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I have two identical graphite strips of dimensions 3.5 cm by 1 cm. Both are 3/4 submerged in aqueous $\ce{NaCl}$ solution (0.34 mol/dm$^3$, 2% wt) separated by a distance of 7 cm. I have attached crocodile clips to each strip and a voltmeter is used to record the potential difference between the two graphite strips.

I have found that:

A. There is a potential difference that ranges between 1 mV and 100 mV; and

B. This potential difference can change quite dramatically within that range over time periods of an hour.

Does anyone know of any chemical or physical reason for these potential differences?

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What type of voltmeter are you using? to verify the answer to my question, you need to determine 1) the precision and 2) the detection limit of the instrument .

The precision is a measure of the error in a typical measurement. The smaller the standard deviation in repeat measurements, the higher the precision. To get a guess at the precision, prepare a system that should generate a known potential difference in the same range as your current measurements (like 100 mV). If the variation in that value is similar to what you are seeing, then you are recording the noise.

The detection limit is the lowest value your instrument can reliably measure. Generally, you cannot reliably measure values smaller than the twice the precision of your instrument. If you determine a standard deviation of 50 mV when determining precision, then 100 mV is your the detection limit. If this is true, you are recording the noise.

The precision and detection limit of an instrument depend on quality (price), but also on purpose. A voltmeter designed to be used by an electrician will have high precision in the range 1-200 V, but low precision below 1 V (because that range is not as important). A sensitive voltmeter used for electrochemical measurements may have high precision between 1 uV and 1 V, and behave miserably outside that range.

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  • $\begingroup$ I am using a Keithley 2000 with detection limit of 0.1μV. I have also examined the precision and it seems to be consistent to around 0.1 mV, so I dont think it is likely to be noise. $\endgroup$ – joshlk Mar 10 '13 at 20:07
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    $\begingroup$ It is also possible that the previous history of the graphite strips is important. The potential difference observed might, in part, be due to impurities in the graphite. $\endgroup$ – Paul J. Gans Apr 8 '13 at 17:29
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    $\begingroup$ @PaulJ.Gans - that is a very good point. Graphite is not an inert material. $\endgroup$ – Ben Norris Apr 8 '13 at 19:07
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It might be asymmetry potential. Asymmetry potentials are due to differences between the electrodes themselves, and are not due to differences in potential within the NaCl solution.

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Also don't rule out thermal gradients at the points where the probes/wiring are attached to the graphite strips. You can be measuring thermal electric effects as well.

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What is the input impedance of your voltmeter? How much DC current does it pass to measure a potential?

In theory, the input impedance is infinite and the DC current is zero, but in practice, this isn't the case.

In the NaCl solution with graphite electrodes, any DC current has to pass through electrolysis or impurities. This will lead to the behavior you are seeing with the impurities making the potential fluctuate.

What are you really after?

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