10.17
Voltammetrie: Übersicht
Die Voltammetrie ist eine elektroanalytische Technik, bei der der durch eine elektrochemische Zelle fließende Strom als Funktion des angelegten Potent…
Bei der Voltammetrie wird der Strom, der durch eine elektrochemische Zelle fließt, als Funktion des angelegten Potentials unter Bedingungen der Konzentrationspolarisation gemessen.
Die voltammetrische Zelle besteht aus drei Elektroden, die in eine verdünnte Lösung des Analyten zusammen mit einem unterstützenden Elektrolyten eingetaucht sind.
Ein Potentiostat variiert das Potential der Arbeitselektrode relativ zur Referenzelektrode. Der resultierende Strom wird als Voltammogramm gegen das Arbeitselektrodenpotential aufgezeichnet.
Arbeitselektroden in der Voltammetrie haben minimale Oberflächen, um die Polarisation zu verbessern.
Im Allgemeinen werden Quecksilber-Arbeitselektroden aufgrund ihres relativ großen negativen Potentialbereichs aufgrund der hohen Überspannung von Wasserstoff auf Quecksilber bevorzugt.
Weitere Vorteile sind die Amalgambildung und die Möglichkeit, die Elektrodenoberfläche durch neue Tropfenbildung zu erneuern.
In verdünnten Lösungen erreicht der Strom, der entsteht, wenn eine elektroaktive Spezies reduziert oder oxidiert wird, einen Grenzwert, der direkt proportional zur Konzentration des Analyten ist. Durch die Messung von Spannung und Strom an der Grenzfläche zwischen der Elektrode und einem elektroaktiven Analyten gibt die Voltammetrie Aufschluss über die Reduktions- oder Oxidationsprozesse des Analyten.
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Q1: What are the three electrodes used in a voltammetric cell?
A voltammetric cell contains a working electrode where redox reactions occur, a reference electrode that maintains constant potential for accurate measurement, and an auxiliary electrode, typically platinum wire, that completes the circuit. These three electrodes are immersed in a dilute solution containing the analyte and supporting electrolyte.
Q2: Why are mercury electrodes preferred as working electrodes in voltammetry?
Mercury electrodes offer a large negative potential range, preventing unwanted hydrogen reduction that would interfere with measurements. They can form amalgams with various metals, enhancing certain redox reactions. Additionally, their surface can be renewed by forming new drops, maintaining measurement accuracy and consistency throughout experiments.
Q3: How does voltammetry determine analyte concentration?
In dilute solutions, the current produced when an electroactive species is reduced or oxidized reaches a limiting value directly proportional to the analyte's concentration. By measuring voltage and current at the electrode-analyte interface, voltammetry reveals reduction or oxidation processes and enables quantitative analysis through this concentration-dependent current response.
Q4: What is a voltammogram and what information does it provide?
A voltammogram is a plot of current versus applied potential at the working electrode. It records the current response as a potentiostat varies the working electrode potential relative to the reference electrode. The voltammogram reveals information about redox-active species, their reduction or oxidation processes, and provides data for identifying and quantifying electroactive analytes in solution.
Q5: What role does concentration polarization play in voltammetry?
Voltammetry operates under conditions of concentration polarization, where current is measured as a function of applied potential. This condition ensures that the current response depends on the analyte concentration at the electrode surface. Working electrodes with minimal surface areas enhance polarization, allowing accurate measurement of electroactive species and reliable quantitative analysis.
Q6: What are the common types of mercury working electrodes used in voltammetry?
The hanging mercury drop electrode (HMDE) and the dropping mercury electrode (DME) are the most common mercury working electrodes. Both electrodes leverage mercury's wide negative potential range and ability to form amalgams. Their surfaces can be renewed by forming new drops, ensuring consistent and reliable results during voltammetric measurements and analysis.
Q7: How does a potentiostat control the working electrode in voltammetry?
A potentiostat varies the working electrode's potential relative to the reference electrode, which maintains a constant reference point. The potentiostat systematically changes the applied potential while measuring the resulting current response. This controlled potential variation allows the instrument to generate a voltammogram that reveals the electrochemical behavior of redox-active species in the solution.