View Full Version : Electrochemistry for dummies (me included)

Photo Engineer
01-13-2009, 01:24 PM
There has been a whole thread on this going on plus dozens of e-mail and PM traffic over the measurment of vAg when making an emulsion.

I am not an electrochemist. I used to ask someone to do this for me or I just used the equipment. I never designed any, nor did the grunt work, but I do know the model and what is going on.

Here is a very simple explanation then for us all.

If you have a beaker of Distilled Water, and stick in electrodes attached to a VOM, you should get infininte resistance. I know, it isn't, due to dissolved CO2 gas and its ionization, but lets start here.

If you add salt, resistance goes down as the conductivity (Ionization) goes up in solution. If the salt is KBr and you add Silver Nitrate, then the salt preciipitates and conductivity goes down again. Simple. The resistance varies with salt, and if salt precipitates, concentration decreaases and resistance goes up!

However, KNO3 is formed, and that is another salt! Oops, resistance goes down due to KNO3 which is proportional to the vanishing KBr and AgNO3. It goes down at about the same rate as any other salt unless you use electrodes sensitive to Silver ion and insensitive to Nitrate ion.

So, you could use anything as an electrode, but most things contaminate emulsions and most things used as electrodes are sensitive to most things in ionic form.

What you need is an electrode that is not affected by Silver or Silver Halide, and one that is not affected materially by Nitrates. Here is an analogy, and it is as far as I can go right now....

Assume a Distilled Water filled beaker with zero conductance or infinite resistance. Add some Gelatin and resistance goes down a tiny fraction. Now, you could zero on this new value to remove gelatin's contribution, but what if the second electrode was made of Gelatin (I know, impossible, but useful for this scenario). Then the original value of infinity would be offset by the Gelatin electrode to start with and addition of Gelatin would make little difference. (I know this is a bad analogy but it works for now)

In this instance, purely an example, the two electrodes might be Silver and a type of Salted Gelatin. The addition of any more gelatin would ignore the second electrode and the salt would work with the conductivity in a preplanned manner.

Now, if you start with Silver as an electrode, the instant the precipitation starts, the electrode is plated with the least soluable Silver Halide present. The result is that your balance or calibration is offset by having a new electrode suddenly form. And so, using a Silver metal electrode, while useful, can cause discontinuities in measurement. In fact, it can go the other way and unplate a Silver Halide electrode. I've seen that too.

So the use of electrodes to measure vAg is very difficult and what you get in the end is a measure of either mV or Ohms. The conversion of these to vAg requires use of the Debye Huckle and Nernst equations or the construction of a simple data table that converts these units to vAg. I can do either of these rather easily.

To show this, I can offer a page from my personal data log.

KBr conc = 0.1 m/l, vAg predicted = -50.17, vAg measured = -50.2

So, overall, practice and modeling can agree. My data log has 3 salts at 3 concentrations and they are all nearly that good. The variation is in KI which is very hard to measure compared to KCl and KBr.

In any event, vAg can be measured and predicted with great accuracy and the effects of Gelatin, Nitrate and other ions can be accounted for including mixed halides, but don't ask me about Ammonia. There is not suitable model for this in emulsion making AFAIK.

vX, pAg and pX should not be meausred due to a variety of problems. For example, 0.1 molar solutions of KBr and KI have nearly the same pX and vX, but the vAg values differ by many millivolts. So, you must be careful what you are measuring.

I hope this helps with better understanding.


01-13-2009, 02:58 PM

This actually helps quite a bit. Thank you for taking a moment to write this up. If you have time, at some point, can you please illustrate further using some typical values in a Nernst and / or Debye-Huckle equation as an example?


Bob M.

Photo Engineer
01-13-2009, 03:16 PM
I did, above Bob. You crank in molarity and get out vAg.


Kirk Keyes
01-13-2009, 03:39 PM
ISE measurements are certainly more complicated than pH measurements.

I understand that a 1 mV change in ISE reading is equivalent to about 4% in concentration for single charged ions. Good thing silver in monovalent, as divalent ions have an error of about 8%.

Here's an even better overview than the one I listed yesterday.

Photo Engineer
01-13-2009, 09:15 PM

Sometimes ISE measurements assume destruction of the sample, but in emulsion making it is assumed that the sample is inviolate! This makes a huge difference.