The Combination pH Electrode
Modern pH electrodes are usually of the "combination" type, meaning that a single cylinder contains both the reference electrode, and a glass membrane electrode. Schematically, the total cell may be expressed as
SCE//test solution ([H3O+]=a1)/glass membrane/[H3O+]=a2, Cl-/AgCl(s)/Ag
Silver-silver chloride or SCE reference electrodes may be as used for internal and external reference electrodes.
The potential of a combination pH electrode is due to the difference in activities of H+ between the test solution and reference solution sides of the glass membrane.
Since a2 is fixed by the internal solution
where K(T) is a temperature-dependent constant. Thus the potential of the combination electrode is proportional to the pH of the test solution.
An "alkaline error" can result when cations other than H+ are present in solution. These cations can exchange for H+ in the gel layer,
In this case the electrode potential is
where Kex is the proton-ion exchange constant.
The glass can be tailored by varying the amount of alumina (Al2O3) and other common constituent to what we call glass, (Na2O, K2O, B2O3, etc.). Addition of Al2O3 increases Kex.
Using alumina, the glass becomes sensitive to alkali metal ions at moderate pH. When a1<<KexaM., typically for pH greater than 4, the glass electrode is more sensitive to alkali metals than to proton and
Solid-State Ion Selective Electrodes
Any solid that exhibits a surface activity toward binding an electro-active species can be used as the basis of an ion selective electrode if it can be made conductive.
The most famous is the fluoride ion selective electrode made from the Eu2+ doped LaF3 crystal (Eu:LaF3). In this semi-conducting crystal, F- ions are bound to the surface. The potential between the analyte and standard internal solution faces of the crystal is proportional to the logarithm of fluoride activities.
The potential is
Here is a list of some of the ion-selective electrodes commercially-available.
Liquid-Membrane Selective Electrodes
Liquids or soluble species that exhibit affinity toward an electro-active species can be used to fabricate a selective electrode. The electrode produces a potential in much the same fashion as the solid-state selective electrodes. The main difference is that complexation takes place with a molecular compound.
The molecular compound must be one that complexes with the species of interest. In addition, the compound cannot be too soluble in water. If the compound is soluble in water, it will just dissolve into the first solution tested. The selective compound can be dissolved into an organic solvent which is not miscible with water.
There must also be a physical barrier between reference and test solutions. This is often accomplished using a membrane that the organic phase, which contains the selective molecule, can permeate. The organic-phase saturated membrane is then suspended between the reference and test solutions. Schematically, the electrode is
Ref/internal standard/organic phase/test solution//Ref
where Ref indicates one of the usual (or unusual) reference electrodes, e.g., SCE or Ag/AgCl, the internal standard has the analyte at a fixed activity, the organic phase is a liquid organic solvent in which the active molecular compound is dissolved, and the test solution is the analyte solution (sample).
Example: the Ca2+ selective electrode is made using an aliphatic diester of phosphoric acid, (RO)2PO2- where R is an aliphatic group of low aqueous solubility. The chemical equilibrium is
2(RO)2PO2- + Ca2+ « [(RO2PO2]2Ca
Using an internal reference solution, the electrode half-potential can be found from
Ag/AgCl(s)/Cl-, Ca2+ (int std.) /(RO)2PO2H/test solution//Ref
Since the activity of [(RO)2PO2]2Ca is related to the concentration of Ca2+ through the equilibrium constant for both reference and external solutions
Some species able to be measured with these electrodes are
Electrodes may also be used as gas sensors.
The electrode images have been reproduced, with permission, from web pages at Orion, a leader in ion selective electrode technology. Other interested in using these images should contact email@example.com directly.
Visit Orion (manufacturer of ISEs) pages to find out much more about ion selective and pH electrodes.
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This page was created by Professor Stephen Bialkowski, Utah State University
Last Updated Tuesday, August 03, 2004