Electroanalytical methods
Techniques measuring potential or current in electrochemical cells.
Electroanalytical methods are techniques in analytical chemistry that examine an analyte by measuring either the voltage or the electric current in an electrochemical cell containing that analyte. They fall into three main categories based on what is controlled and what is measured: potentiometry, amperometry, and coulometry. In potentiometry, the voltage difference between two electrodes is measured with minimal disturbance to the solution. One electrode, the reference, maintains a constant potential, while the other, the indicator electrode, changes potential with the sample’s composition. The difference between them reveals the sample’s composition. Because the measurement is non-destructive and assumes equilibrium between the electrode and solution, it essentially measures the solution’s potential. Indicator electrodes are often made selectively sensitive to a specific ion, such as fluoride in a fluoride-selective electrode, so the potential depends only on that ion’s activity. The time needed for the electrode to reach equilibrium affects accuracy. In aquatic environments, platinum is common due to its fast electron transfer kinetics, though multi-metal electrodes can also improve these kinetics. The most widely used potentiometric electrode is the glass-membrane electrode found in pH meters. A variation, chronopotentiometry, applies a constant current and records potential over time; it was introduced by Weber. Amperometry covers techniques where current is measured as a function of an independent variable, typically time (chronoamperometry) or electrode potential (voltammetry). Chronoamperometry measures current at a fixed potential at different times after polarization begins, usually in unstirred solution at a fixed electrode to avoid convection. Voltammetry, a subclass of amperometry, measures current while varying the applied potential. Different voltammetric techniques are defined by the waveform of that potential variation over time. In chronoamperometry, a sudden potential step is applied to the working electrode, and current is recorded over time. Because it is not exhaustive, microelectrodes are used, and experiments are very short—typically 20 milliseconds to 1 second—to avoid consuming the analyte. Voltammetry applies a constant or varying potential to an electrode surface and measures the resulting current using a three-electrode syste
- field
- Analytical chemistry
- known_for
- Measuring potential and current in electrochemical cells to analyze analytes
- main_categories
- Potentiometry, amperometry, coulometry
- common_technique
- Potentiometry using glass-membrane electrode in pH meter
- subclass_of_amperometry
- Voltammetry and chronoamperometry
- subclass_of_voltammetry
- Polarography using dropping mercury electrode
Lore & Background
Potentiometry passively measures the potential difference between two electrodes, one a reference electrode with constant potential and the other an indicator electrode whose potential changes with sample composition. This non-destructive measurement assumes the electrode is in equilibrium with the solution. Indicator electrodes are often made selectively sensitive to the ion of interest, such as fluoride in fluoride selective electrodes. The time to establish equilibrium affects sensitivity or accuracy. In aquatic environments, platinum is often used due to its high electron transfer kinetics, though electrodes made from several metals can enhance this. The most common potentiometric electrode is the glass-membrane electrode used in a pH meter. A variant, chronopotentiometry, uses a constant current and measures potential as a function of time.
Reader's Guide
Amperometry encompasses techniques where current is measured as a function of an independent variable, typically time (chronoamperometry) or electrode potential (voltammetry). Chronoamperometry applies a sudden step in potential and measures current over time, typically using microelectrodes and short times (20 ms to 1 s) to avoid consuming the analyte. Voltammetry applies constant or varying potential at an electrode's surface and measures current with a three-electrode system, revealing reduction potential and electrochemical reactivity. It is practically non-destructive, consuming only a small amount of analyte at the electrode surface. Polarography is a subclass of voltammetry using a dropping mercury electrode. Coulometry uses applied current or potential to completely convert an analyte from one oxidation state to another, measuring total current passed to determine number of electrons, which indicates analyte concentration or number of electrons transferred. Forms include bulk electrolysis (potentiostatic coulometry) and coulometric titrations.
Did You Know?
- Potentiometry is a non-destructive measurement that affects the solution very little.
- Chronoamperometry typically uses microelectrodes and experiment times of 20 ms to 1 s.
- Polarography is a subclass of voltammetry that uses a dropping mercury electrode as the working electrode.
- Coulometry can determine the number of electrons passed in a redox reaction when the concentration is known.
Frequently Asked Questions
What are Electroanalytical methods in the Electrochemistry And Surface Chemistry canon?
They are a family of analytical-chemistry techniques that identify or quantify a substance by reading the voltage or current generated inside an electrochemical cell that contains the analyte. Rather than relying on optical or mass-based signals, the entire analysis hinges on electrical measurements at an electrode interface.
What are the three main categories of Electroanalytical methods?
The canon sorts them into potentiometry (measuring voltage), amperometry (measuring current), and coulometry (measuring total charge passed). Which category applies depends on whether the experimenter holds the potential steady, holds the current steady, or simply integrates charge over time.
What is the most commonly cited technique under Electroanalytical methods?
Potentiometry using a glass-membrane electrode in a pH meter is the go-to example fans point to. The reference electrode holds a fixed potential while the indicator electrode shifts its voltage in response to the hydrogen-ion activity of the sample, giving a direct, low-disturbance reading.
How does potentiometry differ from amperometry in the Electroanalytical methods framework?
In potentiometry the cell is allowed to settle and the voltage difference between two electrodes is read with almost no current flowing, so the solution is barely perturbed. In amperometry, by contrast, the potential is held at a set value and the resulting current is monitored, making it sensitive to how fast the analyte is consumed at the electrode surface.
What sub-techniques sit beneath amperometry and voltammetry in the canon?
Chronoamperometry and voltammetry are both listed as subclasses of amperometry, and polarography—using a dropping mercury electrode—is treated as a special case of voltammetry. This nested hierarchy is one of the structural details fans love to map out when discussing the chapter.
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