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From Wikipedia:

The potential is applied between the working electrode and the reference electrode while the current is measured between the working electrode and the counter electrode.

Why do you need both a reference electrode and a counter electrode? Why measuring the current between the reference electrode and the working electrode is not enough?

Three-electrode setup: (1) working electrode; (2) auxiliary electrode; (3) reference electrode

Three-electrode setup: (1) working electrode; (2) auxiliary electrode; (3) reference electrode.

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    \$\begingroup\$ Because driving current through electrode 3 would cause chemical reactions to happen at its surface which would mess with your voltage reading. \$\endgroup\$
    – user253751
    Sep 9 '15 at 23:56
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    \$\begingroup\$ @immibis, why electrode 3 is not placed next to electrode 2? the potential difference depends on the distance between 2 and 1, no? \$\endgroup\$
    – Sparkler
    Sep 10 '15 at 0:00
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Normally, you want the applied potential difference between working and reference electrode not to change. When you use two electrode setup, as chemical reaction occurs, there is some current (faradaic) passing through reference electrode which when combined with reference electrode resistance changes the potential at which reference electrode stays, thus changing the potential difference. Therefore, third electrode is introduced to the electrochemical cell. In Cyclic voltametry (CV), the potential is not changing based on reaction occuring (you determine the upper and lower limits for scan). The position of electrodes for CV is important when the solvent is resistive and it is rule of thumb to put reference close to working and not to put counter between them.

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