ELECTROCHEMISTRY · UNIT 04

Weston

Standard Cell

The Cadmium Cell — a primary reference of EMF for calibrating electrical measuring instruments.

B.SC. PHYSICS · EXAM REVISION DECK · 12 SLIDES

Cambridge Instrument Co. Ltd. · Weston Normal Cell

CHAPTER 01 · FOUNDATIONS

What is a Standard Cell?

DEFINITION

A standard cell is a primary electrochemical cell that provides a constant, accurate, and reproducible Electromotive Force (EMF). It is used as a reference source of voltage for calibrating electrical instruments and for measuring unknown EMFs by comparison.

ⓘ

Because its EMF is fixed by construction, a standard cell is never used as a power source — only as a reference in null-balance (potentiometric) measurements.

Characteristics of a Standard Cell

01

⚡

Constant EMF

EMF is accurately known and remains fixed.

02

✦

Low temperature coefficient

EMF changes very little with temperature.

03

⚗

Reproducible

Same construction yields the same EMF every time.

04

↻

Reversible reaction

Cell reaction proceeds in both directions.

05

✦

Long-term stability

EMF holds for years without drift.

WESTON CELL

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CHAPTER 02 · THE WESTON CELL

The Weston Standard Cell

Also called the Cadmium Cell — the most widely used standard cell in the world.

The Weston Standard Cell is the most widely used standard cell because its EMF remains almost constant for many years and changes very little with temperature. Invented by Edward Weston in 1893, it became the international legal volt standard for most of the 20th century.

Edward Weston

☻

1893

▦

International Legal Volt

◯

20th Century

◔

TYPES

Two Types

01

Saturated Weston Standard Cell

Electrolyte stays saturated; international standard.

02

Unsaturated Weston Standard Cell

Electrolyte unsaturated; laboratory reference.

WESTON CELL

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CHAPTER 03 · CONSTRUCTION

Construction of the Saturated Cell

H-shaped glass tube · saturated CdSO₄ electrolyte

⚗

PART 01

Vessel & Sealing

● H-shaped glass tube, sealed at top with wax/cork to prevent evaporation.

● Platinum wires sealed at the bottom of both limbs serve as electrical terminals.

−

PART 02 · ANODE

Negative Electrode (Anode)

● 12.5% Cadmium Amalgam (Cd dissolved in Hg).

● Covered with crystals of hydrated cadmium sulphate CdSO₄·8/3 H₂O.

＋

PART 03 · CATHODE

Positive Electrode (Cathode)

● Pure Mercury (Hg).

● Covered with a paste of Mercurous Sulphate Hg₂SO₄.

● Above the paste: a layer of hydrated CdSO₄·8/3 H₂O crystals.

● Entire H-tube filled with saturated CdSO₄ solution (electrolyte).

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CHAPTER 04 · NOTATION

Cell Representation

The Weston standard cell written in IUPAC cell notation — phases separated by single vertical bars.

CELL NOTATION · IUPAC

Read left → right (anode → cathode)

Cd(Hg)|CdSO₄·8/3 H₂O(s)|CdSO₄ (sat. aq)|Hg₂SO₄(s)|Hg

Anode (oxidation) → ← Cathode (reduction)

⚗

Cd(Hg)

Cadmium amalgam anode (12.5% Cd in Hg).

☂

CdSO₄·8/3 H₂O (s)

Hydrated cadmium sulphate crystals maintaining saturation.

◐

CdSO₄ (sat.)

Saturated aqueous cadmium sulphate electrolyte.

●

Hg₂SO₄ | Hg

Mercurous sulphate paste over pure mercury cathode.

Single | denotes a phase boundary; salt bridge would be ||.

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WESTON CELL

CHAPTER 05 · ELECTROCHEMISTRY

Cell Reactions

Half-reactions at each electrode combine to give the overall Weston cell reaction.

↑

AT ANODE · OXIDATION

Cadmium loses electrons

Cd(Hg) + SO₄²⁻ + 8/3 H₂O → CdSO₄·8/3 H₂O + 2e⁻

Cd → Cd²⁺ + 2e⁻

Cadmium amalgam oxidises, releasing two electrons into the external circuit.

↓

AT CATHODE · REDUCTION

Mercurous sulphate gains electrons

Hg₂SO₄ + 2e⁻ → 2Hg + SO₄²⁻

Insoluble Hg₂SO₄ paste accepts electrons, depositing pure mercury.

⇄

OVERALL CELL REACTION

Net reaction of the Weston cell

Cd(Hg) + Hg₂SO₄ + 8/3 H₂O → CdSO₄·8/3 H₂O + 2Hg

Two electrons transferred per cadmium atom.

◆

Adding the two half-reactions cancels the electrons and the sulphate ions, leaving the net reaction — and regenerating pure Hg, which makes the cell reversible.

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WESTON CELL

CHAPTER 06 · EMF

EMF & Temperature Dependence

⚡

1.0183

EMF AT 20 °C (V)

Saturated Weston cell, International Standard.

✦

−4.06×10⁻⁵

TEMPERATURE COEFFICIENT (V/°C)

EMF decreases slightly as T rises.

⚗

1.0186

UNSATURATED EMF AT 20 °C (V)

Slightly higher; drifts down with time.

TEMPERATURE EQUATION

Valid for laboratory temperature range

E_t = 1.01830 − 4.06×10⁻⁵ (t − 20) − 9.5×10⁻⁷ (t − 20)²

E_t

= EMF at temperature t °C

t

= Temperature in °C

Thanks to the tiny coefficients, the EMF is highly stable across the laboratory temperature range.

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WESTON CELL

CHAPTER 07 · TYPE 01

Saturated Weston Cell

The International Standard Cell.

EMF AT 20 °C

1.0183

V

International Standard since 1908.

◯

SATURATED · TYPE 01

⚙

Characteristics

☂

Excess crystals

Contains solid CdSO₄·8/3 H₂O in excess; electrolyte always remains saturated.

✓

Highly stable

EMF holds constant over very long periods — years to decades.

✦

Long life

Suitable for use as a transfer/working standard in standards labs.

◯

International Standard

Adopted as the International Standard of EMF.

✦

Small temperature coefficient

About −40 μV/°C; correctable using the E_t equation.

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WESTON CELL

CHAPTER 08 · TYPE 02

Unsaturated Weston Cell

The Laboratory Reference Cell.

EMF AT 20 °C

1.0186

V

Slightly higher than saturated; drifts down with age.

Characteristics & Construction

⊘

No excess crystals

Construction identical to saturated cell except no CdSO₄·8/3 H₂O crystals are added.

◐

Unsaturated electrolyte

CdSO₄ solution is unsaturated; concentration can drift over time.

✦

Near-zero temperature coefficient

EMF barely changes with temperature — convenient for lab use.

↘

EMF decreases with time

Cell ages; EMF slowly drifts downward, so periodic recalibration is required.

⚗

Laboratory reference

Commonly used as a working reference in lab potentiometer circuits.

Less suitable as an international standard because of long-term drift.

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CHAPTER 09 · COMPARISON

Saturated vs Unsaturated

Bottom line for exam:

Saturated is the international standard — accurate and stable, but temperature-sensitive. Unsaturated is the lab workhorse — temperature-insensitive, but drifts with age.

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CHAPTER 10 · USAGE

Advantages & Applications

Advantages

✓

Highly accurate and constant EMF (1.0183 V reproducible).

✓

Very low temperature coefficient — minimal thermal correction needed.

✓

Excellent long-term stability over years.

✓

Reversible cell reaction — returns to equilibrium after small current draws.

✓

Industry standard for calibrating voltmeters, potentiometers, and measuring instruments.

Applications

Potentiometer & meter bridge — classical Weston cell application.

◔

Standard source of EMF in calibration labs.

◔

Calibration of potentiometers.

◔

Calibration of voltmeters and panel meters.

◔

Calibration of electrical measuring instruments.

◔

Measurement of unknown EMF by comparison method.

Never draw more than a few microamperes — the cell polarises and EMF shifts.

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CHAPTER 11 · REVISION

Quick Revision

Everything you must remember on one page.

STANDARD EMF

⚡

1.0183 V

At 20 °C (saturated cell).

CELL REPRESENTATION

✦

Cd(Hg) | CdSO₄·8/3 H₂O | CdSO₄ (sat.) | Hg₂SO₄ | Hg

ANODE (OXIDATION)

↑

Cd(Hg) → Cd²⁺ + 2e⁻

Cadmium amalgam loses electrons.

CATHODE (REDUCTION)

↓

Hg₂SO₄ + 2e⁻ → 2Hg + SO₄²⁻

Mercurous sulphate gains electrons.

OVERALL REACTION

⇄

Cd(Hg) + Hg₂SO₄ + 8/3 H₂O → CdSO₄·8/3 H₂O + 2Hg

TEMPERATURE EQUATION

✦

E_t = 1.01830 − 4.06×10⁻⁵(t−20) − 9.5×10⁻⁷(t−20)²

Saturated 1.0183 V · Unsaturated 1.0186 V @ 20 °C.

★Saturated = International Standard

·

★Unsaturated = Laboratory reference

·

★Temp. coefficient ≈ −40 μV/°C

WESTON CELL

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