How Ion Selective Electrodes Work

There are two different types of Electrical Conductivity:

1) In Metals the electric current is carried by Electrons.

2) In Liquids the electric current is carried by Ions.

Every Electrochemical Process (Galvanic Cell, Electrolysis, Electro-Analysis) involves both these types of conductivity. The junctions where they meet and transfer the electrical charge are referred to as Metal-Liquid Interfaces. These interfaces were originally called Electrodes, but now this term is also used for various other devices such as welding electrodes or electro-cardiogram electrodes.

At the Metal-Liquid interface there is an exchange of Electrons in one or other direction (details can be found in standard chemistry text books, in sections on Galvanic or Electrolytic Cells.

(NB: Galvanic [Voltaic] Cells generate electricity; Electrolytic Cells consume electricity).

For example, in a Copper-Silver Galvanic Cell, on one electrode an Oxidation reaction takes place:

Cu (metallic) à Cu 2+ (ionic, in solution) +2 e-

on the other electrode a Reduction reaction takes place: Ag+ (ionic, from solution) + e- à Ag (metallic - deposited on electrode surface)

This explains how the electric current in the wire (Electrons) becomes a current in the liquid (Ions).

An ISE (with an own inner anxiety electrode - some-more sum later) is enthralled in an aqueous resolution containing an ions to be measured, together with a separate, outmost anxiety electrode. (NB: this outmost anxiety can be utterly apart or incorporated in a physique of an ISE to form a Combination Electrode.) An electrochemical circuit is finished by joining electrodes to a supportive milli-volt scale regulating special low-noise cables as well as connectors. An intensity disproportion is grown opposite an ISE surface when a aim ions disband by from a tall thoroughness side to a reduce thoroughness side (a minute outline follows later).

At equilibrium, the membrane potential is mainly dependent on the concentration of the target ion outside the membrane and is described by the Nernst equation (see Glossary at www.nico2000.net).

Briefly, the measured voltage is proportional to the Logarithm of the concentration, and the sensitivity of the electrode is expressed as the electrode Slope - in millivolts per decade of concentration.  Thus the electrodes can be calibrated by measuring the voltage in solutions containing, for example, 10ppm and 100ppm of the target ion, and the Slope will be the slope of the (straight) calibration line drawn on a graph of mV versus Log concentration.

i.e. S = [ mV(100ppm) - mV(10ppm) ] / [Log100 - Log10]

Thus the slope simply equals the difference in the voltages - since Log100-Log10 = 1.Unknown samples can then be determined by measuring the voltage and plotting the result on the calibration graph.

The exact value of the slope can be used as an indication of the proper functioning of an ISE and the following are typical values:

Monovalent:Cations +55 ± 5, Anions -55 ± 5

Divalent:cations +26 ± 3, Anions -26 ± 3

The surface intensity cannot be totalled directly. It needs a Metal-Liquid interface (or a metal-solid resolution interface in complicated "all-solid-state" ISEs) upon both sides of a membrane. Theoretically these could usually be steel wires enthralled in solutions. Though a electrical intensity upon many elementary metal-liquid junctions is not stable; to illustrate a need for a supposed anxiety complement upon both sides of a ISE membrane, with a sold metal-liquid interface that is well known to have a fast potential. A bulk of this intensity need not be well known since it is a same for all measurements of standards and samples and is to illustrate separated during a calibration process.

Nevertheless, it contingency be remarkable that this potential, as well as any others that might be generated during any or all of a metal-liquid or liquid-liquid junctions in a circuit, is a worth that is seen when a electrodes have been enthralled in pristine H2O or any alternative resolution that does not enclose a aim ion. This explains since a totalled voltage is not approaching to be 0 when no aim ion is benefaction as well as additionally since it is not indispensably regularly certain when a aim ion is benefaction - it all depends upon a disproportion in between a ISE voltage as well as a total of all a alternative voltages in a circuit. For example, for a monovalent certain ion, a voltage could be -25 mV in 10ppm as well as +30mV in 100ppm (or even -60 mV in 10ppm as well as -5mV in 100ppm) though this still gives a slant of +55mV per decade of thoroughness as well as indicates that a ISE is functioning correctly. Reversing charges upon top of would report a incident for a monovalent disastrous ion.

It should be remarkable here that soak in pristine H2O should be avoided since it tends to leach out an aim ion from a ISE membrane. This, together with a fundamental instability of a glass connection intensity of a anxiety electrode, will means an inconstant voltage to be totalled in pristine H2O as well as need a ISE surface to be re-equilibrated in a tall thoroughness "pre-conditioning" resolution prior to it will give fast readings again.

In practice, a many usual anxiety complement is a china handle coated with plain china chloride as well as enthralled in a strong resolution (known as a "filling solution") of potassium chloride jam-packed with china chloride. A anxiety electrode is a half-cell that provides a consistent intensity that is contingent usually upon a thoroughness of chloride ions in a stuffing solution. A reversible Redox greeting involves a chloride atoms in a plain china chloride (plated upon a china wire) reception an nucleus as well as a chloride ion starting in to solution, as well as clamp versa. This electrode will give a consistent intensity of +205 mV (relative to a Standard Hydrogen Electrode) with a jam-packed KCl/AgCl resolution during 25°C.

There have been assorted opposite charge-transfer processes, both outward as well as inside a surface for a assorted opposite surface types, as well as most of these have been rarely formidable as well as feeble accepted in detail. For example, even a assumingly elementary potion surface of a pH electrode, which has traditionally been suspicion of as involving a thoroughfare of Hydrogen (H+), or presumably hydroxonium (H3O+) ions, has not long ago been shown, by hot tagging experiments, to engage usually a transformation of Sodium (Na+) ions !

The following descriptions of a Calcium as well as Fluoride ISEs have been standard examples of simple beliefs of ion-selective surface processes. Nevertheless, it contingency be remarkable which these processes might be distant some-more formidable than those described as well as might engage multiform layers of ions during any proviso junction

In a box of an F ISE, an ion-selective surface is a singular clear of Lanthanum Fluoride (LaF3) doped with Europium Fluoride (EuF2) that produces holes in a clear hideaway by that F ions can pass. When enthralled in a fluoride resolution as well as continuous around a voltmeter to an AgCl/KCl outmost anxiety electrode enthralled in a same solution, a disastrous F ions in a resolution pass by a clear surface by normal freeing from tall thoroughness to low thoroughness until there is an balance in between a force of freeing as well as a retreat electrostatic force due to abhorrence in between particles of identical charge. Upon an alternative side of a surface there is an analogous rave of certain ions.

The set up up of disastrous F ions upon a inside of a surface is compensated for by Cl ions in a inner anxiety resolution apropos neutralised by mixing with a Ag/AgCl wire, as well as electrons have been to illustrate forced by a outmost handle to a voltage measuring device (ion scale or mechanism interface). An alternative depot of a voltmeter is continuous to a Ag/AgCl handle of a outmost anxiety electrode. Here, a liquid of electrons causes Ag ions in a stuffing resolution to accept electrons as well as deposition upon a china handle and, consequently, Cl ions to upsurge out in to a representation solution.

Note that, in general, depending upon a concentrations inside and outward a surface and that ion is being measured, all a reactions described on top of could start in a conflicting direction.