Purifcation of Metals by Electrolysis

Galvanic cells and batteries use chemistry to produce electricity, but the opposite is also possible. We can use electricity to drive chemistry. This process is called electrolysis and one of the reasons we do it is to purify metals from mixed ore samples.

Imagine that you were digging in your backyard and found a sample of gold. You might think that you had struck pay-dirt, but if the gold is mixed with other metals, like zinc, then you might not be able to sell it. What you need to do then, is to purify the sample. One way to do so would be through electrolysis.

We’ll start with the setup below.

In this, we have a container holding a solution of gold III nitrate. We’ve shown only the gold III ions since the nitrate ions will not be involved in the chemistry. On the left side we have a wire of pure gold connected to the negative end of our power source. The right side is our impure gold sample connected to the positive end of the power source.

On the positive side, several things are happening. Zinc atoms at the surface can lose 2 electrons and become \(Zn^{+2}\) ions (\(Zn \rightarrow Zn^{+2} + 2~e^{-1}\)) and gold atoms on the surface can lose three electrons to become \(Au^{+3}\) ions (\(Au \rightarrow Au^{+3} + 3~e^{-1}\)). BOTH of these reactions can and do occur. Which is happening at any given moment is determined in large part by which atoms are at the surface of the metal sample.

The negative side is where the “magic” occurs. Initially, only gold III ions are present in solution, but very quickly, the solution will contain both gold III ions and zinc ions. BOTH of these ions are attracted to the negative wire and both of these ions can take in electrons and become neutral metal atoms. But, nature doesn’t look at these reactions equally. Here are the reactions and their reduction potentials:

\(Au^{+3} + 3~e^{-1} \rightarrow Au~~~~~~~~~~1.50~V\)

\(Zn^{+2} + 2~e^{-1} \rightarrow Zn~~~~~~~~~~-0.76~V\)

We know that the more positive a voltage is, the more “natural” the reaction is, that it the more likely it is to occur. In this case there is no contest. Although both the gold and the zinc will react on the right (determined by physical arrangement of the ions) only the gold will form on the left. Thus, the impure metal on the right will slowly dissolve and the wire on the left will grow through the addition of pure gold.