10.17
Woltamperometria: Przegląd
Woltamperometria to technika elektroanalityczna, w której prąd przepływający przez ogniwo elektrochemiczne jest mierzony jako funkcja przyłożonego pot…
W woltamperometrii prąd przepływający przez ogniwo elektrochemiczne jest mierzony jako funkcja przyłożonego potencjału w warunkach polaryzacji stężenia.
Cela woltamperometryczna składa się z trzech elektrod zanurzonych w rozcieńczonym roztworze analitu wraz z elektrolitem pomocniczym.
Potencjostat zmienia potencjał elektrody roboczej w stosunku do elektrody odniesienia. Powstały prąd jest rejestrowany w stosunku do potencjału elektrody roboczej w postaci woltammogu.
Elektrody robocze w woltamperometrii mają minimalne powierzchnie, aby poprawić polaryzację.
Ogólnie rzecz biorąc, elektrody robocze rtęci są preferowane ze względu na ich stosunkowo duży zakres potencjału ujemnego ze względu na wysokie przepięcie wodoru na rtęć.
Dodatkowe zalety to tworzenie amalgamatu i możliwość odnawiania powierzchni elektrody poprzez tworzenie nowej kropli.
W rozcieńczonych roztworach prąd, który powstaje, gdy forma elektroaktywna jest redukowana lub utleniana, osiąga wartość graniczną wprost proporcjonalną do stężenia analitu. Mierząc napięcie i prąd na granicy faz między elektrodą a elektroaktywnym analitem, woltamperometria ujawnia informacje o procesach redukcji lub utleniania analitu.
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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.