# Electrochemistry Concentration Cells

> Concentration cells for JEE Chemistry with EMF from the Nernst equation, electrode and electrolyte types, and their use in measuring Ksp and unknown pH.

- Canonical URL: https://prepelephant.com/topics/jee/chemistry/electrochem-concentration-cells
- Exam / course: JEE · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Electrochemistry Concentration Cells", PrepElephant, https://prepelephant.com/topics/jee/chemistry/electrochem-concentration-cells

## Direct answer

A concentration cell squeezes electricity out of inequality: identical electrodes, different ion concentrations, so both standard potentials cancel and the EMF is purely logarithmic — Ecell = (0.0591/n) log(c2/c1) at 298 K, with the dilute side acting as anode and the concentrated side as cathode. The cell runs spontaneously until the two concentrations equalise, where the EMF decays to zero. Electrode-concentration variants use different gas pressures, and the same algebra doubles as a measuring instrument for the Ksp of a sparingly soluble salt or the pH of an unknown solution.

## What you must remember

- **Master formula:** Ecell = (2.303 RT/nF) log(c2/c1); at 298 K the factor is 0.0591/n volts, so a tenfold ratio delivers 59.1 mV for n = 1 and 29.5 mV for n = 2.
- **Direction rule:** oxidation at the dilute electrode, reduction at the concentrated one; electrons flow from the dilute to the concentrated side through the external circuit.
- **Why E° = 0:** both electrodes are the same couple, so their standard potentials subtract to zero — the EMF arises only from the activity ratio.
- **Electrode concentration cells:** Pt|H2(p1)|H+|H2(p2)|Pt gives E = (0.0591/2) log(p2/p1); the higher-pressure electrode is the cathode.
- **Ksp measurement:** couple Ag|AgCl(s), Cl−(known) against a normal Ag+/Ag electrode; the free [Ag+] = Ksp/[Cl−], and the measured EMF yields Ksp without any titration.
- **pH measurement:** a hydrogen electrode at unknown pH shifts by 0.0591 V per pH unit at 298 K against a fixed reference.
- **Liquid junction:** a salt bridge suppresses junction potentials; the exam formulas assume the bridge is in place.
- **Self-discharge:** as current flows, the anolyte concentration rises and the catholyte falls, so the EMF decays to zero at equality — this is a concentration-difference battery, not a chemical fuel.

## Building the EMF step by step

Set up Cu|CuSO4(0.001 M)||CuSO4(0.1 M)|Cu. The dilute left side is the anode (Cu → Cu2+ + 2e−) and the concentrated right side the cathode. The Nernst equation gives each electrode E = E° + (0.0591/2) log[Cu2+]; subtracting, the E° terms annihilate and Ecell = (0.0591/2) log(0.1/0.001) = 0.0295 × 2 = 0.059 V. Now deploy the same machinery to find the Ksp of AgCl. Build Ag(s)|AgCl(s), KCl(0.1 M)||AgNO3(0.01 M)|Ag: the right side is a plain Ag+/Ag couple, while the left side's [Ag+] is pinned at Ksp/0.1 by the suspended silver chloride. If the voltmeter reads 0.40 V, the ratio [Ag+]right/[Ag+]left = 10^(0.40/0.0591) ≈ 6.6 × 10^6, so [Ag+]left = 0.01/(6.6 × 10^6) ≈ 1.5 × 10^-9 M and Ksp = 1.5 × 10^-9 × 0.1 ≈ 1.5 × 10^-10 — the right order for AgCl, obtained purely from a potential difference.

## How JEE frames concentration cells

Main asks the direct template: identify the anode, write E = (0.0591/n) log ratio, compute. Two slips dominate — inserting E° values from tables (they cancel; a nonzero E° answer signals an error) and inverting the ratio (keep the cathode concentration in the numerator). Advanced dresses the same algebra in measurement contexts: the Ksp determination above, a hydrogen-electrode pH problem, or a cell with transference where the EMF carries the ion transport number. The classic paragraph question asks why a concentration cell cannot do work indefinitely — because the driving inequality erodes as the cell discharges, and at c2 = c1 the free energy change reaches zero. One further tell: these cells convert no chemical species; they only redistribute concentrations, which is why their thermodynamics is entirely Nernst logarithms.

## Frequently asked questions

### Why is the standard EMF of a concentration cell zero?

Both electrodes belong to the same redox couple, so their identical standard potentials cancel; only the concentration ratio drives the reaction.

### Which electrode serves as the anode in a concentration cell?

The more dilute one — oxidation there raises its ion concentration, moving the system towards equality.

### How can a concentration cell determine Ksp?

Couple a normal Ag+/Ag electrode to one dipping in saturated AgCl at known chloride level; the EMF reveals the tiny free silver ion concentration, which equals Ksp divided by the chloride concentration.

### What happens to the EMF as the cell delivers current?

The anolyte concentration rises, the catholyte falls, the log ratio shrinks, and the EMF decays smoothly to zero at equality.

### How does a hydrogen-electrode concentration cell measure pH?

Against a fixed reference, its potential shifts by 0.0591 V per pH unit at 298 K, so the measured voltage reads out the pH directly.