Electrolysis is a process where electrical energy will change in chemical energy. The process happens in an electrolyte, a watery solution or a salt melting which gives the ions a possibility to transfer between two electrodes. The electrolyte is the connection between the two electrodes which are also connected to a direct current. This unit is called electrolyse cell and is shown in the picture below:
If you apply an electrical current, the positive ions migrate to the cathode while the negative ions will migrate to the anode. Positive ions are called cations and are all metals. Because of their valency they lost electrons and are able to pick up electrons. Anions are negative ions. They carry more electrons then normal and have the opportunity to give them up. If the cations have contact with the cathode, they get the electrons they lost back to become the elemental state. The anions react in an opposite way. If they have contact with the anode, they give up their superfluously electrons and become the elemental state. At the electrode, the cations will be reduced and the anions will be oxidated.
To control the reactions in the cell you can choose between different electrode materials. As well as you can use various electrolytes to special reactions and effects. The electrolyte contains the ions, which conduct the current.
For example, if you want to galvanize a piece of metal is used as cathode. The metal ions of the anode migrate to the cathode and become a thin layer on the surface on the part, which needs to be galvanised.
To produce very pure copper you have to choose a polluted copper anode, which will be cleaned by electrolysis. The copper get as Cu2+ ions in solution and precipitate on the cathode a more pure copper layer than before. The pollution from the anode will not react as the copper. Metals with a more noble character than copper will not react. They fall down to the ground a build the anode mud. Metals with a standard potential more less than copper will also be electrolysed and migrate at the cathode.
Also silver is cleaned by this way.
To predict the reactions of the electrolysis the “electrochemical electromotive series” is used. You can find in this table (see a fragment below) the elements ordered by their standard potentials (E0). The standard potential shows the capability, in regard to hydrogen ions, to give up electrons. In this table the standard potential of hydrogen is 0. The other elements have a positive or a negative standard potential. That means elements with a negative potential were easier to oxidate than hydrogen and elements with a positive potential were more difficult to oxidate than hydrogen.
Li/ Li+ -3,04 mV
K/ K+ -2,92 mV
Ca/ Ca2+ -2,87 mV
Na/ Na+ -2,71 mV
Mg/ Mg2+ -2,37 mV
Mn/ Mn2+ -1,18 mV
2 H2O/ H2+ 2OH- -0,83 mV
Zn/ Zn2+ -0,76 mV
Cr/ Cr3+ -0,74 mV
Fe/ Fe2+ -0,56 mV
Fe/ Fe3+ -0,44 mV
Cd/ Cd2+ -0,40 mV
Ti/ Ti2+ -0,34 mV
The table above shows the electrochemical electromotive series (standard potential at 25ºC)
Generally “positive” metals are noble opposite with the “negative” metals. With the exception of a few elements you can predict with the standard potential the occur in the earth’s crust. Noble metals are generally in a state of pure metal while nonnoble metals are normally found as a compound.
Following the thinking above the possibility to oxidise and also to reduce is a funktion of the standard potential. Following strong oxidants (will be reduced) have a negative standard potential and strong reducing agents have a high positive value.
Electrolyse processes are difficult to control. The galvanic corrosion process is normally an unwanted reaction, which causes damages in pipings, frames and a lot of other metal things. The process is convert of the electrolysis technic which aims from the potential differences of different used metals. To reduce the damage you can use for example an other anode, which will be more easily oxidated than the material. Also you can use an ion remove unit.