User:PGChem 300/Ion-selective electrode

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ISE is a type of sensor device that senses changes in signal based on the surrounding environment through time. This device will have an input signal, a property that we wish to quantify, and an output signal, a quantity we can register. In this case, ion selective electrode are electrochemical sensors that give potentiometric signals. ^[1]

ISE Applications: Analysis with ISEs expands throughout a range of technological fields such as biology, analytical chemistry, environmental science and other industrial workplaces like agriculture.

General Theory of Ion-Selective Electrodes

When using ion-selective electrodes, a scientist wants to compare the signal of an analyte to the electrochemical potential shown by the ISE. Different types of electrodes can be used to do this, as described in the sections below. As shown in the general schematic, an ion-selective membrane (consisting of glass, crystalline, liquid, or polymers) selectively allows specific types of ions to travel through, or in other words, is selectively permeable.

All ISE measurements are made with a comparison to an internal reference electrode with a known concentration of the analyte being measured. The external reference electrode is the part of the system that is exposed to the solution. The potential is measured using the following formula:

$E_{cell}=E_{ise}-E_{ref}$

E_ise includes the potential of the internal reference electrode and the ion-selective membrane potential E_m. The E_ise is governed by analyte activity in the internal solution whereas E_m is governed by the activity of the analyte on each side of the selective membrane. Furthermore, the E_ref or external reference portion of the cell is dependent on the half-reaction of the electrode and the liquid junction potential E_j.

Reference Electrodes

The most common types of reference electrodes used in analytical chemistry include the standard hydrogen electrode, the saturated calomel electrode, and the Ag/AgCl electrode.^[2]

The standard hydrogen electrode (SHE) is the primary reference electrode that has a potential of 0 volts at all temperatures and a pressure of 1 atm. The figure on the left highlights the platinum (Pt) wire electrode which is not a part of the reaction (it’s a catalyst) and can serve as either the anode or cathode. The wire is immersed in an acidic solution with an H2 (g) outlet pumping gaseous hydrogen into the solution. On the surface of the Pt electrode, a half-reaction occurs:

${\ce {2H^+{}_{(aq)}+ 2e^- <=> H_2{}_{(g)}}}$

The cell notation is as follows with a single line denoting a phase boundary and a double line representing a salt bridge: ${\ce {Pt | H_2 (1 atm) | H^+(1M) ||}}$

In fieldwork, the SHE is inconvenient, making the Saturated Calomel Electrode (SCE) the second most used reference. However, it contains mercury, making it the less preferred choice of measurement. The electrode, as shown on the right, is connected to an electrical lead. A platinum wire in a paste of Hg/Hg₂Cl₂ is placed in a saturated 3M KCl solution. A small hole of asbestos wire is located on the bottom of the internal electrode. A ceramic frit, acting as the salt bridge, is located on the bottom of the reference electrode. The overall half-reaction is:

${\ce {Hg2Cl2 + 2e^- <-> 2Hg + 2Cl^-}}$

The notation for the cell is: ${\ce {Hg | Hg2Cl2 | KCl(xM) ||}}$

Given its toxicity, the silver chloride electrode is most frequently used even over the SCE. Within the reference electrode, an Ag/AgCl wire is immersed in a KCl-filling solution. A frit at the bottom of the reference electrode plays the role of a salt bridge. The overall half-reaction is:

${\ce {AgCl + e^-<=> Ag +Cl^-}}$

The notation for the cell is: ${\ce {Ag | AgCl | KCl(xM)||}}$

References

^ Meyerhoff, M. E.; Opdycke, W. N. (1986-01-01), Spiegel, Herbert E. (ed.), "Ion-Selective Electrodes", Advances in Clinical Chemistry, vol. 25, Elsevier, pp. 1–47, retrieved 2024-11-04
^ Ion-Selective Electrodes. doi:10.1007/978-3-642-36886-8.pdf.

Cite error: A list-defined reference with the name ":0" has been invoked, but is not defined in the <references> tag (see the help page).

[:0-1] Meyerhoff, M. E.; Opdycke, W. N. (1986-01-01), Spiegel, Herbert E. (ed.), "Ion-Selective Electrodes", Advances in Clinical Chemistry, vol. 25, Elsevier, pp. 1–47, retrieved 2024-11-04

[:1-2] Ion-Selective Electrodes. doi:10.1007/978-3-642-36886-8.pdf.

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