Selective Discharge of Ions: Introduction, Cations, Anions

Selective Discharge of Ions: Electrolysis is a process employed to separate an ionic compound into its ions. The ionic compound is taken in an electrolytic cell comprising a positively charged anode and a negatively charged cathode. The cations that are positively charged species move towards the cathode, and anions that are negatively charged species move towards the anode. But what happens if there are more than one charged species in an electrochemical cell. Let’s explore more about it in this article.

During electrolysis, positively charged cations are drawn to the negatively charged electrode (cathode), while negatively charged anions are drawn to the positively charged electrode (anode). When multiple cations or anions reach their corresponding electrodes, only one of them is discharged. As a result, there is a selection of ions that will be discharged on an electrode. During electrolysis, this is referred to as selective discharge of ions.

Selective Discharge of Cations

The ease with which cations can be discharged is determined by the position of metals in the reactivity series.

The more reactive the metal, the less likely the metal ions are to be discharged preferentially at the cathode during electrolysis.

Potassium and sodium will not be released and will remain in the electrolyte solution. This is due to the fact that they are reactive metals.

The hydrogen ion, as well as ions from less reactive metals like copper and silver, will be discharged first.

In an aqueous solution, hydrogen ions will be preferentially discharged over the ions of the metals above it in the reactivity series. In such cases, hydrogen gas will be liberated at the cathode.

Likewise, if the ions come from a metal that is placed below hydrogen in the reactivity series, then the metal ions will be preferentially discharged. In such cases, metal will be produced.

During electrolysis of aqueous solutions, if a cation has a higher electrode potential (reduction potential) than water, i.e. it has a reduction potential of more than $–0.83\mathrm{V}$ in the standard electrode potential list. It is preferentially discharged at the cathode (e.g. electrolysis of copper and silver salts). However, for cations whose electrode potentials are less than $–0.83\mathrm{V},{\mathrm{H}}_2$ gas is liberated at the cathode (e.g. $\mathrm{N}\mathrm{a},\mathrm{K},\mathrm{C}\mathrm{a}$ salts, etc.).

Selective Discharge of Anions

As the discharge of cations was based on reduction potential, the discharge of anions was based on oxidation potential. The negative reduction potential gives oxidation potential. The anion with higher oxidation potential than water, i.e., it has a reduction potential of more than $–1.23\mathrm{V}$, is preferentially discharged at the anode, e.g. $\mathrm{B}{\mathrm{r}}^{–}(–1.09\mathrm{V}),{\mathrm{I}}^{–}(–0.53\mathrm{V})$. However, for anions that have lower oxidation potential than water, i.e. it has reduction potential less than $–1.23\mathrm{V}$, oxygen gas is liberated at anode, e.g. ${\mathrm{F}}^{–}(–2.87\mathrm{V}),\mathrm{S}{{\mathrm{O}}_4}^{2–}(–2.01\mathrm{V})$, etc.

The order of ease of discharge (oxidation) of certain anions at anode-

${\mathrm{F}}^{–}<\mathrm{S}{{\mathrm{O}}_4}^{2–}<\mathrm{N}{{\mathrm{O}}_2}^{–}<\mathrm{C}{\mathrm{l}}^{–}<\mathrm{B}{\mathrm{r}}^{–}<{\mathrm{I}}^{–}<\mathrm{O}{\mathrm{H}}^{–}$

Ease of discharge of anions increases down the list:

• Sulfate ion, $\mathrm{S}{{\mathrm{O}}_4}^{2–}$
• Nitrate ion, $\mathrm{N}{{\mathrm{O}}_3}^{–}$
• Chloride ion, $\mathrm{C}{\mathrm{l}}^{–}$
• Bromide ion, $\mathrm{B}{\mathrm{r}}^{–}$
• Iodide ion, ${\mathrm{I}}^{–}$
• Hydroxide ion, $\mathrm{O}{\mathrm{H}}^{–}$

Sulfate and nitrate ions will remain in the electrolyte solution as they are not electrolysed during electrolysis.

Among all anions, hydroxide ions have the highest ease of discharge. This is because it gives out electrons most readily and can be easily oxidised during electrolysis. Oxygen gas will be produced.

The half equation for the discharge of hydroxide ions:

$4\mathrm{O}{\mathrm{H}}^{–}(\mathrm{a}\mathrm{q})\to 2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})+{\mathrm{O}}_2(\mathrm{g})+4{\mathrm{e}}^{–}$

If the electrodes used in the electrolysis are inert, the ions that are discharged preferentially are determined by three criteria. These are the following:

i) Position of the Metallic Ion in the Electrochemical Series
Assuming that all other factors remain constant, an ion positioned lower in the electrochemical series is discharged preferentially at the respective electrode over all other ions in the series.

ii) Concentration of Ions in the Electrolyte 
The higher the negative ion concentration in the electrolytic solution, the greater its probability of being discharged at the anode.
For example, in the electrolysis of concentrated $\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}$ solution, the ions available in the electrolyte are:

• Sodium ion, $\mathrm{N}{\mathrm{a}}^{+}$
• Chloride ion, $\mathrm{C}{\mathrm{l}}^{–}$
• Hydrogen ion, ${\mathrm{H}}^{+}$ (due to partial dissociation of water)
• Hydroxide ion, $\mathrm{O}{\mathrm{H}}^{–}$ (due to partial dissociation of water)

As there are two negative ions, both chloride and hydroxide ions will be attracted to the positive anode. However, hydroxide ions should be preferentially discharged as it has the highest ease of discharge compared to other anions.

But, a concentrated solution consists of a lot more chloride ions than hydroxide ions. Therefore, chloride ions are preferentially discharged over the hydroxide ions at the anode.

iii) Nature of the Electrode
If an inert electrode is utilised, such as graphite or platinum, the electrode has no effect on the preferential discharge of an ion at it.

If an active electrode such as $\mathrm{C}\mathrm{u},\mathrm{A}\mathrm{g},\mathrm{N}\mathrm{i}$ is used, then the electrode plays an important role in deciding the ions that will be preferentially discharged at the electrodes. In such cases, anions migrate to the anode but do not get discharged. Instead, the active anode itself loses electrons and form ions.

Summary of Selective Discharge of Ions

The elements in an electrochemical series are grouped in order of their reduction potential. The reduction potential of an element indicates how easy it is to reduce that element. The cation with the highest reduction potential will be the first to receive electrons and be discharged. Similarly, anions with a low reduction potential will readily lose electrons and discharge. The electrolyte is an aqueous solution that contains more than one type of cation and anion. During the electrolysis process, however, only one cation and one anion are released preferentially or selectively.

Important Questions on Selective Discharge of Ions

FAQs on Selective Discharge of Ions

Q.1. What is the preferential discharge of ions?
Ans: Preferential means that in the case of more than one kind of cations and anions in the electrolyte, the one with higher concentration or at a lower position in the activity series will get discharged first.

Q.2. What is discharge potential?
Ans: The reduction potential of an atom determines the discharge potential. It is an ion’s tendency to gain electrons and become reduced. To put it another way, it’s the potential at which an ion is released.

Q.3. What are the factors that influence the selective discharge of ions?
Ans: i) Position of the metallic ion in the electrochemical series: Assuming that all other conditions remain constant, an ion positioned lower in the electrochemical series is discharged preferentially at the respective electrode over all other ions in the series.
ii) Concentration of ions in the electrolyte: The higher the concentration of a negative ion in the electrolytic solution, the greater is its probability of being discharged at the anode.

Q.4. Is discharge potential the same as oxidation potential?
Ans: Their discharge potential measures the ability of ions to discharge at an electrode. The ability of a chemical species to lose/acquire electrons and hence be oxidised/reduced is measured by its oxidation/reduction potential.

Q.5. What is the difference between reduction potential and oxidation potential?
Ans: The main distinction between oxidation potential and reduction potential is that oxidation potential shows a chemical element’s tendency to be oxidised. On the other hand, reduction potential refers to a chemical element’s tendency to be reduced.

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