composition that determines the voltage of the electrode remains constant.

Fabricating Reference Electrodes: The first step in building our reference electrodes is to cut glass tube (or other insulating tube) to length. I use three millimeter ID standard wall glass tube. To cut glass tube, you first score it using a carbide tipped tool (the red router bit in the image above) where you want it to break. The scratch doesn't have to be deep or go all the way around the tube. You then apply a little spit to the scratch (don't ask why, just do it). Grasp the tube in both hands. Your thumbs should be about an inch apart. The scratch on the tub should be on the surface away from your thumbs and be half way between them (that is, it should be pointed away from you). To reiterate, the spacial order goes like this: your eyeballs, your eye protection, your thumbs, your glass tube, the scratch on the tube. Push with your thumbs to bend the tube away from you and it will break cleanly at the scratch. Fire polishing the ends of the tube with a propane torch should give you a rounded edge that will not cut you when you use it (I say should because we use an oxygen / natural gas setup). As described by Yee and Chang (reference below), we use a molecular seive for the solution junction that separate the reference electrode from the test solution. A molecular seive is a porous ceramic pellet that can be used to exchange ions with solutions. 

The image above shows several cut tubes, each with a sieve glued to the end. We have used epoxy and super glue on separate occasions. Both work fine. I dip the end of the tube in glue, then stick a seive ball to it. Ben covers the whole thing in glue and sands it down after it dries (given the high rates of leakage at the joint that I experience, Ben might have the better method). After the glue dries, fill the tube with saturated potassium chloride solution and put it in a capped container containing this same solution. The seive needs to stand in solution for a few days (weeks?) to equilibrate with the salt solution.

Once the seive has had time to equilibrate, a silver wire is inserted in the open end of the tube, down into the KCl solution. The wire should be thick enough to take abuse. I use 0.64 mm diameter 99.9 % metals based wire from Alfa Aesar (cat no 43325, $42 for 2 meters). For accurate values, the silver is oxidized while immersed in chloride salt solution (if accurate voltages are not necessary, you don't need to do this. The silver oxide that comes as standard issue on this metal will give a stable voltage). I am told the AgCl coating can be achieved by dipping the wire in bleach (need ref). Alternatively, Inamdar, et al, recommend the following: Connecting your silver wire to the positive terminal of a 3 volt battery and immerse it in .05 M KCl solution, connect the other battery terminal to a different wire and immerse that one in the same solution, give it about ten min to form the gray-white AgCl layer and remove.

The last steps are to insert the silver wire into the cell and cap it.  A craft bead can be glued to the top, with the wire poking through.  You could put glue or wax in the top, but that would make it hard to re-fill the solution. IMPORTANT NOTE Store your reference electrode in a capped bottle with the seive end immersed in saturated KCl. If it dries out, the electrode will not work (until you equilibrate it again). If some of the water evaporates you may get salt crystals inside the cell (or crusty stuff around the outside). Not a big deal. Just add water so that the thing doesn't dry out.

References

• Inamdar, S. N., Bhat, M. A., Haram, S. K., "Construction of Ag/AgCl Reference Electrode from Used Felt-Tipped Pen Barrel for Undergraduate Laboratory", J. Chem. Ed., 2009, 86, 355.

• Yee, S., Chang, O. K., "A Simple Junction for Reference Electrodes", J. Chem. Ed., 1988, 65, 129