Page 377 - Physical chemistry understanding our chemical world

344 ELECTROCHEMISTRY

Figure 7.19 The electric eel (Electrophorus electricus) is a long, thin fish (3–5 feet) capable of
delivering an electric shock of about 600 V. (Figure reprinted from Ions, Electrodes and Membranes
´
by Ji` ri Koryta. Reproduced by permission of John Wiley and Sons Ltd)
of self-protection or for hunting. The eel either stuns a possible aggressor, or becomes
an aggressor itself by stunning its prey, prior to eating it.
Fundamentally, the eel is simply a living battery. The tips of its head and tail
represent the poles of the eel’s ‘battery’. As much as 80 per cent of its body is an
electric organ, made up of many thousands of small platelets, which are alternately
super-abundant in potassium or sodium ions, in a similar manner to the potentials
formed across axon membranes in nerve cells (see p. 339). In effect, the voltage
comprises thousands of concentration cells, each cell contributing a potential of about
160 mV. It is probable that the overall eel potential is augmented with junction
potentials between the mini-cells.
The eel produces its electric shock when frightened, hungry or when it encounters
its prey. The shock is formed when the eel causes the ionic charges on the surfaces
of its voltage cells to redistribute (thereby reversing their cell polarities), and has the
effect of summing the emf s of the mini-cells, in just the same way as we sum the
voltages of small batteries incorporated within a series circuit. The ionic strength of
seawater is very high, so conduction of the current from the eel to its prey is both
swift and efficient.

Battery terminology

A battery is defined as a device for converting chemical energy into electrical energy.
A battery is therefore an electrochemical cell that spontaneously produces a current
when the two electrodes are connected externally by a conductor. The conductor will
be the sea in the example of the eel above, or will more typically be a conductive