The first step in building our reference electrodes is to cut glass 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 main image) 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. This is the label on the bottle that we use: