Quote Originally Posted by kb3lms View Post
While browsing the web at lunchtime, I found references to this article about constructing a reference electrode out of a used felt-tip pen. So I tracked down the article tonight and have posted it at the link below. The electrode described is for Ag/AgCl, but it seems like the basic design could be adapted by using KBr instead of KCl.


Any thoughts on it's usefulness for measuring vAg or at least pAg? The more I read I see how knowing vAg or pAg is important for making more advanced emulsions so it seems like finding an economical solution for the probes would be useful.

-- Jason
Note that what's described in that article is for a reference electrode.

When making electrode measurements, you need to have both a reference electrode and what's called an ion specific or ion selective electrode.

For pH measurements, you use a what's typically called a "pH" electrode. What's really being said there is that you need a glass electrode (which can simply be thought of as a hydrogen ion specific electrode) and a reference electrode.

Most people never even mention the reference electrode when talking about pH measurements because most pH electrodes nowadays have the reverence electrode built right into the ion specific electrode. The reference electrodes are typically silver chloride electrodes or calomel electrodes. Calomel seems to have lost favor these days as they contain metallic mercury and mercury (I) chloride, both rather toxic compounds.

The silver chloride referece electrode consists of a piece of silver metal that is coated with silver chloride, and it is placed into a concentrated solution of potassium chloride to stabilize the silver chloride that coats the silver wire. The potential that is generated by the silver|silver chloride junction is the "reference" voltage that this type of electrode produces. The standard electrode potential that the silver reference electrode produces against the standard hydrogen electrode (SHE) is 0.230V 10mV.

The silver chloride reference electrode that is discussed in the article linked to above is this type of electrode, not an "silver" electrode that is used for making vAg measurements.

The vAg electrode that PE talks about for making vAg measurements, is simply a piece of high purity silver wire/rod that has been coated with the dominant halide for the emulsion that is to be measured. Like PE mentioned, somewhere here on APUG are a series of post where he and I discussed the construction of a home-made vAg electrode. Basically, it's the silver wire that PE mentioned, I think what I found was 12 or 14 guage - which is nice and sturdy and resists being bent from use. That silver wire was then soldered to a length of coax cable with a BNC connector so it could be directly connected to the "pH" meters that we both had. The silver wire/coax assembly was then inserted into some fire-polished glass tubing with bare silver wire outside the glass and the silver wire solder joint inside the glass and then 5-minute epoxy was then used to set the wire in place in one end of the glass tube and set the cable into the other end.

For use, this vAg electrode was cleaned with fine wet-dry sandpaper and then placed into a solution with the halide of choice, and a small current run through the wire and solution to drive the halides to the vAg electrode to plate it. (As they mention in the article, the vAg electrode can be connected to the positive terminal of a 3-volt battery and then current run through for 10 minutes using a second vAg electrode or a simple siver wire in circuit with the plating solution.) Once this had been done, the vAg electrode was ready for use. This treatment was good for several days if stored in solution and kept in the dark.

Now this vAg electrode would have to be used with a reference electrode - just like the one in the article linked to above, so that you had both an ion specific electrode and a reference electrode.

And actually, the idea of a felt-tip pen as the body of a reference electrode is pretty good, and great for the home lab, as silver chloride reference electrodes are expensive new and hard to find used. Note that the pen-based reference electrode has a "bridge" like PE mentioned above built into it - that's what the agar gel is doing. It's slowing down the mixing of the reference solution (the potassium chloride that surrounds the silver|silver chloride electrode). The felt tip is merely acting as the junction that allows the interior solutions to come into contact with the solution to be tested.

PE also suggests that you want to use a double bridge to minimize the migration of non-chloride halides from the test solution to the reference solution. If bromide or iodide get into the reference solution that surrounds the silver|silver chloride reference electrode, then it will be poisoned and it will no longer serve as a "reference" electrode.