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| Selective Discharge of Ions |
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| Some electrolytes contain more than one type of cations or anions. When one type of ions is discharged in preference to the other, under certain conditions of electrolysis, it is known as selective discharge of ions. |
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| The following are the factors that influence the selective discharge of ions: |
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| a) The position of the ion in the electrochemical series |
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| The ions that are lower in the electrochemical series get discharged in preference to the ones above them. |
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| For e.g., if a solution has potassium ions and copper ions, the copper ions will accept electrons, and get discharged as copper atoms first. The potassium ions will not be affected. |
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| b) Concentration of the ions in the electrolyte |
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| If an electrolyte contains a higher concentration of ions, which are higher in the electrochemical series than those that are lower, then these ions get discharged in preference to the lower ones. |
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| For e.g., a solution of sodium chloride in water contains two types of anions i.e., the chloride (Cl-) ions and the hydroxyl (OH-) ions. Hydroxyl ions are lower in the electrochemical series than chloride ions. But if the concentration of chloride ions is much higher than that of the hydroxyl ions then the chloride ions get discharged first. |
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| Electrolysis of NaCl Solution at different Concentration |
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| c) Nature of electrode |
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| The type of electrode used in electrolysis can affect its end result. If the electrodes used are very inert like platinum or carbon, they do not take part in the reaction and not affect its end result. However in the case of other materials being used as electrodes, they may take part in the reaction and affect the end products. |
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| For e.g., if copper sulphate solution is electrolysed using platinum anode, the product obtained at the anode will be oxygen. But if instead of platinum, copper is used as the anode, then the product obtained will be copper ions and not oxygen. |
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