In many applications, a very pure form of a specific metal is required. One method used to purify a metal is called electrorefining. It uses a Type 3 Electrolytic Cell

The most important thing to know for electrorefining is (click) the anode is an impure sample of the metal we want to refine. Remember the anode is connected to the positive terminal of the power supply. Power Supply + –

And the cathode must be a pure sample of the metal we want to refine. Remember, the cathode is attached to the negative terminal. In this video we’ll use electrorefining of copper as an example. Power Supply + –

On the impure anode we’ll represent an assortment of atoms, mainly copper, but with some impurities Power Supply + – Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE

And on the pure cathode, we’ll represent all copper atoms, as this is pure copper. Power Supply + – Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE

We’ll add some aqueous copper sulphate to this cell. If we’re going to purify copper, we must have copper (2+) cations in the electrolyte. Power Supply + – Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE SO 4 2– O H H H H O H H O H H O H H O H H O H H O H H O O H H Cu 2+

Neither water nor sulphate ions will react in this cell, so we’ll remove them from our diagram. Power Supply + – Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE SO 4 2– O H H H H O H H O H H O H H O H H O H H O O H H H H O Cu 2+

We’ll focus on the anode and we’ll add (click) a section of the right side of our reduction table. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Cu 2+

We’ll zoom in on gold, (click) copper, and (click) zinc Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Cu 2+

We’ll just point out here, (click) that we use the solid copper at 0.34 because this half-reaction represents the reduction of the more stable copper ion, Cu 2+. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Cu 2+

We don’t use the one at 0.52 because this represents the reduction of the Cu + ion, which is unstable in aqueous solution. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Cu 2+ X

Copper is the metal we’re purifying, so the impure sample is mainly copper atoms. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Cu 2+

The gold atoms we added can represent (click) any metals that are above copper on the right side of the reduction table. This includes gold, silver, and mercury Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Cu 2+

Anything we say about gold on this video applies equally to all metals above copper on the right side of the table. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Cu 2+

The zinc atoms we added can represent (click) any metals that are below copper on the right side of the reduction table. These include metals like lead, tin, nickel, cobalt, iron and chromium. Even metals below zinc, that are not shown on this diagram, such as aluminum or manganese. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Cu 2+

Anything we say about zinc in this video applies equally to all metals below copper on the right side of the table. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Cu 2+

Now, if we consider gold, copper, and zinc, we see that (click) gold has an oxidation potential of –1.50 volts (click) copper has a oxidation potential of –0.34 V, and (click) zinc has an oxidation potential of Volts Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Ox. Pot. = V Cu 2+

The question now is, which one of these metals will be oxidized first? Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Power Supply + – Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Ox. Pot. = V Which one of these will be oxidized first? Cu 2+

The answer is, zinc will be oxidized first. It has the highest oxidation potential of all three and is lowest on the right. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Ox. Pot. = V Zn will be oxidized first Power Supply + – Cu 2+

Looking at this zinc atom, it will lose two electrons, to become a Zn 2+ ion, Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Ox. Pot. = V Zn will be oxidized first Power Supply + – Zn 2+ e–e– e–e– Cu 2+

Which will then leave the electrode and go into the electrolyte. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Ox. Pot. = V Zn will be oxidized first Power Supply + – Zn 2+ Cu 2+

The half-reaction for the oxidation of zinc is Zn (s)  Zn e –. Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Ox. Pot. = V Zn will be oxidized first Power Supply + – Zn 2+ Zn (s)  Zn e – Cu 2+

Additional zinc atoms will be oxidized to zinc 2+ ions Au Cu Au Zn Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Ox. Pot. = V Zn will be oxidized first Zn 2+ Zn (s)  Zn e – Zn 2+ Power Supply + – e–e– e–e– Cu 2+

Which will leave the electrode Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Ox. Pot. = V Zn will be oxidized first Zn 2+ Zn (s)  Zn e – Zn 2+ Power Supply + – Cu 2+

This will happen until all of the zinc atoms are gone, and zinc no longer remains as an impurity in the impure copper anode. Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Ox. Pot. = V Zn will be oxidized first Power Supply + – Zn 2+ Zn (s)  Zn e – Zn 2+ Cu 2+

The metal with the next highest oxidation potential is Cu, with an oxidation potential of –0.34 volts. Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Cu will be oxidized next Power Supply + – Zn 2+ Cu 2+

So a copper atom will be oxidized to a Copper 2+ ion, which will leave the electrode and go into solution. Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Cu will be oxidized next Power Supply + – Zn 2+ e–e– e–e– Cu 2+

The half-reaction for the oxidation of copper atoms at the anode is Cu  Cu e –. Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Cu will be oxidized next Power Supply + – Zn 2+ Cu 2+ Cu (s)  Cu e –

Cu 2+ Copper atoms continue to be oxidized into copper (2+) ions and go into the electrolyte solution Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ e–e– e–e– Cu (s)  Cu e –

Cu 2+ click Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ e–e– e–e– Cu (s)  Cu e –

Cu 2+ click Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ e–e– e–e– Cu (s)  Cu e –

Cu 2+ Because gold has such a low oxidation potential, it cannot be oxidized as long as copper atoms are still present in the anode. Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ Cu (s)  Cu e –

Cu 2+ As all the copper atoms surrounding gold atoms are oxidized and leave the electrode, there is nothing left to hold the gold atoms up, (click) so they will simply fall down onto the bottom of the container. Au Cu Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ Cu (s)  Cu e –

Cu 2+ It is important to remember that this gold has not been oxidized. It is still a neutral atom of gold metal. Metals above copper on the right side of the table will fall off of the anode in metallic form and collect below it in the container. Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ Au This gold has not been oxidized. It is still an atom of gold metal Cu (s)  Cu e –

Cu 2+ A combination of metals that are above copper on the right side of the table, which do not get oxidized, such as silver and gold, along with non-metallic impurities that were in the copper, will collect underneath the anode. This (click) is often called anode sludge. Later on, this is refined to extract the precious metals from it. Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ Au The anode “sludge” or anode “slime” Cu (s)  Cu e –

Cu 2+ Copper atoms continue to be oxidized to copper 2+ ions that go into the solution Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ Au Cu 2+ e–e– e–e– Cu (s)  Cu e –

Cu 2+ (click) Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ Au Cu 2+ e–e– e–e– Cu (s)  Cu e –

Cu 2+ ( Click ) Au Cu IMPURE ANODE PURE CATHODE Ox. Pot. = –1.50 V Ox. Pot. = –0.34 V Power Supply + – Zn 2+ Cu 2+ Au Cu 2+ e–e– e–e– Cu (s)  Cu e –

Cu 2+ Now we’ll focus on the species above copper on the right side of the table Au Cu IMPURE ANODE PURE CATHODE Power Supply + – Zn 2+ Cu 2+ Au Cu 2+

We had left sulphate and water out of our diagram, but remember they are still present. Au Cu IMPURE ANODE PURE CATHODE Power Supply + – Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Cu (s)  Cu e –

Cu 2+ Sulphate has a very low oxidation potential, of –2.01 volts. And (click) water with it’s overpotential effect, behaves like its oxidation potential is about –1.38 volts. Au Cu IMPURE ANODE PURE CATHODE Power Supply + – Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H2OH2O H H O Cu (s)  Cu e –

Cu 2+ So we might think that after copper has finished oxidizing, that (click) water will be the next species to oxidize. Au Cu IMPURE ANODE PURE CATHODE Power Supply + – Zn 2+ Cu 2+ Au Cu 2+ H2OH2O SO 4 2– H H O

Cu 2+ However, (click) the whole process is STOPPED before all the copper has been oxidized. So neither the (click) water, the (click) gold, nor the (click) sulphate, will get to be oxidized. Au Cu IMPURE ANODE PURE CATHODE Zn 2+ Cu 2+ Au Cu 2+ H2OH2O SO 4 2– H H O The process is stopped before all the Cu (s) has oxidized SO 4 2– Power Supply + –

Cu 2+ So once the process is stopped, Au Cu IMPURE ANODE PURE CATHODE Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – The only cations in the solution are:  Cations of metals below Cu on the table (Zn 2+ )  Cu 2+ cations

Cu 2+ The only cations in the solution are: Au Cu IMPURE ANODE PURE CATHODE Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – The only cations in the solution are:  Cations of metals below Cu on the table (Zn 2+ )  Cu 2+ cations

Cu 2+ Cations of metals below Cu on the table like (Zn 2+ ), Au Cu IMPURE ANODE PURE CATHODE Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – The only cations in the solution are:  Cations of metals below Cu on the table (Zn 2+ )  Cu 2+ cations

Cu 2+ And Cu 2+ cations Au Cu IMPURE ANODE PURE CATHODE Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – The only cations in the solution are:  Cations of metals below Cu on the table (Zn 2+ )  Cu 2+ cations

Cu 2+ There are no cations of gold or any other metals above Cu (s) on the table. Au Cu IMPURE ANODE PURE CATHODE Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – The only cations in the solution are:  Cations of metals below Cu on the table (Zn 2+ )  Cu 2+ cations  There are no cations of Au or any other metals above Cu (s) on the table.  These will not be oxidized.

Cu 2+ These will not be oxidized, so they will not form any cations. They remain as neutral atoms. Au Cu IMPURE ANODE PURE CATHODE Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – The only cations in the solution are:  Cations of metals below Cu on the table (Zn 2+ )  Cu 2+ cations  There are no cations of Au or any other metals above Cu (s) on the table.  These will not be oxidized.

Cu 2+ Now, we’ll reveal the cathode again Au Cu IMPURE ANODE Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – PURE CATHODE

Cu 2+ Remember, reduction of Cations occurs at the Cathode Au Cu IMPURE ANODE Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – Reduction of Cations occurs at the Cathode PURE CATHODE

Cu 2+ The reduction potential of Cu2+ ions is volts, Au Cu Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – IMPURE ANODE PURE CATHODE

Cu 2+ Is considerably higher than that of Zn 2+, at –0.76 volts. Au Cu Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – IMPURE ANODE PURE CATHODE

Cu 2+ So Cu2+ ions will be reduced at the cathode Au Cu Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – IMPURE ANODE PURE CATHODE

Cu 2+ But Zn 2+ ions will be not be reduced as long as Cu 2+ ions are present, because (click) Cu 2+ has a higher reduction potential than Zn 2+. So we’ll (click) remove zinc ions from our diagram. Au Cu Zn 2+ Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – IMPURE ANODE PURE CATHODE

Cu 2+ Remember, while the cell is operating, Au Cu Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – IMPURE ANODE PURE CATHODE

Cu 2+ Copper atoms are being oxidized at the anode and copper 2+ ions are continuously added to the solution, Au Cu Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – IMPURE ANODE e–e– e–e– Cu 2+ Cu (s)  Cu e – PURE CATHODE

Cu 2+ so Cu 2+ ions in the solution are NEVER depleted. Au Cu Cu 2+ Au Cu 2+ SO 4 2– H H O Power Supply + – IMPURE ANODE Cu 2+ Cu 2+ ions in the solution are NEVER depleted. Cu (s)  Cu e – PURE CATHODE

Cu 2+ Focusing on the cathode, Au Cu Cu 2+ Au Cu 2+ H H O IMPURE ANODE Cu 2+ Power Supply + – PURE CATHODE

Cu 2+ As the cell operates, Copper 2+ ions will be attracted to the negative cathode and will be reduced to a neutral copper atoms Au Cu Zn 2+ Cu 2+ Au Cu 2+ H H O IMPURE ANODE Cu 2+ Power Supply + – e–e– e–e– Cu PURE CATHODE

Cu 2+ The equation for the reduction of Cu 2+ is Cu e  Cu (s). Au Cu Zn 2+ Cu 2+ Au Cu 2+ H H O IMPURE ANODE Cu 2+ Power Supply + – Cu Cu e –  Cu (s) PURE CATHODE

Cu 2+ This process will continue… Au Cu Zn 2+ Cu 2+ Au Cu 2+ H H O IMPURE ANODE Cu 2+ Power Supply + – e–e– Cu e–e– Cu e –  Cu (s) PURE CATHODE

Cu 2+ And copper 2+ ions continue to be reduced to neutral copper atoms as more ions come in from the anode, where they are formed. Au Cu Zn 2+ Cu 2+ Au Cu 2+ H H O IMPURE ANODE Cu 2+ e–e– Cu e–e– Cu e –  Cu (s) Cu Cu 2+ Power Supply + – PURE CATHODE

Cu 2+ We see that only copper metal is produced and builds up on the pure copper cathode, increasing its mass. Au Cu Zn 2+ Cu 2+ Au Cu 2+ H H O IMPURE ANODE Cu 2+ Power Supply + – Cu Cu e –  Cu (s) Cu Cu 2+ Only Cu (s) can build up on the pure copper cathode, so the mass of the pure copper will increase as the electrolysis process continues PURE CATHODE

Cu 2+ So the impure copper anode loses mass as (click) the small amount of zinc it has gets oxidized, the (clk) large amount of copper it has oxidizes, and(clk)inactive metals drop off without being oxidized. Au Cu PURE CATHODE Zn 2+ Cu 2+ Au Cu 2+ IMPURE ANODE Cu 2+ Power Supply + – Cu Cu e –  Cu (s) Cu Cu 2+ Cu (s)  Cu e –

Cu 2+ Looking at the cathode, (click) ONLY Cu 2+ cations from the solution are reduced and stick to its surface. So the pure cathode stays pure increasing in mass and size. In this way, impure copper is purified using a type 3 electrolytic cell. Au Cu Zn 2+ Cu 2+ Au Cu 2+ IMPURE ANODE Cu 2+ Power Supply + – Cu Cu e –  Cu (s) Cu Cu 2+ Cu (s)  Cu e – PURE CATHODE