Redox series
When there is more than one anion or cation ion present in the electrolyte, how do we decide which ion will be discharged. The ion that is preferentially discharged depends on its position in the redox series. The redox series is presented below. Students should take note of the position of the hydrogen ion. In the presence of almost all other ions except copper and silver ions, hydrogen ions will be preferentially discharged.
There is a similar series for anions, albeit a shorter one. Similarly for hydroxide ions, unless the other ion present is bromide or iodide ion, hydroxide ions will be preferentially discharged.
Concentration
The concentration of ions present may also affect which ion will be discharged. Take for example if both lead ion and hydrogen ion are attracted to the cathode and lead ion is present in much higher concentration, lead ion will be preferentially discharged although it is higher in the redox series. In other words, high concentration can promote the discharge of an ion higher in the redox series.
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