10.2
Elettrodi Panoramica
Le misurazioni elettrochimiche vengono condotte in una cella elettrochimica formata da vari componenti che controllano e misurano la corrente e il pot…
Il potenziale e la corrente all'interno delle celle a due elettrodi sono tipicamente misurati utilizzando una combinazione di un elettrodo di riferimento e di un elettrodo di lavoro o di un elettrodo indicatore.
In un test potenziometrico, l'elettrodo indicatore risponde proporzionalmente alla concentrazione dell'analita attraverso una reazione redox.
I tipi comuni di elettrodi indicatori includono elettrodi in metallo inerte, elettrodi in metallo reattivi reversibili ed elettrodi a membrana ionoselettivi.
Gli elettrodi di riferimento, come l'elettrodo di calomelano saturo e l'elettrodo di cloruro d'argento-argento, hanno un potenziale costante a una data temperatura, indipendentemente dalle variazioni nella composizione dell'analita. Funzionano meglio quando la corrente che li attraversa è bassa, fornendo un potenziale di riferimento fisso per l'elettrodo indicatore.
Il potenziale relativo dell'elettrodo indicatore viene misurato rispetto al potenziale di riferimento fisso fornito dall'elettrodo di riferimento.
Tuttavia, per i metodi in cui è necessario un flusso di corrente, è possibile aggiungere un terzo elettrodo, noto come elettrodo ausiliario o controelettrodo. Questi elettrodi ausiliari consentono alla corrente di fluire attraverso la cella per completare il circuito elettrico.
View the full transcript and gain access to JoVE Core videos
Q1: What is the role of a reference electrode in electrochemical measurements?
A reference electrode provides a constant potential at a given temperature, independent of analyte composition changes. It establishes a fixed reference point against which the working electrode's potential is measured. Reference electrodes like the saturated calomel electrode and silver-silver chloride electrode work best when minimal current flows through them, ensuring their potential remains stable during potentiometric analysis.
Q2: How do indicator electrodes respond to analyte concentration?
Indicator electrodes respond proportionally to analyte concentration through redox reactions occurring at their surfaces. Common types include inert metal electrodes like platinum, reversibly reactive metal electrodes like silver, and ion-selective membrane electrodes. The potential change of the indicator electrode reflects the activity of electroactive species in solution, enabling concentration determination in potentiometric tests.
Q3: Why is a third electrode needed in some electrochemical methods?
A third electrode, called an auxiliary or counter electrode, is added when current flow is required through the electrochemical cell. While reference and working electrodes measure potential, the auxiliary electrode completes the electrical circuit and facilitates current passage. This enables dynamic methods where current alters the concentration of species in the cell, such as controlled potential coulometry electrolytic methods.
Q4: What distinguishes inert metal electrodes from reactive metal electrodes?
Inert metal electrodes like platinum do not participate in redox reactions; they only facilitate electron transfer. Reversibly reactive metal electrodes like silver can participate in redox reactions and respond to specific ions such as Ag+, halides, and other species that react with the electrode material. This reactivity makes them suitable as indicator electrodes for detecting particular analytes.
Q5: How do ion-selective membrane electrodes function in potentiometric analysis?
Ion-selective membrane electrodes contain specialized membranes that selectively respond to specific ions in solution. These electrodes generate a potential proportional to the concentration of target ions, enabling selective analyte detection. They represent an important class of indicator electrodes used in potentiometric measurements where specificity for particular ionic species is required.
Q6: What happens to reference electrode potential when current flows through it?
Reference electrodes are designed to maintain constant potential only when minimal current flows through them. If significant current passes through a reference electrode, its potential becomes unstable and unreliable. This is why reference electrodes work best in potentiometric methods with low current, and why auxiliary electrodes are used in high-current applications to protect the reference electrode's stability.
Q7: How are working and reference electrodes used together in a two-electrode cell?
In a two-electrode cell, the working electrode's potential is measured relative to the reference electrode's fixed potential. The working electrode responds to analyte concentration changes through redox reactions, while the reference electrode provides the stable baseline for comparison. This configuration allows potentiometric tests to quantify analyte concentration by monitoring the potential difference between the two electrodes.