CSIR-NET Electrochemical Methods — Potentiometry and Voltammetry

By Aniket Bhardwaj · 3 October 2026 · CSIR-NET Chemistry

This is not the conductance article. Conductometric titrations and Kohlrausch's law of independent migration are covered separately on this site — this article is about the other half of electroanalytical chemistry: methods that measure potential (potentiometry) or current as a function of applied potential (voltammetry and polarography). Keeping these apart is exactly the distinction CSIR-NET examiners test for.

Potentiometry — finding the equivalence point from an E vs V curve

In a potentiometric titration, a cell potential (E) is measured continuously as titrant volume (V) is added. The equivalence point is the volume at which the potential changes most rapidly with volume added — found from the point where the first derivative, ΔE/ΔV, is maximum.

Worked example 1 — locating the equivalence point by the first-derivative method

Q. A potentiometric titration gives the following data near the endpoint. Find the equivalence-point volume.

V (mL)	E (mV)	ΔE/ΔV (mV/mL), midpoint V
19.0	200	—
19.5	260	120, at 19.25
20.0	400	280, at 19.75
20.5	540	280, at 20.25
21.0	600	120, at 20.75

The maximum ΔE/ΔV (280 mV/mL) occurs equally over the two central intervals, so the equivalence point is taken as the midpoint between them: (19.75 + 20.25) ÷ 2 = 20.0 mL. This bracketing approach — find the steepest interval(s), then average their midpoints — is exactly how a first-derivative endpoint is located from tabular titration data in an exam setting.

Cyclic voltammetry — the 59 mV/n reversibility test

A cyclic voltammogram scans the potential forward and backward across a redox couple's formal potential, recording current at each point. For an electrochemically reversible one-electron process at 298 K, the separation between the anodic and cathodic peak potentials is close to a fixed value, essentially independent of scan rate:

ΔE_p = E_p,a − E_p,c ≈ 0.059/n V (n = number of electrons transferred, 298 K)

Worked example 2 — testing a couple for reversibility

Q. A cyclic voltammogram of a one-electron redox couple shows E_p,a = +0.245 V and E_p,c = +0.186 V. Is the couple electrochemically reversible?

ΔE_p = 0.245 − 0.186 = 0.059 V = 59 mV.

For n = 1, the theoretical reversible separation is 0.059/1 = 59 mV — an exact match, so the couple behaves as electrochemically reversible under these conditions. If ΔE_p had instead grown noticeably larger than 59 mV as the scan rate was increased, that growth would indicate quasi-reversible or irreversible electron-transfer kinetics rather than a genuinely reversible couple.

Polarography and the Ilkovic equation

Classical polarography uses a dropping mercury electrode (DME); the diffusion-limited current follows the Ilkovic equation, which depends on the diffusion coefficient D, the mercury flow rate m, and the drop time t:

i_d ∝ n D^1/2 m^2/3 t^1/6 C

Different textbooks quote different numerical constants in front of this proportionality (607 and 708 both appear, depending on the exact unit convention used for D, m, t and concentration) — always check which convention a given source or question is using rather than assuming a single "correct" number.

Worked example 3 — how drop time affects diffusion current

Q. If the drop time of the DME is doubled while everything else is held constant, by what factor does the diffusion current change?

Only the t^1/6 term depends on drop time, so:

i_d,new / i_d,old = (2t)^1/6 / t^1/6 = 2^1/6 = 1.122

Doubling the drop time increases the diffusion current by only about 12.2% — the one-sixth power makes i_d only weakly sensitive to drop time, which is precisely why the DME gives reproducible currents even though individual mercury drops vary somewhat in their exact lifetime.

Common mistakes that cost marks

Confusing peak potential (E_p) with half-wave potential (E_1/2). E_p is a voltammetric (peak-shaped response) concept; E_1/2 is a polarographic (sigmoidal wave) concept — they are related but not the same quantity, and are read off different types of curve.
Treating ΔE_p = 59/n mV as valid at every scan rate. The criterion holds for a genuinely reversible couple; as scan rate increases, electron-transfer kinetics can make ΔE_p grow, signalling quasi-reversibility — the 59 mV benchmark is a test at a given (usually low) scan rate, not a universal constant.
Forgetting the reference electrode's own potential. A measured E value is relative to whatever reference electrode was used (SCE, Ag/AgCl, etc.); converting to a standard reduction potential on the SHE scale requires adding or subtracting the reference electrode's known potential.
Picking one Ilkovic-equation constant and presenting it as the only correct one. As noted above, 607 and 708 both appear in respected sources under different unit conventions — state which convention you are using.

Technique comparison table

Technique	What is measured	Typical electrode/setup
Potentiometry	Cell potential at (near) zero current	Indicator electrode + reference electrode
Cyclic voltammetry	Current vs. applied potential, forward and reverse scan	Working, reference and counter electrode
Polarography	Diffusion-limited current vs. potential	Dropping mercury electrode (DME)
Conductometry (separate topic)	Bulk solution conductance	Conductivity cell, AC bridge

Cross-check a Nernst-equation-based cell potential alongside a voltammetry question. The Nernst Equation calculator handles the concentration dependence and the 0.0592/n term at 298 K in seconds.

Open the Chemistry Calculator Suite →