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I'm trying to make a bioanode for use in a microbial fuel cell. I have a piece of carbon cloth in an acetate solution with river sediment. I'd like to know the potential of the anode, so I connected it to an Ag/AgCl reference electrode also in the sediment/acetate solution (note that ref electrode has 3M KCl, about 0.2V relative to SHE). Between the reference electrode and anode, I have a multimeter connected with the positive going to the Ag/AgCl reference electrode and negative going to the carbon cloth anode.

The multimeter reads +0.2V. Does this mean my anode potential is 0.4V (0.2V from multimeter reading + 0.2V for reference electrode correction)? OR, does this mean that my anode potential is 0V (-0.2V multimeter reading + 0.2V for reference electrode correction). I'm not clear if the positive side of the multimeter and negative side are connected correctly to the electrodes. If they are connected incorrectly, then the voltage sign should be flipped.

Thank you!

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Coming to your experiment, there is a nice description along with sufficient details which shows you how to make connections.

Microbial Fuel Cells & Bacterial Power https://agsci.oregonstate.edu/sites/agscid7/files/bioenergy/fuel_cell_interactive.pdf

Coming to the confusion: Electrochemists do not measure electrode potentials by using a multimeter as shown in general chemistry textbooks. The numbers shown on a multimeter display only qualitatively showing us that there is a potential difference between these two electrodes. Secondly, carbon cloth is the least reproducible electrodes in terms of its potential (heavily dependent on the history of how it was made). You won't find its values in standard tables. Therefore we cannot compare your experimental values with standard conditions (in the tables, the values are calculated under controlled conditions) because your system is nowhere close to standard conditions. It is like comparing apples and oranges.

An interesting part for you is, how to decide which one is cathode and which one is anode of your cell? Connect the electrodes in such a way that you get a positive potential difference on the multimeter. If you happen to get it, you have made the right connections i.e., negative terminal of your cell is connected to the negative terminal of your multimeter and so on.

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