Co-deposition of hydrogen:

Consider a case in that a solution contains Cu²⁺ and Pb²⁺ ions.  Lead (II) starts to deposit at point A on the cathode potential curve.  Therefore lead (II) ion would co-deposit well before copper deposition was finished and would interfere.

Co-deposition of hydrogen during electrolysis often leads to spongy and flaky deposits which do not adhere to the electrode.  Like a condition is unsatisfactory for analytical reasons.  The use of depolarisers frequently improves the effectiveness of the procedure.  A depolariser is a substance that gets reduced at the cathode without gasification or gets oxidised at the anode before oxygen evolution and stabilises the potential of the working electrode through minimizing concentration polarisation.  Therefore copper deposited from a nitric acid solution is smoother and more adherent since nitrate ions act as cathodic depolarisers and avoid evolution of hydrogen. Commonly reducing agents such as hydrazine hydrochloride will act as anodic depolarisers.

The cathodic depolariser, nitrate ion gets decreased at a potential less negative than at point C as

NO^-₃ + 10H⁺ + 8e ? NH₄⁺ + 3H₂