Why is the standard hydrogen electrode rarely used in routine work?

It requires a continuous supply of pure hydrogen gas at controlled pressure and a clean platinized surface that is easily poisoned, so robust secondary references like Ag/AgCl or saturated calomel are used instead and related back to the SHE scale.

What is the liquid junction potential and why does it matter?

It is a small potential arising where two solutions of different composition meet at the reference electrode's salt bridge; it adds an uncertain offset to measurements, which is why junctions are designed to minimize and stabilize it.

Reference Electrodes

A reference electrode maintains a stable, reproducible potential against which the potential of a working electrode can be measured and controlled.

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Definition

An electrode with a well-defined, stable equilibrium potential, used as a fixed point of reference for measuring or controlling the potential of another electrode.

Scope

This topic covers the principle and practice of reference electrodes: the primary standard hydrogen electrode that defines the zero of the potential scale, and practical secondary references such as the silver/silver chloride and saturated calomel electrodes. It includes the requirements of reversibility and non-polarizability, the role of the liquid junction, sources of drift and contamination, and conversions between reference scales.

Core questions

What properties make an electrode suitable as a stable potential reference?
How is the absolute zero of the electrochemical potential scale defined by the standard hydrogen electrode?
Why are silver/silver chloride and calomel electrodes used in place of the hydrogen electrode in practice?
How are potentials converted between different reference electrode scales?

Key theories

Standard hydrogen electrode (SHE): A platinized platinum electrode in contact with hydrogen gas at unit fugacity and protons at unit activity, defined to have exactly zero potential at all temperatures, anchoring the conventional scale of standard electrode potentials.
Non-polarizable reference behavior: A good reference electrode sustains an equilibrium redox couple at high exchange current so that small measurement currents leave its potential essentially unchanged, providing a stable fixed point.

Clinical relevance

Reference electrodes are indispensable in potentiometric sensors including pH meters and blood-gas analyzers, in corrosion monitoring of buried and submerged structures, in battery testing, and in any voltammetric experiment where the working-electrode potential must be known and controlled.

History

The hydrogen electrode emerged as the reference standard in the early 20th century as electrode-potential tables were compiled; the practical calomel and silver/silver chloride electrodes were developed because the hydrogen electrode is cumbersome, requiring gas handling and easily poisoned platinum.

Key figures

Wilhelm Ostwald
David J. G. Ives

Seminal works

bard2001
ives1961
inzelt2013

Frequently asked questions

Why is the standard hydrogen electrode rarely used in routine work?: It requires a continuous supply of pure hydrogen gas at controlled pressure and a clean platinized surface that is easily poisoned, so robust secondary references like Ag/AgCl or saturated calomel are used instead and related back to the SHE scale.
What is the liquid junction potential and why does it matter?: It is a small potential arising where two solutions of different composition meet at the reference electrode's salt bridge; it adds an uncertain offset to measurements, which is why junctions are designed to minimize and stabilize it.