The Polarity of Overpotential Theodms Solomon Addis Ababa University, P.O.Box 1176, Addis Ababa, Ethiopia
I n a recent article in this Journal, Moran and Gileadi (1) discussed the problems encountered regarding the issue of polarity in electrochemistry. One of the sections in that article dealt with the polarity of overpotential, and a very clear and instructive presentation was given. Here I will elaborate on a certain aspect of the defmition and polarity of overpotential that is a cause of a lingering confusion. Defintions of Overpotential Overpotential is defined as where AT (or simply q) is the potential difference across an electrode/solution interface when current is flowing; and Acp,, (or simply cp,,) is the potential difference across the electrode solution interface a t equilibrium, when there is no current flowing.
When current is flowing, thereby producing more Zn2+, the potential rp increases from its reversible value. Thus, the anodic overpotential qa= (9- rprev) is positive. When one considers the anodic (oxidation) reaction Cu + c$+ + 2ethat takes place during charging, q, will still be positive. I t can similarly be shown that cathodic overpotential is always negative. Thus, we see a necessary consequence of the definition of overpotential: q, is always positive, whereas qcis always negative. In this light, one must be careful not to be confused by a statement in the article by Moran and Gileadi (I): Thr ovrrpotcnr~aldrvrloprd at rneh rlrctrode rhnngrs sign rn gomg from the drwm tl, the drwrng mode
Polarity of the Electrode Because a n electrochemical cell (in either the driving or driven mode) usually consists of an anode and a cathode, one can refer to an anodic overpotential q, or a cathodic overpotential q. As Moran and Gileadi (1)have pointed out, the polarity of a given electrode (e.g., the zinc electrode or the copper electrode in the Daniel cell) remains the same whether i t is discharging (driving) or being charged (driven). However, the electrode at which oxidation takes placeby definition, the anode-bewmes the cathode (i.e., where reduction occurs) when the cell is being charged. Taking the Daniel cell a s a n example, we can distinguish between the following two cases.
Discharging Zn + cu2++2n2++ CU Charging Cu + zn2+
