Ion-selective electrode

Key takeaways

Ion-selective electrode belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ion-selective electrode to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ion-selective electrode from memory before moving on to harder problems.

Reference excerpt

An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a simple membrane-based potentiometric device which measures the activity of ions in solution. It is a transducer (or sensor) that converts the change in the concentration of a specific ion dissolved in a solution into an electrical potential. 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. The voltage is theoretically dependent on the logarithm of the ionic activity, according to the Nernst equation. Analysis with ISEs expands throughout a range of technological fields such as biology, chemistry, environmental science and other industrial workplaces like agriculture. Ion-selective electrodes are used in analytical chemistry and biochemical/biophysical research, where measurements of ionic concentration in an aqueous solution are required.

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 c e l l = E i s e − E r e f {\displaystyle E_{cell}=E_{ise}-E_{ref}}

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

Reference electrodes The most common types of reference electrodes used in analytical chemistry include the standard hydrogen electrode, the saturated calomel electrode, and the silver chloride electrode. 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:

2 H +

+ ( aq ) 2 e − ↽ − − ⇀ H 2

( g ) {\displaystyle {\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: Pt | H 2 ( 1 atm ) | H + ( 1 M ) | | {\displaystyle {\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:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Ion-selective electrode

Start with the simplest possible case. Write down what Ion-selective electrode claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ion-selective electrode before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ion-selective electrode ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ion-selective electrode

In research: Ion-selective electrode appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ion-selective electrode in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ion-selective electrode is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electroanalytical chemistry devices, Electrodes, so understanding it makes those chapters shorter.
In everyday life: Look for Ion-selective electrode outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Ion-selective electrode in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ion-selective electrode means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ion-selective electrode out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ion-selective electrode in simple terms?

An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a simple membrane-based potentiometric device which measures the activity of ions in solution. It is a transducer (or sensor) that converts the change in the concentration of a specific ion dissolved in a solution in…

Why does Ion-selective electrode matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ion-selective electrode?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ion-selective electrode.