8.2
Verkehrsnummer
Die Transportzahl ist der Anteil des Gesamtstroms, den ein Ion in einer Elektrolytlösung transportiert. Sie ist definiert als das Verhältnis des Strom…
Die Transportzahl ist der Anteil des gesamten Stroms, der von Kationen oder Anionen in einer elektrolytischen Lösung transportiert wird.
Hittorfs Methode erklärt die Transportzahlen, indem sie die Ionenwanderung in einer Zelle untersucht, die in anodische, zentrale und kathodische Kompartimente unterteilt ist.
Wenn ein Potentialunterschied angelegt wird, bewegen sich Kationen zur Kathode und Anionen zur Anode, was zu Konzentrationsänderungen in der Nähe jeder Elektrode führt. Das zentrale Kompartiment dient als Referenzbereich, der weniger von Elektrodenreaktionen betroffen ist.
Das Kationen-zu-Anion-Mobilitätsverhältnis entspricht dem Abfall der Konzentration um die Anode gegenüber der Kathode.
Die Kationentransportzahl wird definiert als der Konzentrationsfall um die Anode geteilt durch den Gesamtfall um beide Elektroden, während die Anionentransportzahl eins minus die Kationentransportzahl ist.
Ein alternativer Ansatz ist die bewegliche Grenzmethode, bei der ein elektrischer Strom die Grenze zwischen dem Hauptelektrolyten HCl und dem Indikatorelektrolyt CdCl₂ verschiebt.
Wenn ein angewandtes Potential Q die Grenze um l cm verschiebt und lA cm3 verdrängt, das c-Gramm-Äquivalente von HCl enthält, kann die Transportzahl der H⁺-Ionen mathematisch berechnet werden.
View the full transcript and gain access to JoVE Core videos
Q1: What is transport number and why does it matter in electrochemistry?
Transport number is the fraction of total current carried by a specific ion in an electrolyte solution. It represents the ratio of current carried by that ion to the total current flowing through the solution. Understanding transport numbers is essential for predicting ionic mobility and how fast ions move under an electric field, directly connecting to electrochemical reaction behavior.
Q2: How does Hittorf's method measure transport numbers?
Hittorf's method divides an electrolyte solution into anodic, central, and cathodic compartments. When potential is applied, cations migrate toward the cathode and anions toward the anode, causing measurable concentration changes near each electrode. The central compartment serves as a reference region unaffected by electrode reactions, allowing independent measurement of concentration changes that reflect ion mobility ratios.
Q3: What is the relationship between cation mobility and concentration changes in Hittorf's method?
The cation-to-anion mobility ratio equals the concentration fall around the anode to that around the cathode. The cation transport number is defined as the concentration fall around the anode divided by the total fall around both electrodes. This relationship allows researchers to calculate transport numbers from measured concentration changes in gram equivalents.
Q4: How does the moving boundary method determine transport numbers?
The moving boundary method tracks the movement of a sharp boundary between a principal electrolyte and an indicator electrolyte under an applied electric field. For example, to measure H⁺ transport in HCl, CdCl₂ serves as the indicator because its cation has lower mobility than H⁺ and shares the same anion. By measuring boundary displacement over time, the transport number can be calculated mathematically.
Q5: Why is the central compartment important in Hittorf's method?
The central compartment in Hittorf's method serves as a reference region that remains relatively unaffected by electrode reactions. This allows researchers to measure concentration changes in the anodic and cathodic compartments independently and accurately. The central compartment's stability ensures that observed concentration changes reflect only ion migration caused by the applied potential.
Q6: How is anion transport number calculated from cation transport number?
Anion transport number is obtained by subtracting the cation transport number from unity. Since cations and anions together carry all the current in an electrolyte solution, their transport numbers must sum to one. This relationship provides a simple way to determine anion transport once cation transport is measured or calculated.
Q7: What role does ionic mobility play in transport number determination?
Ionic mobility describes how fast an ion moves under an electric field influence. The extent of concentration change near electrodes in Hittorf's method directly reflects the relative mobilities of cations and anions. Transport numbers quantify this mobility difference, connecting the physical behavior of ions in solution to the electrical transport properties measured during electrochemical reactions.