Gas station without pumps

2012 September 6

Making Ag/AgCl electrodes

Filed under: Circuits course — gasstationwithoutpumps @ 12:11
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I mentioned in  Better measurement of conductivity of saline solution that I would try buying fine sliver wire and making Ag/AgCl electrodes, either with bleach or by electroplating on chloride from salt water. In Measuring Ag/AgCl electrodes, I described a simpler technique of sticking two Ag/AgCl gel electrodes I had for the EKG lab face-to-face.  That worked well, but I ended with the thought “Putting small Ag/AgCl electrodes into a saline solution, however, might have quite a different effect, where the resistance of the electrodes plays a more substantial role in the electronic behavior of the system. So maybe I do still need to order some silver wire and make my own Ag/AgCl electrodes (either by bleaching or by electroplating).”

Two pieces of fine silver wire clipped into slots on the sides of the 1″ diameter cup.

So I did buy some 18-gauge fine silver wire from FusionBeads.com, and I tried making a holder for it, since fine silver is so soft. My first thought was to make some sort of plastic clip to hold the wires a fixed distance apart that I could dip into saline solution, but I did not come up with anything for that yet. My second thought was to make the holder and the cup be the same item. I drilled a 1″ hole part way through a cutting board with a Forstner bit, and cut two slits in it with a hack saw to hold the wire. (I did this almost two weeks ago, but just got around to testing the holder today—I suppose I feared that it wouldn’t work.)

The wire can be pressed firmly into the slots and stays there well—I can even pick up the whole assembly by either wire. Unfortunately, water does not stay in the cup but wicks through the slot and slowly siphons out.  While I can get water to stay in long enough that I could probably do electroplating, this device would be too likely to cause problems in the electronics lab, with salt water getting all over the place.  Even though both Steve and I agree on the need for secondary containment for both the water baths of the thermistor lab and the salt solution for the electrode lab, I’d prefer a system that doesn’t leak.

I then tried using the same piece of plastic to hold the wires, realigning the wires so that they are flush on one side and both come out the other side. I could then dangle this from clip leads in a cup of salt water, very similar to the way the stainless steel electrodes were used. If we used a smaller silver wire (say 24 gauge, instead of 18 gauge), then the students could wrap it easily around a framework that has a couple of notches for spacing, rather than clipping the wires into slots. I think that would be easier to manage, and the wire would be a bit cheaper (1/4 the price per foot).

Warner Instruments recommends electroplating for about a minute at 1mA/cm2 of surface area. Since the 18-gauge wire has a diameter of 0.1024 cm (according to Wikipedia’s table of AWG wire gauges) or 0.10 cm with my calipers, if I immerse it 3.5 cm deep in saline solution, each electrode would have an area of 1.1 cm2, and so need about 1.1mA of current.  They recommend either using normal saline solution (a 0.9%  or 0.154M NaCl solution, about the same osmolarity as blood, and so commonly found in hospitals) or 1M KCl.  I suspect that any sufficiently large chloride concentration would do.

I used a 5V battery pack (4 NiMH cells), a variable resistor, and an ammeter to make my current source.  I adjusted the resistor until I got about 1.2 mA through the electrode in a 1M NaCl solution (it turned out to be about 2.6kΩ).  The electrode gradually darkened, but it took more like 3 minutes to get a fairly uniform coating, rather than 1 minute.  One important point—I had handled the wire a lot, and so it had finger grease on it, which resulted in a mottled plating after 1 minute. Wiping the wires off with a paper towel and continuing the plating resulted in a much more uniform plating. Students should be warned to clean the electrode before electroplating.

For characterizing the electrodes, I did the same electronic setup as before, with a sine wave driving the electrodes in series with a resistor.  I used a 10.0Ω resistor this time, since I expected low resistance for the high-concentration saline solution, and I didn’t want the voltage drop across the electrodes to get too small to measure.

Resistance of electrodes as a function of frequency for three different salt concentrations. The variation with frequency is much larger for high salt concentrations.

The frequency dependence is much lower than for the polarizable stainless steel electrodes, and doesn’t fit the R2+(C1||R1) model well.

Because the frequency dependence is much higher for large salt concentrations, I think that students should be instructed to do this lab with 1M NaCl. If they have time, they can repeat it at lower concentrations. Only at the highest frequencies is the conductance proportional to the concentration. With the Agilent multimeters in the lab, the students should be able to go up to 1MHz, but my ancient handheld Fluke meter doesn’t do true RMS AC at those frequencies. I’d be interested to see what happens at higher frequencies—does the power law continue?. (Actually the Agilent meter is only spec’ed up to 300kHz, but there is an implication in a footnote that the it can handle up to 1MHz with somewhat higher error.)

I’m curious about what causes the frequency dependence, and why the exponent on the power law changes with the concentration.

 

5 Comments »

  1. […] It takes a lot of power to shake large objects, and I’m still waiting for the 20W LP-2020A+ Lepai Tripath 20W amplifier to arrive, but in the meantime I decided to characterize the impedance of the transducer using the same setup I used for characterizing Ag/AgCl electrodes. […]

    Pingback by Characterizing tactile transducer « Gas station without pumps — 2012 September 13 @ 11:34 | Reply

  2. […] mentioned in Making Ag/AgCl electrodes that I was not really happy with the holder I had made for 18-gauge silver wires for Ag/AgCl […]

    Pingback by New holder design for Ag/AgCl electrodes « Gas station without pumps — 2012 November 18 @ 19:52 | Reply

  3. […] in the magnitude of impedance with frequency for polarizable stainless steel electrodes and Making Ag/AgCl electrodes for non-polarizable silver/silver-chloride electrodes. For polarizable electrodes, we probably want […]

    Pingback by More on automatic measurement of conductivity of saline solution | Gas station without pumps — 2013 December 21 @ 17:48 | Reply

  4. why do plating get better impedance??

    Comment by mona — 2014 August 5 @ 03:08 | Reply

    • I believe that plating is supposed to provide a more uniform coating of AgCl than soaking in bleach does, though I have not done any experimenting to determine whether this is true. Reversing the current to plate the other electrode results in stripping off some of the plating from the first electrode, so you need to overplate the first electrode in order to plate the second electrode. If you use pure Ag electrodes without an AgCl plating, you only get good current flow in one direction (Cl- + Ag => AgCl + e-).

      I have found that students handle the sliver wire too much and don’t wipe off their fingerprints, so that they often get streaky plating. In a real lab one would be cleaning the silver wire thoroughly before plating or soaking in bleach.

      Comment by gasstationwithoutpumps — 2014 August 5 @ 08:51 | Reply


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