1.5 Factors affecting Preferential Discharge
Factors affecting products of Electrolysis

There are TWO considerations:
A. IF the electrolyte is MOLTEN   [ MX ] 
(and carbon/graphite INERT electrodes are used)
the electrolytic reactions are simple and straightforward.

During electrolysis,
M positive ions migrate to Cathode:  M + (l) + e ==> M (l)
X -  negative ions migrate to Anode:   2X - (l) -  2e ==> X2 (g)
B. IF the electrolyte is AQUEOUS
the electrolytic reactions become more complicated.
Water comes into the picture as water ionises to form H + and OH - ions. Thus, selection of ions for preferential discharge is based on
the following three factors:
1. POSITION of the ION in the Electrochemical Series

Cations           K   Na   Ca    Mg    Al    Zn    Fe    Sn    Pb       Cu    Ag
graphic
Anions            SO4       NO3       CO3       Cl        Br        I         OH
graphic
Increasing ease of discharge
Note:
*If two or more positive ions migrate to the cathode, it is the ion lowest in the ECS
 (i.e. of the least electropositive metal) that is discharged preferentially.
Examples Copper ions are discharged in preference to Hydrogen ions; Hydrogen ions are discharged in preference to Sodium ions.
It will be recalled that the reactivity series gives the order of the tendency of metals to form ions. Metals at the top of the series have the greatest tendency to form ions and are therefore the most difficult to discharge at the cathode.

* If two or more negative ions migrate to the anode, the ion lower in the ECS is discharged preferentially.

2. CONCENTRATION of the ions in the electrolyte
IMPORTANT NOTE:  A very dilutesolution of any electrolyte,
on electrolysis, liberates oxygen at the anode.

However, if the concentration of a particular ion is high, then this can alter the preferential discharge.
For example, if dilute hydrochloric acid is electrolysed, hydrogen gas is produced at the cathode and oxygen gas at the anode. However, when concentrated hydrochloric acid is electrolysed, hydrogen gas is still liberated at the cathode, but chlorine gas (and NOT oxygen gas) is now produced at the anode instead. The higher concentration of the chloride ions favor their discharge over the hydroxide ions.
NOTE: Ions of Sodium, Potassium, Calcium, Sulfate and Nitrate ions are NOT discharged even if they are in high concentrations.
3. ELECTRODE material used
Inert electrodes (made of graphite or platinum) do not affect the products of electrolysis ; only  Reactive / Active electrodes do affect.
Example
Electrodes
Electrolyte
At Cathode
At Anode
Carbon (inert)
aqueous copper(II) sulfate
copper metal deposited
oxygen liberated
Copper (active)
aqueous copper(II) sulfate
copper metal deposited
copper electrode  dissolves
Created: 1 Dec 2006 / 1.25 pm