Potentiometry in Electrochemistry Quiz

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Questions and Answers

What is the primary purpose of potentiometry in electrochemistry?

To determine the concentration of a gas in a solution

To analyze the voltage drop across a resistor

To measure the potential of electrochemical cells without significant current flow (correct)

To quantify the rate of a chemical reaction

In a metal/ion half-cell in potentiometry, what happens to the metal ions when they reach the metal rod?

They precipitate as solid metal

They ionize and enter the solution

They gain electrons and deposit as reduced form (correct)

They cause a reduction in the cell potential

What influences the change in potential for an analyte in potentiometry?

Temperature and humidity conditions

The concentration of the electroactive species (correct)

The color of the solution

The size and shape of the electrode

Which component is essential for maintaining the stability of the potential measurement in potentiometry?

Salt bridge Signup and view all the answers

Which equation is typically used to describe the relationship between potential and concentration in potentiometry?

Nernst equation Signup and view all the answers

What type of species is specifically analyzed using potentiometry?

Electroactive species Signup and view all the answers

What role does the reference electrode play in potentiometry?

It provides a stable reference potential Signup and view all the answers

When the concentration of metal ions increases in a galvanic cell, what is the expected effect on the potential?

The potential will increase Signup and view all the answers

What is the purpose of the salt bridge in an electrochemical cell?

To allow charge transfer without mixing solutions Signup and view all the answers

What factor primarily causes the formation of a junction potential in a salt bridge?

Different migration rates of anions and cations Signup and view all the answers

What is a practical way to minimize junction potential in an electrochemical cell?

Select a salt bridge with cations and anions of similar mobility Signup and view all the answers

According to Ohm's law, what does E represent in the equation E = RAC I?

The electromotive force of the cell Signup and view all the answers

What might limit the accuracy of direct potentiometric measurements in an electrochemical cell?

The junction potential at the salt bridge Signup and view all the answers

Which of the following salts would be most appropriate for constructing a salt bridge in electrochemical applications?

KCl Signup and view all the answers

What is the typical magnitude of a potential developed at the junction of a salt bridge?

Up to a few millivolts Signup and view all the answers

What physical structure can a salt bridge take?

A bent tube or inverted U-shaped tube Signup and view all the answers

What function does the reference electrode serve in a potentiometric measurement?

It provides a known constant potential for comparison. Signup and view all the answers

Why is a potentiometer used in measuring the e.m.f. of a galvanic cell?

To ensure measurements are taken under zero current conditions. Signup and view all the answers

What is e.m.f. in the context of a galvanic cell?

The difference in potential between the two electrodes. Signup and view all the answers

Which component connects the two electrode solutions and completes the circuit in a potentiometric setup?

The salt bridge. Signup and view all the answers

What does the change in e.m.f. of the cell indicate during potentiometric measurement?

Changes in the indicator electrode's potential related to ion concentration. Signup and view all the answers

In what scenario is a potentiometer particularly useful?

When measuring e.m.f. without causing polarization. Signup and view all the answers

How does a voltage divider function within a potentiometric setup?

It opposes the galvanic cell's potential. Signup and view all the answers

Which statement about the indicator electrode is correct?

Its potential can be directly linked to the ion concentration. Signup and view all the answers

What is the purpose of using an inert electrode in redox systems?

To facilitate electron transfer without participating in the reaction. Signup and view all the answers

In the Nernst equation, what effect does increasing the concentration of the reduced species have on the electrode potential (E25°C)?

It increases the electrode potential. Signup and view all the answers

According to the Nernst equation, when the ionic concentration is 1 molar, what is the relationship of E25°C to E0?

E25°C is equal to E0. Signup and view all the answers

For the ion/ion half-cell of Fe3+/Fe2+, what does the term 'oxidized form' refer to?

Fe3+. Signup and view all the answers

What does the variable 'n' represent in the Nernst equation?

The number of electrons transferred in the half-reaction. Signup and view all the answers

In a solution with a high concentration of a reduced species, how would you expect the Nernst equation to affect the system?

It would favor the reduction reaction. Signup and view all the answers

In the equation E25°𝐶 = E0 + 0.059 log [oxidized form]/[reduced form], what can be inferred when the ratio of the concentrations of the oxidized and reduced forms is less than 1?

The electrode potential (E25°C) will be negative. Signup and view all the answers

What is indicated when the standard electrode potential E0 is positive?

The reaction is spontaneous in the forward direction. Signup and view all the answers

What is a requirement for a reference electrode?

It must have a constant potential. Signup and view all the answers

Which of the following advantages does the Normal Hydrogen Electrode (NHE) have?

Its potential is considered to be zero. Signup and view all the answers

What role does a salt bridge play in electrochemical cells?

It allows migration of ions towards the electrodes. Signup and view all the answers

What is a disadvantage of the Normal Hydrogen Electrode?

It is difficult to maintain hydrogen gas at one atmosphere. Signup and view all the answers

Which of the following is a primary reference electrode?

Normal Hydrogen Electrode Signup and view all the answers

What is the half-cell presentation formula for the Normal Hydrogen Electrode (NHE)?

Pt(s)|H2(g,1atm)|H+(aq),1M || Signup and view all the answers

What interferes with the catalytic activity of platinum in the Normal Hydrogen Electrode?

Catalytic poisons like S2- Signup and view all the answers

Which equation describes the electrode reaction of the Calomel electrode?

Hg2Cl2 + 2e- ⇌ 2Cl- + 2Hg Signup and view all the answers

What is the role of the silver ion in the reaction involving silver chloride?

It is reduced to silver metal. Signup and view all the answers

What does the Nernst equation relate to in the context of the silver-silver chloride electrode?

It relates the potential to the concentration of ions in solution. Signup and view all the answers

What is the relationship between the solubility product Ksp of AgCl and the concentrations of silver and chloride ions?

Ksp of AgCl equals the product of the concentrations of silver and chloride ions. Signup and view all the answers

How does the concentration of chloride ions influence the electrode potential in a silver-silver chloride electrode?

The electrode potential is directly proportional to the logarithm of the chloride ion concentration. Signup and view all the answers

What happens to the electrode potential E25°C (Ag/Ag+) if the concentration of Ag+ decreases?

It decreases. Signup and view all the answers

At what standard condition does the E°(Ag/Ag+) potential equal 0.059 V?

In a saturated solution of KCl. Signup and view all the answers

Which of the following equations correctly represents the dissociation of sparingly soluble AgCl?

AgCl ⇌ Ag+ + Cl- Signup and view all the answers

What is the significance of the terms in the Nernst equation in relation to Ksp?

It provides a way to express concentrations in terms of each other. Signup and view all the answers

Study Notes

Electrochemistry: Potentiometry

Potentiometry is a method of analysis.
The concentration of an ion or substance is determined by measuring the potential developed when a sensitive electrode is immersed in a solution of the species being analyzed.
The process measures potential in electrochemical cells without significantly drawing current.

Overview of Today's Lecture

Introduction to potentiometry
Potentiometer instrument
Salt bridge and junction potential
Reference electrodes

Potentiometry

Potentiometry is a method of analysis.
It determines the concentration of an ion or substance by measuring the potential developed when a sensitive electrode is immersed in the solution.
Electrochemical cells are used without drawing significant current.

Basics of Potentiometry

Analyte is an electroactive species within a galvanic cell.
The analyte's concentration affects the potential, following the Nernst equation.
Case 1: Metals—metal/ion half-cell.
- The metal (M°) is immersed in a solution of its ions (Mⁿ⁺).
- Solution pressure (oxidation): The tendency of M° to go into solution.
- Ionic pressure (reduction): The tendency for Mⁿ⁺ ions to deposit on the M° rod.
Case 2: Non-metals—non-metal/ion half-cell.
- An inert electrode (e.g., platinum) is needed for electron transfer.
- The Nernst equation for non-metals: E_25°C = E₀ + (0.059/n) * log[N^n-]
Case 3: Redox systems—ion/ion half-cell.
- An inert platinum electrode is used.
- Example: Fe³⁺/Fe²⁺ E_25°C = E₀ + (0.059/n) * (log[oxidized form]/[reduced form])

Some Notes on the Electrode Potential

If the ionic concentration is 1 molar, the potential (E_25°C) equals the standard electrode potential (E₀).
The potential's sign is similar to the electrode's charge.
The potential of a single electrode cannot be directly measured but is measured against a reference electrode with a fixed potential within an electrochemical cell.

Instrument: Components for Potentiometric Measurement

Indicator electrode: The electrode used to determine ion concentration.
Reference electrode: Its potential is constant and known, used in conjunction with the indicator electrode to measure EMF.
EMF (=electromotive force): E = E_cathode - E_anode
Potentiometer: The device that measures the EMF developed in the electrochemical cell.
Salt bridge: Connects the two electrode solutions and completes the circuit.

Potentiometer: Measurement of EMF

Potentiometer measures EMF under zero current (no current withdrawal or imposition) to prevent polarization or chemical changes within the cell.
A galvanic cell, a voltage divider, and a galvanometer are its key components.
The variable potential from the divider is changed until zero current (as detected by the galvanometer) is reached. The potential at this point matches that of the galvanic cell.

Salt Bridge

A liquid junction between two half-cells.
Allows charge transfer without mixing the solutions.
Usually a bent or U-shaped tube filled with agar gel saturated with a salt (e.g., KCl, KNO₃).
Ions in the salt bridge should not pass into the half-cells to avoid potential interference with the intended measurements.

Salt Bridge: Junction Potential

A potential developed at the boundaries (interfaces with anode/cathode solutions) of the junction.
Occurs from differences in anion and cation migration rates within the salt bridge solution.
Unbalanced charge distribution at the boundaries generates a small potential.
Junction potential puts a fundamental limitation on accuracy in direct potentiometric measurements.

To reduce liquid junction potential

Choose an electrolyte for the salt bridge where its cations and anions have similar mobilities.
Employ high concentrations of the salt in the bridge solution to reduce the effect of differences in ion migration rates between the salt bridge and the electrode solutions.

Reference Electrodes

Requirements for reference electrodes:
- Constant potential
- Known and definite potential
Types:
- Primary: Normal Hydrogen Electrode (NHE)
- Secondary: Calomel, Silver/Silver Chloride

Normal Hydrogen Electrode (NHE)

Electrode reaction: 2H⁺ + 2e^- = H₂
Half-cell presentation: Pt(s) | H₂(g, 1 atm) | H⁺(aq, 1M) ||
Design: Diagram of the NHE setup details is essential for demonstration.
Nernst equation: E_25°C = E₀ + (0.059/2) * log[H⁺]²
Potential in standard conditions: E_25°C = 0V

Normal Hydrogen Electrode (NHE): Disadvantages

Difficult to use (practical limitations)
Vulnerable to catalytic poisons
Cannot maintain constant H₂ pressure
Needs replating

Calomel Electrode

Electrode reaction: Hg₂Cl₂ + 2e^- = 2Hg + 2Cl^-
Half-cell presentation: Hg | Hg₂Cl₂, KCl(sat/1N/0.1N)

Silver/Silver Chloride Electrode

Electrode reaction: AgCl + e^- = Ag + Cl^-
Half cell presentation: Ag | AgCl, KCl(sat/1N/0.1N)
Nernst equation: E= E₀ + (0.059/1) * log(K_sp/[Cl^-])

Thank you

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Description

Test your knowledge on the principles and applications of potentiometry in electrochemistry. This quiz covers topics such as half-cells, metal ions, potential measurements, and the roles of electrodes and salt bridges. Challenge yourself to deepen your understanding of this essential analytical technique.