Iron, the major reactive component of steel, has a standard reduction potential of negative 0.447 volts. Using the standard electrode potentials shown in the table, determine which of the following metals could be used as a sacrificial anode in the cathodic protection of an underground steel storage tank. (A) Zn, (B) Ag, (C) Au, (D) Cd, or (E) Ni.
Before we press on with the question, let’s just make sure we know the names for all these metals. Zn is zinc, Ag is silver, Au is gold, Cd is cadmium, and Ni is nickel. And the chemical symbol for iron is Fe. The question gives us the standard reduction potential for iron. This is the same as the standard electrode potential, as electrode potentials are always written in their reduction form. The scenario we’ve been given is an underground steel storage tank. And our job is to find an appropriate material that will prevent it from rusting away. We’ll do this by picking a suitable material for the sacrificial anode.
The sacrificial anode is the material that oxidizes instead of the iron. The iron is protected from rusting because it’s turned into a cathode by the electrochemical cell. A cathode is where reduction occurs. By turning the iron into a cathode, we either prevent or reverse any oxidation. We can bolt a lump of our sacrificial material anywhere that has electrical contact with our underground steel storage tank. Instead of our underground steel storage tank rusting away, our sacrificial anode will gradually become oxidized instead.
Our task is to pick which of the five metals could potentially be used in this application. Standard electrode potentials are the potential you’d expect if a half equation were paired with a standard hydrogen electrode. The lower the value of a standard electrode potential, the more likely the reaction will go in reverse. The conversion of H+ ions into H2 gas is our standard measure for the standard electrode potential, and it’s given the value of 0.00 volts. Any standard electrode potential half equation will go in the opposite direction compared to hydrogen if the electrode potential is negative. So if we pair together the half equations for zinc and hydrogen, we would expect zinc to transform into Zn2+ and H+ to transform into H2. And the case would be very similar for cadmium and nickel.
Since zinc would be being oxidized, it will appear on the left in our cell notation. And the potential for our cell would be the standard electrode potential for hydrogen, 0.00 volts, minus the standard electrode potential for zinc, negative 0.7618 volts, giving us a positive cell potential of 0.7618 volts. This positive value indicates that the reaction is indeed spontaneous. But in this case, we are not looking at hydrogen. We are looking at iron, which already has a very negative standard electrode potential. So what we need to find is a metal that when paired with iron, will make sure that iron is the cathode, even though iron has a very negative standard electrode potential.
What if we used silver? Silver has a standard electrode potential of positive 0.7996 volts. If we try to insert it as the cathode, we actually end up with a negative cell potential, which indicates that actually things would happen in reverse. A negative cell potential is nonspontaneous because it’s energetically unfavorable for the reaction to occur. So if we did mix iron and silver together, iron would act as the sacrificial anode for the silver, not the other way around. So silver and gold are definitely out. Their standard electrode potentials are too positive.
But if we look a cadmium and nickel, even though their electrode potentials are negative, they’re still more positive than the standard reduction potential of iron. So they won’t act as sacrificial anodes. This leaves zinc as our only sensible candidate. When we insert zinc as our anode, we end up with a cell potential that’s positive at 0.3148 volts. This means that if we bolted some zinc onto the side of our underground storage tank, any iron oxidation that occurs would be reversed. And instead, the zinc would corrode.
None of the other metals have a standard electrode potential that is low enough to achieve this effect. In fact, it will be the iron acting as a sacrificial anode for them rather than the other way around.