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A cell with iron and zinc

Both iron and zinc have negative standard potentials, -0.44 V and -0.76 V. The less negative one, iron, still becomes the cathode, and zinc becomes the anode. The cell voltage is still cathode minus anode, and it is positive. This is why a zinc anode can protect iron: zinc is oxidised rather than the iron. The same principle is used on ship hulls. Keep the signs. A minus minus a more negative number gives a positive voltage.

E^\circ = E^\circ(\mathrm{Fe}) - E^\circ(\mathrm{Zn})

iron is the cathode

Symbols

$\mathrm{Fe}^{2+}/\mathrm{Fe}$	-0.44	V
$\mathrm{Zn}^{2+}/\mathrm{Zn}$	-0.76	V

Example

Iron is the cathode and zinc is the anode.

$E^\circ = -0.44 - (-0.76) = 0.32$ V.

The sacrificial anode must have a lower potential than the metal it is to protect, and it is used up.

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Part of Chemistry 2: Electrochemistry.