How Ion-Selective Electrodes Work

Electrical Conductivity: The Basics

There are two fundamentally different types of electrical conductivity:

In Metals: electric current is carried by electrons.
In Liquids: electric current is carried by ions.

Every electrochemical process — galvanic cells, electrolysis, electro-analysis — involves both types. The junctions where they meet and transfer electrical charge are Metal-Liquid Interfaces, historically called electrodes.

In a copper-silver galvanic cell, for example, one electrode undergoes oxidation:

Cu (metallic) → Cu²⁺ (ionic) + 2e⁻

While the other undergoes reduction:

Ag⁺ (ionic) + e⁻ → Ag (metallic)

This is how current carried by electrons in a wire becomes current carried by ions in solution.

The ISE Electrochemical Circuit

ISE electrochemical circuit diagram — Figure 1 — The electrochemical circuit for an ISE measurement, showing the ISE, external reference electrode, and millivolt meter connected in a complete circuit through the test solution.

An ISE (with its own internal reference) is immersed in an aqueous solution alongside a separate, external reference electrode. The complete electrochemical circuit is formed when these are connected to a sensitive millivolt meter via low-noise cables.

A potential difference develops across the ISE membrane as target ions diffuse through from the high-concentration side to the lower-concentration side. This potential is what we measure.

The external reference electrode may be fully separate or incorporated into the ISE body to form a combination electrode. The ELIT system uses detachable plug-in electrodes for maximum flexibility — see elyxir.co.uk/products.

The Nernst Equation

At equilibrium, the membrane potential is described by the Nernst equation. In simplified form, the measured voltage is proportional to the logarithm of the activity (effective concentration) of the target ion:

E = E₀ + (2.303RT/nF) × log(A)

Where: R = gas constant, T = temperature (K), n = ionic charge, F = Faraday constant, A = ion activity

The electrode slope — millivolts per decade of activity/concentration — is a key indicator of electrode health:

Ion Type	Acceptable Slope
Monovalent cations	+55 ± 5 mV/decade
Monovalent anions	−55 ± 5 mV/decade
Divalent cations	+26 ± 3 mV/decade
Divalent anions	−26 ± 3 mV/decade

The slope decreases as the electrode ages or becomes contaminated — a lower slope means higher measurement errors.

Full glossary of terms and the Nernst equation derivation at nico2000.net

The Reference Electrode

The membrane potential cannot be measured directly — it must be measured relative to a stable reference. The most common system is silver wire coated with solid silver chloride immersed in saturated KCl/AgCl filling solution:

AgCl (s) + e⁻ ⇌ Ag (s) + Cl⁻

This half-cell provides a constant potential of +205 mV relative to the Standard Hydrogen Electrode at 25°C, regardless of what ions are in the test solution.

The reference electrode potential need not be known precisely — it cancels out during calibration because it is constant for all standard and sample measurements.

Elyxir supplies two reference electrode types:

Model	Type	Best Used With
ELIT 001	Single junction AgCl/KCl	Ba, Ca, F, NO₂, Na, ClO₄
ELIT 003N	Double junction lithium acetate	All ions (minimal interference)

Calcium ISE: Membrane Mechanism

ISE membrane diagram — Figure 2 — Membrane potential development across an ISE: ion diffusion creates a charge separation that drives the measurable potential difference.

The calcium ISE has a PVC membrane impregnated with an organic molecule that selectively binds and transports Ca²⁺ ions. The internal solution contains a fixed concentration of calcium chloride.

On immersion, Ca²⁺ ions begin diffusing across the membrane from high to low concentration.
Positive charge builds up on the inside; negative charge increases outside.
The resulting electric field opposes further diffusion.
Equilibrium is reached when diffusion pressure equals the electrostatic repulsion — this is the membrane potential.

All other junction potentials in the circuit (liquid junction, metal-liquid interfaces) are assumed constant during calibration and cancel out.

Fluoride ISE: Crystal Membrane

The fluoride ISE uses a fundamentally different mechanism: a single crystal of Lanthanum Fluoride (LaF₃) doped with Europium Fluoride (EuF₂). The doping creates holes in the crystal lattice through which F⁻ ions can pass selectively.

F⁻ ions pass through the crystal by normal diffusion — high to low concentration.
Equilibrium develops between diffusion force and electrostatic repulsion.
The only significant interference is OH⁻, which reacts with lanthanum to release extra F⁻. Eliminated by maintaining pH 4–8 with a buffer.
The LaF₃ crystal is chemically robust and extremely selective.

Key Insight

Unlike the calcium ISE (where ions are transported by an organic carrier molecule), the fluoride ISE works by physical passage of ions through crystal defects. The result is exceptionally high selectivity — one of the most ion-specific ISEs available.

Browse all ELIT ISE models including fluoride, calcium, nitrate, and 17 others at elyxir.co.uk/products