Gas station without pumps

2016 December 16

Two-electrode vs. four-electrode impedance spectroscopy

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 16:49
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Four electrodes with 1cm spacing.

Four electrodes with 1cm spacing.

Today I decided to revisit the water-conductivity experiments for the course, now that I have an easy way to do proper impedance spectroscopy (including phase information as well as magnitude), using the network analyzer function of the Analog Discovery 2 USB oscilloscope. I wanted to look at 4-electrode measurements, as well as the 2-electrode measurements we’ve done in the past.

First, I made myself a 4-electrode device, by cutting some ⅛” stainless-steel welding rod (316L rod for TIG welding) into 15cm pieces, drilling 4 ⅛” holes in a scrap of cutting-board plastic, and driving the rods through the holes with a hammer.

I then immersed the short end in tap water (using a mason jar, so that the long end stuck out the top) and used alligator clips to attach wires from the electrodes to a breadboard.

I connected the function generator through a series 1kΩ resistor to one of the end electrodes and ground to the other end electrode.  Channel one of the oscilloscope measured the voltage across the 1kΩ resistor (hence the current in milliamps).

Channel two of the oscilloscope was connected to either the two end electrodes (making a 2-electrode measurement similar to what we’ve done for years in the class), or to the two middle electrodes, for a 4-electrode measurement.  The idea of a 4-electrode measurement is that there is an electric field established in the bulk material by the outer electrodes, and the middle electrodes can measure that field without interference from surface effects that occur on the electrodes that are providing the current.

I used the network analyzer function to sweep from 2Hz to 10MHz.  I exported the data so that I could plot it as impedance (rather than as just the dB ratio of the two measured voltages).  For the 2-electrode measurement, we are measuring the impedance of the water and electrodes (voltage across the electrodes divided by the current through them), but for the middle electrodes, we’re looking at the voltage across the middle electrodes, divided by the current through the end electrodes.

The voltage across the middle electrodes is nearly a constant, up to about 1MHz, where wiring inductance starts to matter. The surface chemistry interferes with measurement of bulk properties at low frequencies for the 2-electrode measurement.

The voltage across the middle electrodes is nearly a constant, up to about 1MHz, where wiring inductance starts to matter. The surface chemistry interferes with measurement of bulk properties at low frequencies for the 2-electrode measurement.

The plot of the phase shows even better why 4-electrode measurement is useful:

The capacitive nature of the two-electrode system is seen at low frequencies, but the 4-electrode system has a resistive, nearly 0° phase shift (up to the point where the inductance of the wiring to the reference impedance starts to matter).

The capacitive nature of the two-electrode system is seen at low frequencies, but the 4-electrode system has a resistive, nearly 0° phase shift (up to the point where the inductance of the wiring to the reference impedance starts to matter).

I don’t think I’ll switch to 4-electrode measurements this year (if for no other reason than that I’d have to make a dozen new electrode sets), but I’ll keep it in mind for next year.

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2 Comments »

  1. Hello,

    Why do you think that the 4 electrode method is better ? From the impedance vs frequency plot I can see that the impedance value is lower. How does this make the approach better ? Have you also plotted Nyquist plots for the measurements ?

    Comment by Agnivo Gosai — 2017 February 24 @ 15:20 | Reply

    • The 4-electrode method is flatter with respect to frequency—the electrolyte (water and ions) is basically resistive, all the capacitive effects come from surface chemistry at the electrodes. If we are trying to measure bulk properties of the electrolyte, then the surface effects are not signal, but undesired distortion.

      The actual value of the resistance is going to differ, because I do not have the same electrical field in both cases. For measuring ionic concentration, one always has to calibrate with a known concentration, to correct for geometric effects, in any case.

      Comment by gasstationwithoutpumps — 2017 February 24 @ 21:34 | Reply


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