Potentiometry & Ion-Selective Electrodes (Lecture Notes)

Practice flashcards covering potentiometry, reference and indicator electrodes, Nernst equation, junction potentials, and ion-selective electrodes based on the provided lecture notes.

What is the defining principle of potentiometry?

Measuring the potential of electrochemical cells without drawing appreciable current.

In potentiometry, what is the electroactive species?

An electroactive species (analyte) that participates in redox by donating or accepting an electron at an electrode.

In a potentiometric setup, what are the roles of the working (indicator) electrode and the reference electrode?

The working (indicator) electrode participates in the redox; the reference electrode provides a fixed, known potential.

What is the fixed-potential half-cell in potentiometry called?

Reference electrode.

Name three common reference electrodes.

Silver/Silver Chloride (Ag/AgCl), Saturated Calomel Electrode (SCE), and Standard Hydrogen Electrode (SHE).

What is a double-junction reference electrode used for?

To minimize interaction between the reference solution and the sample, improving stability and lifetime.

What does SCE stand for and what is it?

Saturated Calomel Electrode; a standard reference electrode based on the Hg/Hg2Cl2 couple.

What is the role of the Standard Hydrogen Electrode (SHE)?

A universal reference potential used as a reference point for comparing other electrodes (not easily miniaturized).

What is the general form of the Nernst equation?

E = E0 − (RT/nF) ln Q, where Q is the reaction quotient (activities or concentrations).

In the Nernst equation, what does n represent?

The number of electrons transferred in the redox reaction.

At 25°C, what is the practical slope of the Nernst equation per electron for a log10 form?

0.05916 V per electron (per decade of concentration).

How is Ecell typically written for a potentiometric cell including junction potential?

Ecell = Eindicator − Ereference + Ej (junction potential); in many cases, Ecell ≈ Eindicator − Ereference when Ej is small.

In a typical potentiometric cell, which electrode acts as the anode and which as the cathode?

Reference electrode acts as the anode; indicator/working electrode acts as the cathode.

What is a junction potential and where does it arise?

A voltage difference at the interface between dissimilar electrolyte solutions (e.g., at a salt bridge terminus).

Why is saturated KCl used in salt bridges?

Because K+ and Cl− have similar mobilities, minimizing junction potential and providing stability.

What is the typical potential change per pH unit for a pH glass electrode at 25°C?

Approximately 59.16 mV per pH unit.

Name two common pH-electrode errors.

Alkaline (alkaline) error at high pH and acid error at low pH.

List the four main membrane types used in Ion-Selective Electrodes (ISEs).

Glass membranes; solid-state membranes (inorganic crystals or conductive polymers); liquid-based membranes with a hydrophobic liquid ion exchanger; compound electrodes with analyte-selective membranes.

How do Ion-Selective Electrodes generate a signal?

Diffusion of the target ion across the membrane creates a potential difference at the membrane–solution interface, related to the ion’s activity; calibrated with standards.

What provides selectivity in a potassium (K+) ISE?

An ionophore in the membrane, such as Valinomycin, which preferentially binds K+.

What is the typical arrangement inside an ISE for reference and sensing?

An inner reference and an outer reference electrode with the ion-selective membrane separating the analyte solution.

What is the typical slope for a H+ (pH) ISE under standard conditions?

About 59.16 mV per pH unit (β ≈ 1).

What does β (beta) represent in pH electrode calibration?

Electromotive efficiency; a factor close to 1 that describes the slope/response in calibration.

Name two common sources of error that require recalibration for pH electrodes.

Junction potential drift and changes in response due to drift over time; also electrolyte leakage and hydration state can affect accuracy.

What is the typical accuracy range for pH measurement with glass electrodes?

Limited by calibration standards and junction potentials; commonly on the order of a few hundredths to a few tenths of a pH unit.

Which hospital tests are commonly performed with Ion-Selective Electrodes (ISEs) as part of the Chem 7 panel?

K+, Na+, Cl−, and total CO2 (with others like glucose, urea, and creatinine using other sensing methods).