1. Home
  2. Electronics Design
  3. Making an electrode for a conductivity meter
  4. Page 3

Making an electrode for a conductivity meter

Check up on the Warburg impedance. It is made up of two components of equal magnitude, one real, the other imaginary, giving a 45 deg. phase angle. The term arises from sinusoidal concentration changes near the electrode as the applied (ac) potential oscillates. The concentration changes are determined by the diffusion coefficients of the two substances involved. In fact, the Warburg term is totally diffusion controlled. The total Faradaic impedance is made up of a resistive term arising from the electron transfer, in series with the Warburg term. So I was wrong about the Warburg impedance being a mix of reaction and diffusion terms; it is purely diffusion controlled. It is the overall Faradaic impedance that is the mix of both. Your impedance spectroscopic measurement would measure the total electrode impedance, which now also includes the double layer capacity AND some uncompensated solution resistance.

Carbon is too hard to get pure and it does corrode - people were glad to find the dimensionally stable anodes to replace graphite, which was not. You don't want your condictivity meter to have a slowly varying cell constant, and you don't want the probe to contaminate the solution. Even glassy carbon is porous to some extent.

[...]

We will have to agree to disagree. As I wrote, any Faradaic effects will only lower the interfacial impedance even more, but in any case, due to the large capacitance of the double layer, it is already much smaller than the solution resistance in magnitude, and so we can measure the latter. A slow electron transfer reaction would not matter at all, because the Faradaic reaction has a very small effect on interface impedance at these kHz frequencies. There is no dc component, and the solutions whose conductivity are measured can be anything, so we never know what electrochemical reactions there might be, if any. A low overvoltage does not apply - low, for what reaction? All we have here is a small ac signal, working on the capacitive double layer, with some minor electron transfer possibly happening.

--
Dieter Britz,   Kemisk Institut, Aarhus Universitet, Danmark.
Reply to
Dieter Britz
Loading thread data ...

Hello

"Nick J." a écrit dans le message de news:

4GjCd.274729$5K2.200722@attbi_s03...

No that's not the reason why very small amplitude signal must be used in EIS. I suggest you to have again a look to both descriptions of EIS principle measurement and to relationship between current density and overvoltage (i.e. Butler-Volmer relation), in Bard & Faulkner's book. In the former, it will be said that determined impedance is the ratio between the excitation signal and the response signal (deltaE/deltaI) and that the main assumption is that the response is linear. In the latter, you'll see that the variation of charge transfer resistance vs current density is logarithmic. Thus, the only way for both items to be compliant is to use very small amplitude signal. Note that, here, we talk about the sinusoidal signal and that you can superimposed this signal to any DC signal. However, taking into account the initial poster's aim (i.e. measuring electric conductivity), this restriction is not applicable for you... on the condition that you excitation signal has a high enough frequency that charge transfer resistance doesn't interfere with your measurement or is negligible.

Best regards.

--
Enlevez ".nospam" de mon adresse e-mail pour me répondre.
--------------------------------------------------------------------------------------------------------
Remove ".nospam" from my email address to reply me.
Reply to
n2mp

to

it

assumption

variation

Thank you for clarifying this point. It was many years ago that I looked at the MacDonald EIS book but now that you mention it, I do recall that it did say (in so many words) that a small excitation signal must be used in order to permit a mathematical analysis that assumes linearity. I've just skimmed the B&F sections you suggested and it makes more sense now, although the implication of Butler-Volmer's non-linearity for EIS was not explicitly stated in chap 10. Thats a good insight.

charge

Yes, excitation voltages on the order of several hundred mV's are commonly used in conductivity meters, as can be seen on an O-scope connected across a Thornton 770MAX and 230-211 probe. Higher voltages afford better signal-to-noise ratios, especially for DI water (18.2 megohm-cm) which doesn't pass much current. Of course the trick to getting accurate results is how the response signal is handled, as is true with most of today's uP and DSP-based instruments.

Reply to
Nick J.

Hi Enlevez

I understand the overvoltage principle and know that it applies to biased ac (10-20mV). But if you had NO bias and just applied AC peak to peak say volt what would the outcome be? If the p-p voltage was lower than the cell "overpotential" (not overvoltage) then there would be know effect apart from the oscillations of ions species?

Cheers

Wayne

--------------------------------------------------------------------------------------------------------

Reply to
WayneL

Hello,

"WayneL" a écrit dans le message de news: zJBCd.323$ snipped-for-privacy@newsfe5-win.ntli.net...

I don't understand your questions... For me, an humble electrochemist, there is no difference at all between overvoltage and overpotential... What's true for DC is also for AC. The only difference is that, if the AC signal's frequency is too high for the system to react, then the slow phenomenas won't appear. And that's why EIS is a powerful analytical technique : as AC signal's frequency progressively decreases, the slowlier phenomenas appear and impedance changes. In addition, 1V seems a too high AC signal to me to be sure that the E=f(I) evolution remains linear. However, this depends on the impedance of your system. For some milliohm impedance systems (ex: batteries), it will surely be too high. For very low conductivity systems (ex : very diluted solutions), it might not be enough...

Best regards.

--
Enlevez ".nospam" de mon adresse e-mail pour me répondre.
--------------------------------------------------------------------------------------------------------
Remove ".nospam" from my email address to reply me.
Reply to
n2mp

Hi According to the Electrochemistry Dictionary

formatting link

Does this help?

Wayne

overpotential The difference in the electrode potential of an electrode between its equilibrium potential and its operating potential when a current is flowing. The overpotential represents the extra energy needed (an energy loss that appears as heat) to force the electrode reaction to proceed at a required rate (or its equivalent current density). Consequently, the operating potential of an anode is always more positive than its equilibrium potential, while the operating potential of a cathode is always more negative than its equilibrium potential. The overpotential increases with increasing current density, see Tafel equation. The value of the overpotential also depends on the "inherent speed" of the electrode reaction: a slow reaction (with small exchange current density) will require a larger overpotential for a given current density than a fast reaction (with large exchange current density). Also referred to as "polarization" of the electrode. See also overvoltage. An electrode reaction always occurs in more than one elementary step, and there is an overpotential associated with each step. Even for the simplest case, the overpotential is the sum of the concentration overpotential and the activation overpotential.

overvoltage The difference between the cell voltage (with a current flowing) and the open-circuit voltage (ocv). The overvoltage represents the extra energy needed (an energy loss that appears as heat) to force the cell reaction to proceed at a required rate. Consequently, the cell voltage of a galvanic cell (e.g., a rechargeable battery during discharging) is always less than its ocv, while the cell voltage of an electrolytic cell (e.g., a rechargeable battery during charging) is always more than its ocv. Occasionally also referred to as "polarization" of the cell. The overvoltage is the sum of the overpotentials of the two electrodes of the cell and the ohmic loss of the cell. Unfortunately, the terms "overvoltage" and "overpotential" are sometimes used interchangeably.

--------------------------------------------------------------------------------------------------------

Reply to
WayneL

see The difference between a "galvanic cell" and "electrolyte cell"

Wayne

Reply to
WayneL

Does any of this apply if the systtem is not a cell (often incorrectly called a "battery") but is instead a conductivity meter?

Reply to
Guy Macon

Hello,

The difference between overvoltage and overpotential is very subtle and, as far as I'm concerned, it's the same thing and as said at the end of your definition, I use them interchangeably, since for me, it's obvious that both electrodes have overvoltage. To answer your previous question, if the applied AC signal is lower than OCV

  • summ (overpotential), then no current will flow in the solution because no reaction will occur. That's undoubtful. However, it comes to my mind that you'll be able to measure a "faradaic current" (as far as I remember, that's the term) in the external circuit, due to the charge/discharge of the Helmoltz double layer capacitance located at each electrode/solution interface, due to electrons and positive charges (deficit of electrons) accumulated on the electrode side of each interfaces that flow in the electric circuit. The ions in the electrolyte would not be involved in this phenomena.

Best regards.

--
Enlevez ".nospam" de mon adresse e-mail pour me répondre.
--------------------------------------------------------------------------------------------------------
Remove ".nospam" from my email address to reply me.
Reply to
n2mp

Hello,

"Guy Macon" a écrit dans le message de news: snipped-for-privacy@corp.supernews.com...

Yes, everything talked about above will happen in a conductivity meter, and in any system involving electrochemical reactions, as far as current is flowing in the solution. Now, since the latest discussions reminds me what I heared about the "faradaic current", a question raises in my mind, to which I have no answer : is the AC signal in a conductivity meter adjusted so that no electrochemical reaction occur at the electrode ? Then the current used for the response would be the faradaic current : it seems practically difficult, taking into account the wide range of conductivities that a standard conductivity meter would have to measure, but why not ?...

Best regards.

--
Enlevez ".nospam" de mon adresse e-mail pour me répondre.
--------------------------------------------------------------------------------------------------------
Remove ".nospam" from my email address to reply me.
Reply to
n2mp

I was under the impression that a conductivity meter measuring ultrapure water doesn't have any electrochemical reactions.

The one I designed (which was low-cost and low-accuracy) simply took a DC reading, then took another DC reading with polarity reversed. That worked fine.

From my webpage at [

formatting link
]:

"...One interesting technical challenge was measuring the resistivity of the water. 100% pure water has a bulk resistivity of around 18 Megohms per cubic centimeter. This resistivity is also strongly affected by the temperature of the water. A Microcontroller has the ability to measure resistance by measuring the rise-time of a digital pulse through an RC circuit, but the range it measures is usually only a few Kilohms, not the Megohms I needed to measure. I managed to get the range up to 250 Kilohms by careful capacitor selection, but the impedance of the Microcontroller pins wouldn't allow me to go any higher than that. I solved this problem by designing a parallel plate sensor with an area of 200 square centimeters and a distance between plates of one centimeter. This converted a bulk resistivity of 18 Megohms per cubic centimeter to a resistance of 90 Kilohms - well within the capability of the Microcontroller. I used the same Microcontroller to measure a thermistor to detect the water temperature and wrote a calibration table in software to compensate for temperature and sensor nonlinearity. The Microcontroller had limited memory, so I wrote the table with 11 temperature values from

0 degrees C to 100 degrees C in 10 degree steps and 21 resistivity values from 0 ohms to 20 Megohms in 1 Megohm steps. From these values, I extrapolated 1 degree and 0.1 Megohm steps, which were displayed on the LCD. The same sensor also served as a water level detector; when the tank level got too low, there was air between the plates instead of water and the open sensor reading would trigger the addition of more ultra-pure water. I wrote code so that if the water stayed on for too long without filling the tank it would give up and display an error. I used the same scheme so that if the water stayed on for too long without increasing the resistivity to an acceptable level, it would give up and display an error.

"Another interesting challenge occurred when the prototype started acting strangely. Over the course of several hours, the bulk resistivity of the ultra-pure water as measured by the Microcontroller would rise above the theoretical maximum of 18 Megohms per cubic centimeter, while the commercially available resistivity meter would show no change. Eventually the Microcontroller software would erroneously conclude that the water level was below the plates and would add ultra-pure water. As soon as a small amount of additional ultra-pure water was added, the reading would immediately go back to normal. I found that stirring the water made the problem go away - it only happened when the water was very still. It turns out that the ions in the ultra-pure water were moving away from one plate and collecting on the other plate. I programmed the Microcontroller to swap which plate was grounded and which plate was exited at each reading. At this point, the sensing system tracked the commercial meter perfectly."

--
Guy Macon http://www.guymacon.com/>
Reply to
Guy Macon

With due respect, wrong. Overpotential applies to a single electrode (reaction), while overvoltage is the sum of the overpotentials of the two electrodes' reactions. Thus (again drawing from cold fusion electrolysis), the overpotential at the cathode of Fleischmann & Pons' cathode was claimed to be 0.8 V, which gave them the figure of the astronomic 10^{18} atm H2 (if I remember the figure correctly). They didn't say how they measured that (it's nontrivial). The overvoltage over the whole cell was more, but they didn't state that, rightly not being interested in it. Cell voltage was, I think, around 4-10 V, so a large component there was solution (iR) resistance.

That is again wrong. At small ac polarisation, you'd get next to no Faradaic reaction, but you do get a current. Since the double layer capacity is large and the frequency is rather high, the interface impedance is very small compared with solution resistance. So a conductance meter in effect measure an almost pure iR.

That is not external. See the above. I will now not repeat this information.

--
Dieter Britz,   Kemisk Institut, Aarhus Universitet, Danmark.
Reply to
Dieter Britz

Hello everyone,

"Dieter Britz" a écrit dans le message de news: crjn9h$ep$ snipped-for-privacy@news.net.uni-c.dk...

Actually, we agree on physical phenomena... I was just saying that I was confusing (wrongly, I now realize) both overvoltage and overpotential terms, probably because english is not my mother language.

I may appear as narrow-minded but I'm teased by something. If you work with a working and a counter electrode, you must at least apply a signal which amplitude is above the value of the difference between each equilibrium potential. Otherwise, you need to apply your AC signal over a DC signal which will fix the steady state. That's how an impedance is measured in battery or electrolysis cell : you fix a steady state (often the battery OCV) on which you add your measuring AC signal. Now, if both electrodes are identical, then the difference (i.e. OCV) should be 0 (and I think they are in a conductivity meter), then I agree that, if overvolltage is negligible, then you'll only measure the i*R.

What I'm talking about happens only in the case where both electrodes are different, when you apply an AC signal below the difference of the equilibrium potentials and when the system can't work reversibly (like would work a battery for instance). Then no electrochemical reaction occur and no current flow in the electrolyte, but a current flows in the external circuit due to the charge/discharge of the double layer capacitances at the interfaces. That's in my electrochemistry courses but Bard & Faulkner should talk about that in the description of double layer capacitance.

Best regards.

--
Enlevez ".nospam" de mon adresse e-mail pour me répondre.
--------------------------------------------------------------------------------------------------------
Remove ".nospam" from my email address to reply me.
Reply to
n2mp

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.