Ok, here is a summary of several months work and a list of equipment needed.

First, you need a high impedence VOM such as the Radio Shack 22-812, which reads in the MV range or +/- 1V on the total scale and which can reverse polarity automatically or via a switch so that you can span the range from -300 to +300 mv. as quickly as possible.

Second, you need a silver electrode of about 20 grams weight and quite thick (about $15 - 20 depending on the spot price of silver). This would be good, heavy quality silver wire or rod of 99.99 fine. Yes, that is 99.99 fine. Anything less will cause problems at some time or another.

Third, you need a double junction Silver, Silver Chloride, KCl electrode such as the HI5132 or HI73127. You will need filling solution for the former, and storing solution for both.

That should allow you to set up your experiments quite nicely. It will not help you make emulsions unless you build a bridge and I am working on that right now.

Now, with average cleanliness, I get results as follows:

With 0.1 M NaBr I have a range of -36 mv to -70 mv with lots of noise. With scrupulous cleanliness and fresh electrodes, conditioned and cleaned with my best effort, I get -65 to -69 mv, with the calculated value of -62.6 mv.

With 0.1 M NaCl I have a range of +22 mv to +50 mv with lots of noise. Cleaning the equiment brings my range down to 33 mv to 36 mv, with the calculated value of 36.5 mv.

The moral of this is that cleanliness is essential. The Silver electrode corrodes and the reference electrode does not like distilled water very much and must be kept conditioned. The type of reference electrode is not important, nor is the bridge (at this time - still working on the bridge though - more later).

Since the reference electrode leaks saturated KCl, this is not a suitable method for making an emulsion. The bridge is critical for best results.

I hope to summarize the 1 M salt results and the KI results in the near future for you.

At the present time, it seems that you will need 3 Silver electrodes, one each for the 3 major salts. Also, you will need 3 bridges for the same 3 major salts. And, this will not work very well in the presence of any silver halide solvents such as Ammonium Ion or the like. This is more due to the uncertainty of calculation of a correct value than anything else.

BTW, I have verified these results with 4 VOMs, with and without stirring, but all at 20 deg C. And, in the final circuit, to stabilze the system, I may have a battery and potentiometer to allow for zeroing the system with a standard salt along with a slope control for changing salt concentrations from high to low. At the present time, if I zero the system on a standard salt, I often see a 20 mv offset (slope) as the salt becomes more and more dilute. I am working to figure out if this is a slope problem or a contamination problem or both.

PS, the above, when detailed, has added about 6 pages to the book. You see? I have not forgotten it either.

PE