Electrochemistry: Ag/AgCl Reference Electrode

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Questions and Answers

What is the standard reduction potential ($E^o$) for the silver-silver chloride (Ag/AgCl) electrode?

+0.268 V

+0.197 V

+0.222 V (correct)

+0.241 V

The electrode potential of a Ag/AgCl reference electrode depends primarily on which factor?

The concentration of $K^+$ ions.

The concentration of $Ag^+$ ions.

The concentration of $AgCl$ solid.

The concentration of $Cl^-$ ions. (correct)

What is the main purpose of using a high concentration of KCl in the internal solution of a Ag/AgCl reference electrode?

To increase the solubility of AgCl.

To minimize variations in the $Cl^-$ concentration over time. (correct)

To enhance the conductivity of the electrode.

To prevent the formation of AgCl precipitates.

What type of substances are considered interferents for the Ag/AgCl reference electrode?

Substances that react with $Ag^+$, forming precipitates or complexes. Signup and view all the answers

What is the electrode potential (E) of a Ag/AgCl electrode in saturated KCl?

+0.197 V Signup and view all the answers

What is the primary function of the slow outward flow of the electrolyte in a reference electrode?

To prevent the entry of sample components and contamination of the internal electrode. Signup and view all the answers

Why are double junction reference electrodes used?

To prevent interferences from the components of the sample matrix. Signup and view all the answers

Which of the following is a critical characteristic of an effective indicator electrode?

It should respond rapidly and reproducibly to changes in analyte concentration. Signup and view all the answers

What is the electrode potential (E) of a Calomel electrode in saturated KCl?

+0.241 V Signup and view all the answers

Why do metal indicator electrodes have limited analytical applications, especially with complex samples?

They exhibit low selectivity and can be easily interfered with. Signup and view all the answers

Which of the following is true about the standard hydrogen electrode (SHE)?

It is used as a primary reference to which all other electrode potentials are related. Signup and view all the answers

What distinguishes ion-selective electrodes (ISEs) from metal indicator electrodes?

ISEs are based on a selective interaction with the analyte or a concentration difference, whereas metal electrodes works through redox. Signup and view all the answers

Which of the following describes the response mechanism of metal indicator electrodes?

A redox reaction involving the analyte at the electrode surface. Signup and view all the answers

An ion-selective electrode's response is dependent on the analyte concentration. Where does the comparison of analyte concentration take place?

Between the electrode internal solution and the sample solution. Signup and view all the answers

What is the purpose of the porous setto in the reference electrode diagram?

To allow electrical contact while minimizing direct mixing of solutions. Signup and view all the answers

What is the purpose of the intermediate chamber in the reference electrode?

To prevent the reference electrolyte from directly contacting the sample. Signup and view all the answers

Which of the following practices helps maintain the accuracy of pH measurements?

Ensuring that the calibration and sample measurements are performed at the same temperature. Signup and view all the answers

What is the primary function of the thermal probe in a pH meter?

To correct for pH variations due to temperature changes. Signup and view all the answers

What does ATC stand for in the context of pH measurement?

Automatic Temperature Compensation Signup and view all the answers

Why is it important to remove the air inlet cap from a pH electrode before use?

To allow for pressure equalization inside the electrode. Signup and view all the answers

What is the recommended method for drying a pH electrode after washing it with distilled water?

Blot it dry gently with a tissue. Signup and view all the answers

Why should a glass electrode be stored in an aqueous solution?

To prevent dehydration of the glass membrane. Signup and view all the answers

What should you do if a glass electrode has dried out?

Recondition it in dilute acid for several hours. Signup and view all the answers

What specific pretreatment is advised if an electrode is to be utilized at a reading above a pH of $9$?

Soak in a high-pH buffer Signup and view all the answers

What is the primary mechanism by which $H^+$ ions interact with the glass membrane of a pH electrode?

They diffuse into the membrane, replacing metal ions within the hydrated gel regions. Signup and view all the answers

What is the function of the silver wire coated with $AgCl(s)$ inside a glass electrode?

To provide a stable and known potential as an internal reference electrode. Signup and view all the answers

What is the purpose of the external reference electrode in a potentiometric pH measurement using a glass electrode?

To provide a second stable potential for measuring the potential difference across the glass membrane. Signup and view all the answers

What is a key structural characteristic of the silicate glass used in pH electrodes?

An irregular network of $SiO_4$ tetrahedra connected through oxygen atoms. Signup and view all the answers

Which of the following components is essential for the operation of an ion-selective electrode (ISE) based on a crystal membrane?

A single crystal or polycrystalline material acting as the membrane. Signup and view all the answers

What is the role of metal ions such as $Li^+$, $Na^+$, $K^+$, and $Ca^{2+}$ within the silicate glass network of a pH electrode?

They are coordinated to oxygen atoms and can be replaced by $H^+$ ions. Signup and view all the answers

What distinguishes an ion-selective electrode (ISE) based on a polymer membrane from one based on a crystal membrane?

Polymer membrane ISEs utilize a polymer containing an ion exchanger, while crystal membrane ISEs use a crystalline material. Signup and view all the answers

In composite electrodes, what is the primary function of the second membrane?

To isolate the analyte from the electrode, often to prevent interference or fouling. Signup and view all the answers

Which membrane composition is utilized in an ion-selective electrode for fluoride (F-) analysis?

LaF3 crystal doped with Eu2+ Signup and view all the answers

What role does the hydrophobic structure surrounding the binding site play in ion-selective electrodes with polymer or liquid membranes?

Ensures the ion exchanger's solubility in the organic solvent or polymer. Signup and view all the answers

Which of the following components is essential for the function of a liquid membrane ion-selective electrode used for Ca2+ detection.

A porous hydrophobic material soaked with an organic solvent containing an ion exchanger Signup and view all the answers

Which ionophore is primarily associated with K+ (Potassium) selectivity in ion-selective electrodes?

Valinomycin Signup and view all the answers

In the context of Ca2+ selective electrodes with liquid membranes, what is the primary purpose of the relatively long aliphatic chains (C8 – C10) attached to the ion exchanger?

Increase its solubility in the organic solvent. Signup and view all the answers

What is the effect of a low pH environment on a liquid membrane ion-selective electrode used for Ca2+ detection?

Decreases the electrode's sensitivity to Ca2+ due to protonation of the ion exchanger. Signup and view all the answers

What is the source of interference when measuring $Ca^{2+}$ with the ion selective electrode?

Other divalent cations, especially $Fe^{2+}$ Signup and view all the answers

Based on the provided equation, how does the electrode potential (EISE) change with a tenfold increase in $Ca^{2+}$ concentration?

It increases by 0.059/2 V Signup and view all the answers

What is a significant limitation of ion selective electrodes when used for analytical measurements?

They are susceptible to contamination and interference from proteins and organic molecules. Signup and view all the answers

When using ion selective electrodes, what is the primary reason for adding a high concentration of an inert salt to both standards and samples?

To ensure that all solutions have the same ionic strength, minimizing variations in response. Signup and view all the answers

In potentiometry with ion selective electrodes, what is the main reason for eliminating or masking ligands in the sample?

To prevent the formation of complexes that affect the free analyte ion concentration. Signup and view all the answers

Why is calibration with standard addition particularly useful when working with ion selective electrodes?

It compensates for differences in ionic strength and matrix effects between samples and standards. Signup and view all the answers

What is a major advantage of using ion selective electrodes for analysis?

They can be used for real-time monitoring of complex systems due to their rapid and non-destructive analysis capabilities. Signup and view all the answers

Which analytical technique is typically used to measure $pO_2$, exploiting a membrane permeable to oxygen?

Amperometry. Signup and view all the answers

Which approach minimizes errors arising from ionic strength differences between samples and standards?

Employing the saturation approach by adding excess electrolyte to all samples and standards. Signup and view all the answers

In potentiometry, if a fraction of the analyte is present in a complex, how does this affect the measurement, and what action should be taken?

It leads to an underestimation of the total analyte concentration, so ligands must be eliminated or masked. Signup and view all the answers

Study Notes

Potentiometry

Potentiometry is an electrochemical analytical technique that measures voltage in an electrochemical cell to determine the concentration of chemical species.
Electrodes, specifically ion selective electrodes, enable rapid, sensitive, and selective measurements. Measurements are performed using laboratory or portable instruments.
Electric rays and other fishes, generate strong electric shocks using electric organs. Nerve terminals rapidly release acetylcholine to cause sodium ions to surge through membranes, generating a voltage across the membrane. Human technology is inspired by this process.

Introduction

Electricity can be used to power chemical reactions, and vice-versa, chemical reactions can be used to create electricity in potentiometry.
Electric charge is measured in Coulombs (C). The flow of charge is balanced: negative charge flow is countered by positive charge flow.
Electric potential difference (voltage) is the work per unit charge. Current (I) is the rate of charge flow (1A = 1C/s).

Electrochemical Cells

Electrochemistry uses electrical measurements to study chemical systems. Electrochemical cells contain two half cells joined by a salt bridge.
Ions flow to oppositely charged electrodes. Electrons flow between electrodes and an external circuit.
The potential difference, or voltage, is measured between electrodes using a potentiometer which has high resistance to limit current flow.

Standard Potentials

Standard potential (E°) shows the driving force for redox reactions under standard conditions (all species have activity=1 and a reference electrode is used).
Redox reaction refers to the transfer of electrons between species.
Specific half-cell reactions have their own characteristic E° values. These dictate their behaviour as either cathodes or anodes.

Nernst Equation

The Nernst equation relates the cell potential (E) to the concentrations of reactants. It accounts for activities (better said concentrations) of the involved species in a chemical reaction.
The reaction quotient (Q) in the equation shows the relative ratio of product and reactant concentrations. At equilibrium, Q=K, leading to E=0

Reference Electrodes

Reference electrodes maintain stable potentials, independent of the analyte being measured.
Commonly used reference electrodes use a metal and its corresponding insoluble salt. E.g. AgCl/Ag.
The potential depends on the concentration of the anion that forms the insoluble salt with the metal, and in most cases, a known solution with high concentration of the anion (e.g., KCl in a saturated solution) is used for this purpose.

Indicator Electrodes

Indicator electrodes (often called working electrodes) respond to variations on the analyte concentration.
Metal indicator electrodes show their response based on a redox reaction involving the analyte.
Ion selective electrodes (ISEs) do not use redox reactions. They respond based on selective interaction between the analyte and a membrane. The difference in analyte concentration inside the electrode and in external solution is measured.

pH Measurement with Glass Electrodes

Glass electrodes are the most common ion selective electrodes for pH measurements. The electrode measures the potential difference between an internal reference electrode and an external analyte solution using a glass membrane.
The internal reference is fixed (e.g H+), so potential variations depend entirely on the analyte solution pH. This makes pH readily measurable when using glass electrodes.

pH Combination Electrode

pH combination electrodes combine both a glass electrode and a reference electrode in a single unit.
The reference electrode is typically a silver-silver chloride electrode that provides a stable reference potential.

Errors in pH Measurements with Glass Electrodes

Standards: pH measurements depend on the accuracy of known buffers.
Junction potential: the potential difference at the junction between the probe's internal and external solutions, can influence the result, but it is usually stable with well buffered solutions.
Temperature effects: temperatures variations affect the pH measurement as well as the calibration standards, thus, precise calibration at the same temperature as the sample is needed.

Ion Selective Electrodes for Other Ions

Many other ISEs are available for measuring various ions. These include modified composition glass electrodes with varying compositions or crystalline, polymer, or liquid membranes and ion exchangers.
Methods like standard addition or saturation can be implemented to mitigate errors related to the differences between the sample and standards ionic strength.

Potentiometric Gas-Sensing Probes

Probes for gases like CO2 can be used to measure gas concentrations.
They use a membrane that is selectively permeable to the target gas, which causes a pH change in the inside solution, detected by a glass electrode.

Use of Ion Selective Electrodes

ISEs allow for non-destructive analysis using fast and direct measurements on many different ions.
They can be used in turbid or coloured solutions, avoiding the need for complex sample preparation.

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Description

Test your knowledge on the Ag/AgCl reference electrode and its properties. This quiz covers standard reduction potentials, concentration effects, and the characteristics of effective indicator electrodes. Dive into the fascinating world of electrochemistry and enhance your understanding of reference electrodes.