Selective Discharge of ions


Cations: K+, Na+, Ca2+, Mg2+, Zn2+, Fe2+, Pb2+, H+, Cu2+, Ag+
Anions: SO42-, NO3- , Cl-, Br-, I-, OH-

In an electrolyte which has an electrolysis reaction going on, more than one type of ions with the same kind of charge will be attracted to the Cathode or Anode.
How then do we know which ion will be discharged?

The ion's position in the ELECTROCHEMICAL SERIES will help to tell us which one of the ions will be preferentially discharged.
The lower the position of the ion in the E.C.S. it will be more preferentially discharged as compared to the other ions above it in the E.C.S , at the electrode they are attracted to.

*Note: It would be good to memorize the E.C.S

2. Concentration

The higher the concentration of that particular type of ions (eg. Ca2+ is less concentrated as compared to Mg2+) it will help to promote its discharge.

3. Nature of electrode

The nature of electrodes used may influence the choice of ions discharged


(a) electrolysis of concentrated (aq) NaCl

At the cathode:

Platinum Cathode - hydrogen gas evolved
Mercury Cathode - Sodium is preferentially discharged and sodium amalgam is deposited