A glass electrode is a type of
ion-selective electrode
An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a transducer (or sensor) that converts the activity of a specific ion dissolved in a solution into an electrical potential. The voltage is theoretically dependen ...
made of a doped glass membrane that is sensitive to a specific ion. The most common application of ion-selective glass electrodes is for the measurement of
pH. The pH electrode is an example of a glass electrode that is sensitive to hydrogen ions. Glass electrodes play an important part in the instrumentation for chemical analysis and physicochemical studies. The voltage of the glass electrode, relative to some reference value, is sensitive to changes in the
activity
Activity may refer to:
* Action (philosophy), in general
* Human activity: human behavior, in sociology behavior may refer to all basic human actions, economics may study human economic activities and along with cybernetics and psychology may stu ...
of a certain type of ions.
History
The first studies of glass electrodes (GE) found different sensitivities of different glasses to change the medium's acidity (pH), due to the effects of the alkali metal ions.
In 1906, M. Cremer, the father of
Erika Cremer
Erika Cremer (20 May 1900, Munich – 21 September 1996, Innsbruck) was a German physical chemist and Professor Emeritus at the University of Innsbruck who is regarded as one of the most important pioneers in gas chromatography, as she secon ...
, determined that the
electric potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
that arises between parts of the fluid, located on opposite sides of the glass membrane is proportional to the concentration of acid (hydrogen ion concentration).
In 1909,
S. P. L. Sørensen
Søren Peter Lauritz Sørensen (9 January 1868 – 12 February 1939) was a Danish chemist, famous for the introduction of the concept of pH, a scale for measuring acidity and alkalinity.
Personal life
Sørensen was born in Havrebjerg, Den ...
introduced the concept of
pH, and in the same year
F. Haber and
Z. Klemensiewicz reported results of their research on the glass electrode in The Society of Chemistry in
Karlsruhe
Karlsruhe ( , , ; South Franconian German, South Franconian: ''Kallsruh'') is the List of cities in Baden-Württemberg by population, third-largest city of the German States of Germany, state (''Land'') of Baden-Württemberg after its capital o ...
.
In 1922, W. S. Hughes showed that the alkali-silicate GE are similar to hydrogen electrodes, reversible concerning H
+.
In 1925,
P.M. Tookey Kerridge developed the first glass electrode for analysis of blood samples and highlighted some of the practical problems with the equipment such as the high resistance of glass (50–150 MΩ). During her PhD, Kerridge developed the miniature glass electrode, maximizing the surface area of the tool by heat treating the platinum with platinum chloride at red heat, thus enabling a much larger signal; her design was the predecessor of many of the glass electrodes used today.
Applications
Glass electrodes are commonly used for
pH measurements. There are also specialized ion sensitive glass electrodes used for the determination of the concentration of lithium, sodium, ammonium, and other ions.
Glass electrodes have been utilized in a wide range of applications including pure research, control of industrial processes, analysis of foods and cosmetics, measurement of environmental indicators, and microelectrode measurements such as cell membrane electrical potential and soil acidity.
Types
Almost all commercial electrodes respond to single-charged
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s, like H
+, Na
+, Ag
+. The most common glass electrode is the
pH-electrode. Only a few
chalcogenide glass
Chalcogenide glass (pronounced hard ''ch'' as in ''chemistry'') is a glass containing one or more chalcogens (sulfur, selenium and tellurium, but excluding oxygen). Such glasses are covalently bonded materials and may be classified as covalent ne ...
electrodes are sensitive to double-charged ions, like Pb
2+, Cd
2+ and some others.
There are two main glass-forming systems:
silicate matrix based on a molecular network of
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
(SiO
2) with additions of other metal oxides, such as Na, K, Li, Al, B, Ca, etc. and
chalcogenide : 220px, Cadmium sulfide, a prototypical metal chalcogenide, is used as a yellow pigment.
A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements ...
matrix based on a molecular network of AsS, AsSe, AsTe.
Interfering ions
Because of the
ion-exchange
Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...
nature of the glass membrane, it is possible for some other ions to concurrently interact with ion-exchange centres of the glass and distort the linear dependence of the measured electrode potential on pH or other electrode functions. In some cases, it is possible to change the electrode function from one ion to another. For example, some silicate pPNAelectrodes can be changed to pAg function by soaking in a silver salt solution.
Interference effects are commonly described by the semiempirical
Nicolsky-
Shultz
Schulz is a common German and Jewish-Ashkenazi family name from Germany, particularly Northern Germany. The word ''Schulz'' originates from the local official title of Schultheiß or ''(Dorf-)Schulz(e)'', meaning village headman or constable / sh ...
-Eisenman equation (also known as
Nikolsky-
Shultz
Schulz is a common German and Jewish-Ashkenazi family name from Germany, particularly Northern Germany. The word ''Schulz'' originates from the local official title of Schultheiß or ''(Dorf-)Schulz(e)'', meaning village headman or constable / sh ...
-Eisenman equation), an extension to the
Nernst equation
In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempera ...
. It is given by
: