What is the role of Cadmium Amalgam in the cell?

Cadmium amalgam (12.5% Cadmium in Mercury) acts as the negative electrode (anode). During operation, cadmium undergoes oxidation, losing electrons to become Cd²⁺ ions.

Why is the Weston Cell constructed in an H-shaped glass tube?

The H-shape keeps the two electrodes completely separated while allowing the saturated Cadmium Sulphate electrolyte to connect them. It also prevents the chemical contents of one side from mechanically mixing with the other.

Which of the following is a key difference between saturated and unsaturated Weston cells?

Saturated cell has excess CdSO₄ crystals; unsaturated does not

Saturated cell uses zinc; unsaturated uses cadmium

Unsaturated cell is used as international standard

Saturated cell has a higher EMF than unsaturated

JAtone

Weston Standard Cell

A standard cell provides a constant, accurate, and reproducible EMF.
The Weston Standard Cell uses a Cadmium Amalgam anode and a Mercury/Mercurous Sulphate cathode.
Saturated cells have excess CdSO₄·8/3H₂O crystals; unsaturated cells do not.
EMF of a saturated cell is 1.0183 V at 20°C.
The cell has a very low temperature coefficient.

Weston Standard Cell

1. Standard Cell

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 measuring unknown EMF.

Characteristics of a Standard Cell

A standard cell should have:

Constant and accurately known EMF.

Very low temperature coefficient (EMF changes very little with temperature).

Highly reproducible EMF.

Reversible cell reaction.

Long-term stability.

2. Weston Standard Cell

The Weston Standard Cell (Cadmium Cell) is the most widely used standard cell because its EMF remains almost constant for many years and changes very little with temperature.

Types

1.Saturated Weston Standard Cell

2.Unsaturated Weston Standard Cell

3. Construction of Saturated Weston Standard Cell

Construction

Constructed in an H-shaped glass tube.

The tube is sealed at the top with wax/cork to prevent evaporation and contamination.

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

Negative Electrode (Anode)

Contains:

12.5% Cadmium Amalgam (Cd dissolved in Hg)

Covered with crystals of hydrated cadmium sulphate

CdSO₄ · 83H₂O

Positive Electrode (Cathode)

Contains:

Pure Mercury (Hg)

Covered with a paste of Mercurous Sulphate

Hg₂SO₄

Above this paste is a layer of hydrated cadmium sulphate crystals.

Electrolyte

The entire H-shaped tube is filled with a saturated solution of Cadmium Sulphate.

CdSO₄

Construction Summary

Part	Material
Vessel	H-shaped glass tube
Negative electrode	12.5% Cadmium amalgam
Positive electrode	Mercury + Mercurous sulphate paste
Electrolyte	Saturated CdSO₄ solution
Crystals	CdSO₄·8/3H₂O
Electrical contacts	Platinum wires

4. Cell Representation

Cd(Hg) | CdSO₄·83H₂O(s) | CdSO₄(sat.) | Hg₂SO₄(s) | Hg

5. Cell Reactions

(A) At Anode (Oxidation)

Cadmium loses electrons.

Cd(Hg) + SO₄²⁻ + 83H₂O → CdSO₄·83H₂O + 2e⁻

Simplified:

Cd → Cd²⁺ + 2e⁻

(B) At Cathode (Reduction)

Mercurous sulphate gains electrons.

Hg₂SO₄ + 2e⁻ → 2Hg + SO₄²⁻

(C) Overall Cell Reaction

Cd(Hg) + Hg₂SO₄ + 83H₂O → CdSO₄·83H₂O + 2Hg

6. EMF of Weston Standard Cell

At 20°C

E = 1.01830 V

(Usually written as 1.0183 V.)

Temperature Dependence

The EMF at temperature t°C is

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

where,

E_t = EMF at temperature t °C

t = Temperature in °C

Temperature Coefficient

Approximately

-4.06×10⁻⁵ V/°C

Meaning

As temperature increases, EMF decreases slightly.

The change is extremely small, making the Weston cell highly stable.

7. Saturated Weston Cell

Characteristics

Contains excess solid CdSO₄·8/3H₂O crystals.

Electrolyte always remains saturated.

Highly stable.

Long life.

Used as the International Standard Cell.

EMF

1.0183 V at 20°C

8. Unsaturated Weston Cell

Construction

Same as saturated cell except:

No excess CdSO₄·8/3H₂O crystals are present.

The electrolyte is unsaturated.

Characteristics

Lower temperature coefficient (almost zero).

EMF is slightly higher.

EMF slowly decreases with time.

Less suitable as an international standard.

Commonly used as a laboratory reference cell.

EMF

1.0186 V at 20°C

9. Difference Between Saturated and Unsaturated Weston Cell

Saturated Weston Cell	Unsaturated Weston Cell
Contains excess CdSO₄·8/3H₂O crystals	No CdSO₄ crystals
Electrolyte remains saturated	Electrolyte is unsaturated
EMF = 1.0183 V	EMF ≈ 1.0186 V
Temperature coefficient is small	Temperature coefficient is almost zero
Very stable for long periods	EMF slowly decreases with time
Used as International Standard	Used mainly in laboratories

10. Advantages of Weston Standard Cell

Provides a highly accurate and constant EMF.

Very low temperature coefficient.

Excellent long-term stability.

Reversible cell reaction.

Used for calibration of voltmeters, potentiometers, and other electrical measuring instruments.

11. Applications

Used as a standard source of EMF.

Calibration of potentiometers.

Calibration of voltmeters.

Calibration of electrical measuring instruments.

Measurement of unknown EMF by comparison.

Quick Revision

Standard EMF

1.0183 V (20°C)

Cell Representation

Cd(Hg) | CdSO₄·83H₂O | CdSO₄(sat.) | Hg₂SO₄ | Hg

Anode

Cd(Hg) → Cd²⁺ + 2e⁻

Cathode

Hg₂SO₄ + 2e⁻ → 2Hg + SO₄²⁻

Overall Reaction

Cd(Hg) + Hg₂SO₄ + 83H₂O → CdSO₄·83H₂O + 2Hg

Temperature Equation

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

EMF at 20°C

Saturated Cell: 1.0183 V

Unsaturated Cell: 1.0186 V

A standard cell provides a constant, accurate, and reproducible EMF.
The Weston Standard Cell uses a Cadmium Amalgam anode and a Mercury/Mercurous Sulphate cathode.
Saturated cells have excess CdSO₄·8/3H₂O crystals; unsaturated cells do not.
EMF of a saturated cell is 1.0183 V at 20°C.
The cell has a very low temperature coefficient.

Contents

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Weston Standard Cell PPT

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About Weston Standard Cell

Weston Standard Cell is a fundamental concept in physical chemistry. Understanding the mechanisms, reaction conditions, and stereo-chemical outcomes is crucial for mastering organic chemistry. Our curated resources provide step-by-step visualizations to help you excel.

Weston Standard Cell

A standard cell provides a constant, accurate, and reproducible EMF.
The Weston Standard Cell uses a Cadmium Amalgam anode and a Mercury/Mercurous Sulphate cathode.
Saturated cells have excess CdSO₄·8/3H₂O crystals; unsaturated cells do not.
EMF of a saturated cell is 1.0183 V at 20°C.
The cell has a very low temperature coefficient.

Weston Standard Cell

1. Standard Cell

Characteristics of a Standard Cell

A standard cell should have:

Constant and accurately known EMF.

Very low temperature coefficient (EMF changes very little with temperature).

Highly reproducible EMF.

Reversible cell reaction.

Long-term stability.

2. Weston Standard Cell

The Weston Standard Cell (Cadmium Cell) is the most widely used standard cell because its EMF remains almost constant for many years and changes very little with temperature.

Types

1.Saturated Weston Standard Cell

2.Unsaturated Weston Standard Cell

3. Construction of Saturated Weston Standard Cell

Construction

Constructed in an H-shaped glass tube.

The tube is sealed at the top with wax/cork to prevent evaporation and contamination.

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

Negative Electrode (Anode)

Contains:

12.5% Cadmium Amalgam (Cd dissolved in Hg)

Covered with crystals of hydrated cadmium sulphate

CdSO₄ · 83H₂O

Positive Electrode (Cathode)

Contains:

Pure Mercury (Hg)

Covered with a paste of Mercurous Sulphate

Hg₂SO₄

Above this paste is a layer of hydrated cadmium sulphate crystals.

Electrolyte

The entire H-shaped tube is filled with a saturated solution of Cadmium Sulphate.

CdSO₄

Construction Summary

Part	Material
Vessel	H-shaped glass tube
Negative electrode	12.5% Cadmium amalgam
Positive electrode	Mercury + Mercurous sulphate paste
Electrolyte	Saturated CdSO₄ solution
Crystals	CdSO₄·8/3H₂O
Electrical contacts	Platinum wires

4. Cell Representation

Cd(Hg) | CdSO₄·83H₂O(s) | CdSO₄(sat.) | Hg₂SO₄(s) | Hg

5. Cell Reactions

(A) At Anode (Oxidation)

Cadmium loses electrons.

Cd(Hg) + SO₄²⁻ + 83H₂O → CdSO₄·83H₂O + 2e⁻

Simplified:

Cd → Cd²⁺ + 2e⁻

(B) At Cathode (Reduction)

Mercurous sulphate gains electrons.

Hg₂SO₄ + 2e⁻ → 2Hg + SO₄²⁻

(C) Overall Cell Reaction

Cd(Hg) + Hg₂SO₄ + 83H₂O → CdSO₄·83H₂O + 2Hg

6. EMF of Weston Standard Cell

At 20°C

E = 1.01830 V

(Usually written as 1.0183 V.)

Temperature Dependence

The EMF at temperature t°C is

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

where,

E_t = EMF at temperature t °C

t = Temperature in °C

Temperature Coefficient

Approximately

-4.06×10⁻⁵ V/°C

Meaning

As temperature increases, EMF decreases slightly.

The change is extremely small, making the Weston cell highly stable.

7. Saturated Weston Cell

Characteristics

Contains excess solid CdSO₄·8/3H₂O crystals.

Electrolyte always remains saturated.

Highly stable.

Long life.

Used as the International Standard Cell.

EMF

1.0183 V at 20°C

8. Unsaturated Weston Cell

Construction

Same as saturated cell except:

No excess CdSO₄·8/3H₂O crystals are present.

The electrolyte is unsaturated.

Characteristics

Lower temperature coefficient (almost zero).

EMF is slightly higher.

EMF slowly decreases with time.

Less suitable as an international standard.

Commonly used as a laboratory reference cell.

EMF

1.0186 V at 20°C

9. Difference Between Saturated and Unsaturated Weston Cell

Saturated Weston Cell	Unsaturated Weston Cell
Contains excess CdSO₄·8/3H₂O crystals	No CdSO₄ crystals
Electrolyte remains saturated	Electrolyte is unsaturated
EMF = 1.0183 V	EMF ≈ 1.0186 V
Temperature coefficient is small	Temperature coefficient is almost zero
Very stable for long periods	EMF slowly decreases with time
Used as International Standard	Used mainly in laboratories

10. Advantages of Weston Standard Cell

Provides a highly accurate and constant EMF.

Very low temperature coefficient.

Excellent long-term stability.

Reversible cell reaction.

Used for calibration of voltmeters, potentiometers, and other electrical measuring instruments.

11. Applications

Used as a standard source of EMF.

Calibration of potentiometers.

Calibration of voltmeters.

Calibration of electrical measuring instruments.

Measurement of unknown EMF by comparison.

Quick Revision

Standard EMF

1.0183 V (20°C)

Cell Representation

Cd(Hg) | CdSO₄·83H₂O | CdSO₄(sat.) | Hg₂SO₄ | Hg

Anode

Cd(Hg) → Cd²⁺ + 2e⁻

Cathode

Hg₂SO₄ + 2e⁻ → 2Hg + SO₄²⁻

Overall Reaction

Cd(Hg) + Hg₂SO₄ + 83H₂O → CdSO₄·83H₂O + 2Hg

Temperature Equation

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

EMF at 20°C

Saturated Cell: 1.0183 V

Unsaturated Cell: 1.0186 V

A standard cell provides a constant, accurate, and reproducible EMF.
The Weston Standard Cell uses a Cadmium Amalgam anode and a Mercury/Mercurous Sulphate cathode.
Saturated cells have excess CdSO₄·8/3H₂O crystals; unsaturated cells do not.
EMF of a saturated cell is 1.0183 V at 20°C.
The cell has a very low temperature coefficient.

Contents

Available Files2

Weston Standard Cell PPT

PresentationPhysical Chemistry

Preview Download

Weston Standard Cell PDF

PDF DocumentPhysical Chemistry

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About Weston Standard Cell

SELF TEST

Practice MCQs

Question 1 / 5Score: 0

What is the EMF of a saturated Weston Standard Cell at 20°C?

LEARNING SUPPORT

Weston
Standard Cell FAQ

Its EMF remains almost constant for many years and changes very little with temperature.

The saturated cell contains excess solid CdSO₄·8/3H₂O crystals, keeping the electrolyte saturated. It is more stable long-term, while the unsaturated cell has a temperature coefficient of almost zero but slowly decreases in EMF over time.

The EMF of a saturated Weston Standard Cell is highly accurate at exactly 1.0183 V at 20°C.