Instrumental Analysis CHM 456 - Lecture No. 5-6: Amperometry & Voltammetry

A set of Q&A flashcards covering electrolysis basics, three-electrode setup, amperometry, glucose sensors, oxygen sensors, transport phenomena, and voltammetry concepts drawn from Lecture No. 5-6 notes.

What is electrolysis?

A process in which a chemical reaction is forced to occur at an electrode by an applied potential.

In an electrolysis setup, which electrode is the site of the reaction of interest?

The working electrode.

What is the other electrode in an electrolysis setup called?

The counter electrode.

What is the IUPAC convention for cathodic current in electrochemistry?

Cathodic current is negative.

What is the relationship q = I t?

Charge equals current times time.

How are moles of electrons related to current, time, and Faraday's constant?

Moles of electrons = I t / F, where F is Faraday's constant.

In a galvanic cell with no current, how is Ecell defined?

Ecell = Ecathode − Eanode.

In a cell with current, what additional factors reduce the measured cell potential?

Ecell = Ecathode − Eanode − I R − overpotential.

What is overpotential?

The voltage required to overcome the activation energy for a reaction at an electrode.

How does overpotential relate to driving rate?

The faster you drive the reaction, the greater the overpotential required.

What does electric current measure in electrochemical reactions?

The rate of charge transfer (rate of electron transfer).

Do cathode and anode both contribute to overpotential?

Yes, both can contribute to overpotential.

What is the ohmic potential and its formula?

Ohmic potential is the voltage to overcome solution resistance: Eohmic = I R.

What is concentration polarization?

A phenomenon driven entirely by mass transport of reactant to the electrode surface.

What are the three components of a three-electrode cell and their roles?

Working electrode (site of reaction), Counter electrode (auxiliary electrode), Reference electrode (controls potential).

What is a potentiostat?

An instrument that controls the potential difference between the reference and working electrode.

What does amperometry measure?

The current between electrodes driven by an applied potential.

In amperometric glucose sensors, what is the role of the mediator?

The mediator transfers electrons from the enzyme reaction to the working electrode, creating a current proportional to glucose.

What are the benefits of using a mediator in amperometric sensors?

Solves O2 variation, transfers electrons directly to the electrode, and lowers the operating potential to reduce interference; current reflects analyte concentration.

Describe the Clark oxygen sensor principle.

O2 diffuses through a nonpolar membrane and is reduced at a gold cathode; Ag guard keeps the electrode negative to prevent O2 entry.

What are the three mass-transport processes that bring species to the electrode surface?

Diffusion, Convection, and Migration.

What is the effect of a rotating disc electrode (RDE)?

It reduces the diffusion layer thickness to a few microns, improving control of mass transport and giving currents related to diffusion properties.

What does voltammetry measure and how is potential applied?

Voltammetry measures current as a function of applied potential, which is varied with time (not held constant).

What waveform types are used in voltammetry?

Linear sweep (scan), Square Wave, and Cyclic (triangular) voltammetry.

Which electrodes are typically used for oxidation vs. reduction in cyclic voltammetry?

Inert metals (Pt, Au) are common for oxidation; glassy carbon can perform both oxidation and reduction.

In cyclic voltammetry, what does peak separation tell you about reversibility?

For a reversible couple, Ep and Ea are equal in magnitude and opposite in sign; E1/2 is the average of the peak potentials; ipa/ipc ≈ 1.

What is the Randles–Ševčík equation used for?

Describes the current for a reversible, diffusion-controlled electron transfer; i_p ∝ sqrt(scan rate) for diffusion-controlled processes.

How can you distinguish diffusion-controlled from surface-controlled processes in CV using scan rate?

If ip ∝ sqrt(v), the process is diffusion-controlled; if ip ∝ v, the process is surface-adsorption-controlled.