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Summary

In this lecture on potentiometry, the instructor explains the principles behind potentiometric measurements, highlighting the roles of working and reference electrodes. It covers the three types of metal working electrodes, including metal electrodes, redox electrodes, and those based on ion-selective membranes. The discussion also includes how membrane potentials are created and utilized in potentiometric measurements, focusing on the construction and function of ion-selective electrodes. The relationship between the cell potential and the concentration of ions, including examples for different analytes, is also explored, providing a comprehensive understanding of this electrochemical technique.

Takeaways

• 😀 Potentiometric measurements involve two electrodes: the working electrode, whose potential depends on the solution composition, and the reference electrode, which maintains a constant potential regardless of solution changes.
• 😀 Three commonly used reference electrodes are the standard hydrogen electrode, the calomel electrode, and the AgCl electrode.
• 😀 The first type of metal working electrode is a metal immersed in a solution containing metal ions, where its potential depends on the concentration of those ions, as explained by the Nernst equation.
• 😀 The second type of metal working electrode involves a metal immersed in a solution containing ions in equilibrium with metal ions, such as an Ag electrode in a solution with AgCl, where its potential depends on the concentration of chloride ions.
• 😀 The third type is the redox electrode, where a metal immersed in a solution with ions undergoing redox reactions (e.g., platinum in a solution of Fe2+ and Fe3+) generates a potential based on the concentration of those ions.
• 😀 Membrane electrodes, which are selective for certain ions, are also widely used in potentiometric measurements. They form a potential difference due to selective ion movement across the membrane.
• 😀 When a membrane allows ions like H+ and Cl- to pass, their mobility differences cause a potential difference across the membrane, leading to the formation of membrane potential.
• 😀 Ion-selective electrodes are constructed with membranes that are selective for a specific ion, and the potential difference is measured between the working electrode and the reference electrode.
• 😀 The potential difference between the two electrodes can be influenced by the membrane potential, the reference electrode potential, and interface potential, and is related to the concentration of the analyte in the solution.
• 😀 The relationship between cell potential and analyte concentration can be expressed through an equation involving the charge of the ion (Z) and the logarithmic concentration of the analyte, such as for H+, Ca2+, SO4 2-, and NO3- ions.

Q & A

What is the difference between a working electrode and a reference electrode in potentiometry?

The working electrode is an electrode whose potential changes depending on the composition of the solution, while the reference electrode has a constant potential regardless of changes in the solution's composition.

What are the three types of reference electrodes commonly used in potentiometric measurements?

The three reference electrodes commonly used are the Standard Hydrogen Electrode, Calomel Electrode, and AgCl Electrode.

What characterizes a metal working electrode?

A metal working electrode is one whose potential depends on the concentration of metal ions in the solution. There are three types of metal working electrodes: the first type depends on the concentration of metal ions, the second type depends on equilibrium concentrations of metal ions, and the third type is a redox electrode where the metal can undergo redox reactions.

How does a second type metal working electrode differ from the first type?

A second type metal working electrode is one where the potential depends on the concentration of ions in equilibrium with the metal. For example, the potential of an Ag electrode in a solution with AgCl depends on the concentration of chloride ions (Cl-) due to the dissolution equilibrium of AgCl.

What is a redox electrode and how does it work?

A redox electrode is an inert metal electrode immersed in a solution containing ions that can undergo redox reactions. The potential of such an electrode depends on the concentrations of the ions involved in the redox process, such as the potential of a platinum electrode in a solution containing Fe2+ and Fe3+ ions.

What is a membrane electrode and how is the potential formed on it?

A membrane electrode uses a membrane that is selectively permeable to certain ions. When two solutions of different ion concentrations are separated by this membrane, an ion gradient develops, creating a potential difference. This potential is called membrane potential and can be utilized in potentiometric measurements.

Why does H+ ion move faster than Cl- ions across the membrane in the described example?

H+ ions move faster than Cl- ions because H+ has higher mobility. This difference in mobility causes a net movement of H+ from the left to the right side of the membrane, resulting in a positive charge accumulation on the right side.

What is the purpose of the internal solution in a membrane electrode?

The internal solution in a membrane electrode ensures that the concentration of the analyte ions inside the electrode remains constant. This helps maintain a stable reference for accurate measurements during potentiometric analysis.

What is the role of the external comparison electrode in potentiometric measurements?

The external comparison electrode is placed in the analyte solution and serves as a reference for the working electrode. The potential difference between the working electrode and the external comparison electrode is measured to determine the concentration of the analyte.

How is the relationship between cell potential and analyte concentration derived?

The relationship between cell potential and analyte concentration is derived using the Nernst equation. It considers the membrane potential, reference electrode potentials, and the ion concentration inside and outside the membrane. This equation helps calculate the cell potential based on the analyte concentration.