Analytical Electrochemistry : The Basic Concepts

1
Analytical Electrochemistry
The Basic Concepts

• Module Description
• Whats this all about?
• Goals and Objectives
• Why take the time to look at this?
• Fundamentals
• What is analytical electrochemistry and what
  makes it work?
• Voltammetric Methods
• What are they and what are they good for?
• Experimental Hardware
• What do I need to do electrochemistry?
• Experiments
• Ready to give it a try?

Introduction Electrochemistry is something
that is seldom studied and yet is all around us,
including the control circuitry of our body. We
are familiar with lightning that reverberates
with thunder in a rainstorm, with batteries that
power flashlights and hybrid autos, and with
sensor devices such as smoke and carbon monoxide
detectors or glucose analyzers for monitoring
diabetes. All rely on or exhibit some basics of
electrochemistry. To understand electrochemical
phenomenon, we need to have an understanding of
some basic concepts and the language that conveys
these concepts. It is the goal of this module to
get you started so that you can explore further
as you wish. Web-links and hardcopy references
are provided to assist you in that process. Good
luck!
Richard S. Kelly Department of Chemistry, East
Stroudsburg University, East Stroudsburg, PA
18301 This work is licensed under a Creative
Commons Attribution-Noncommercial-Share Alike 2.5
License
2
Analytical Electrochemistry
The Basic Concepts

• Back to Introduction
• Goals and Objectives
• Why take the time to look at this?
• Fundamentals
• What is analytical electrochemistry and what
  makes it work?
• Voltammetric Methods
• What are they and what are they good for?
• Experimental Hardware
• What do I need to do electrochemistry?
• Experiments
• Ready to give it a try?
• References and Links

Module Description This module
gently introduces a topic whose mere mention
often strikes fear in the hearts of students and
teachers alike. Whether because of some
intimidating mathematical aspects of the subject,
or a lack of time to provide an adequate basis
for understanding, it is often left off of course
syllabi in analytical chemistry. This learning
module is designed as an introduction to the
underlying theory and general practice of several
common techniques in analytical electrochemistry.
It is intended to contain sufficient background
material so that educators or new practitioners
in the field can use the material in a
stand-alone way. References and links are
provided so that users who so desire can pursue a
more extensive study of each topic presented.
Sufficient coverage is provided such that the
module could serve as a two- to three-week
segment of a course in analytical chemistry or
instrumental methods.
2
3
Analytical Electrochemistry
The Basic Concepts

• Back to Introduction
• Module Description
• Whats this all about?
• Fundamentals
• What is analytical electrochemistry and what
  makes it work?
• Voltammetric Methods
• What are they and what are they good for?
• Experimental Hardware
• What do I need to do electrochemistry?
• Experiments
• Ready to give it a try?
• References and Links

• Goals and Objectives
• Each part of this module describes certain
  aspects of the experience that is analytical
  electrochemistry.
• The goal is to include enough information that
  you can understand and use electrochemical
  techniques effectively without getting bogged
  down in theory (or lost in the double layer).
• After study of this module, you should be able
  to
• Discuss the physical basis underlying
  voltammetric methods.
• Describe an electrochemical cell and its
  contents.
• Visualize how a potentiostat works.
• No longer cringe at the mention of
  electrochemistry!

3
4
Analytical Electrochemistry
The Basic Concepts

• Back to Introduction
• Goals and Objectives
• Why take the time to look at this?
• Module Description
• Whats this all about?
• Voltammetric Methods
• What are they and what are they good for?
• Experimental Hardware
• What do I need to do electrochemistry?
• Experiments
• Ready to give it a try?
• References and Links

• I. Fundamentals of Electrochemistry
• A. Electrochemical thermodynamics the study
  of an interface
• 1. What is potential?
• 2. The double-layer and charging current
• B. The Electrode Process moving electrons
  around
• 1. Plotting Conventions
• 2. Reversibility chemical and
  electrochemical
• 3. Mass Transport getting stuff to the
  electrode surface

4
5
Analytical Electrochemistry
The Basic Concepts

• I. Fundamentals of Electrochemistry
• A. Electrochemical thermodynamics the study of
  an interface
• 1. What is potential?
• 2. The double-layer and charging current
• B. The Electrode Process moving electrons
  around
• 1. Plotting Conventions
• 2. Reversibility chemical and electrochemical
• 3. Mass Transport getting stuff to the
  electrode surface
• Back to Introduction

A. Electrochemical
Thermodynamics Electrochemistry can be defined
as the study of phenomena at electrified
interfaces.1 As such, two things are fundamental
to electrochemical measurements a) Interface
boundary between two distinct, chemically
different phases b) Electric field existing
across the interface Generally, the interface
occurs between a metallic conductor (the
electrode) and a fluid, ionic conductor (the
solution). The electric field usually results as
a consequence of contact between a solid
electrode and the solution. In electrochemistry,
the electric field is most often one that is
under user control, for example by utilizing a
device called a potentiostat, which will be
discussed in a later section.
5
6
Analytical Electrochemistry
The Basic Concepts

• I. Fundamentals of Electrochemistry
• A. Electrochemical thermodynamics the study of
  an interface
• 1. What is potential?
• 2. The double-layer and charging current
• B. The Electrode Process moving electrons
  around
• 1. Plotting Conventions
• 2. Reversibility chemical and electrochemical
• 3. Mass Transport getting stuff to the
  electrode surface
• Back to Introduction

B. The Electrode Process As
you have probably realized, the fundamental basis
for most electrochemical techniques is the
measurement of current or voltage changes between
two electrodes in solution. Given that basis, it
follows that analytical electrochemistry is
broken down into two major categories 1)
techniques that measure current following a
change in potential, and 2) techniques that
measure potential under conditions of no current
flow. Techniques of the first type are known as
voltammetric methods, while those in the second
are referred to as potentiometric methods. We
will concern ourselves here with voltammetry,
leaving a discussion of potentiometry including
measurement of pH for a separate learning
module. Voltammetry is defined as the measurement
of current which flows at an electrode as a
function of the potential applied to the
electrode.3 Current-potential curves are the
electrochemical equivalent of absorbance-wavelengt
h curves recorded in spectrophotometric
experiments. Once you gain an understanding of
the voltammetric process (hopefully by the time
you finish this module), you will recognize that
voltammograms have the potential (no pun
intended) to yield qualitative, quantitative,
thermodynamic, and kinetic information about
redox active species.
6
7
Analytical Electrochemistry
The Basic Concepts

• Back to Introduction
• Goals and Objectives
• Why take the time to look at this?
• Module Description
• Whats this all about?
• Fundamentals
• What is analytical electrochemistry and what
  makes it work?
• Experimental Hardware
• What do I need to do electrochemistry?
• Experiments
• Ready to give it a try?
• References and Links

• II. Voltammetric Methods
• A. Basics of Voltammetry applying a potential
  and measuring a current
• 1. Potential Step Methods
• a) Chronoamperometry current as a function
  of time and applied potential
• b) Chronocoulometry charge as a function of
  time and applied potential
• continued

7
8
Analytical Electrochemistry
The Basic Concepts

• Back to Introduction
• Goals and Objectives
• Why take the time to look at this?
• Module Description
• Whats this all about?
• Fundamentals
• What is analytical electrochemistry and what
  makes it work?
• Experimental Hardware
• What do I need to do electrochemistry?
• Experiments
• Ready to give it a try?
• References and Links

• II. Voltammetric Methods
• 2. Potential Sweep Methods
• a) Linear Sweep Voltammetry current as a
  function of time and applied potential
• b) Cyclic Voltammetry current as a function of
  applied potential sweep in two directions
• c) Anodic Stripping Voltammetry current as a
  function of applied potential following
  deposition of species of interest
• previous page

8
9
Analytical Electrochemistry
The Basic Concepts

• II. Voltammetric Methods
• A. Basics of Voltammetry applying a potential
  and measuring a current
• 1. Potential Step Methods
• a) Chronoamperometry
• b) Chronocoulometry
• 2. Potential Sweep Methods
• a) Linear Sweep Voltammetry
• b) Cyclic Voltammetry
• c) Anodic Stripping Voltammetry
• Back to Introduction

A. Basics of Voltammetry
Electrochemical techniques in which current is
measured as a function of the applied potential
in an electrochemical cell are called
voltammetric methods. In previous sections we
have introduced the concepts underlying electron
transfer reactions which can occur at the surface
of solid electrodes as the potential across the
solution interface is changed. In this section
we will introduce a number of electrochemical
techniques that use the controlled application of
potential to accomplish analytical objectives.
9
10
Analytical Electrochemistry
The Basic Concepts

• 1. Potential Step Methods
• a) Chronoamperometry -
• current as a function of time and applied
  potential
• b) Chronocoulometry -
• charge as a function of time and applied
  potential
• Back to Introduction

1. Potential Step Methods Methods of this
type involve stepping the electrode potential
from a value where little or no faradaic current
is observed to one sufficient to oxidize or
reduce an electroactive species in the vicinity
of the electrode. Large amplitude step methods
described in this section involve a change in
potential capable of instantly converting
essentially all of the electroactive material at
the electrode surface to its redox partner. In
contrast, small amplitude potential methods,
described elsewhere in this module, involve small
changes in surface redox concentrations as a
function of changing potential, with the
concentrations of the members of the redox pair
described by the Nernst Equation. We will
include here two of the most frequently used
large amplitude potential step (potentiometric)
methods, chronoamperometry and chronocoulometry.
Chronoamperometry involves the measurement of
current passing in the electrochemical cell at a
fixed potential as a function of time (i vs.
t). Chronocoulometry is the measure of total
charge (the integrated i-t response, Q), also as
a function of time.
10
11
Analytical Electrochemistry
The Basic Concepts

• 2. Potential Sweep Methods
• a) Linear Sweep Voltammetry -
• current as a function of applied potential
  and scan rate
• b) Cyclic Voltammtery
• current as a function applied potential and scan
  rate sweep in two directions
• c) Anodic Stripping Voltammetry
• current as a function of applied
• potential following deposition of
• species of interest
• Back to Introduction

2. Potential Sweep Methods Potential sweep
methods are those utilizing an applied potential
that changes with time as the excitation signal.
The most common waveform is that of a linear
sweep, beginning at a potential far removed from
the E0 for the species of interest and
increasing in magnitude at a constant rate. The
current passing at the working electrode is
measured as a function of the applied potential,
with electron transfer accomplished by scanning
the potential through the regions on either side
of the E0. The most common potential sweep
methods are linear sweep voltammetry (LSV),
cyclic voltammetry (CV), and anodic stripping
voltammetry (ASV).
11
12
Analytical Electrochemistry
The Basic Concepts
b. Cyclic Voltammetry i) Introduction ii)
Important parameters in CV 1. Peak location 2.
Current ratios 3. Scan rate dependence of peak
current iii) Chemical reactions coupled to
electron transfer 1. ErCi 2. ErCr 3. ErCi 4.
CrEr 5. ErCiEr iv) Additional considerations
1. Capacitive current 2. Solution resistance
Back to Introduction
b. Cyclic Voltammetry Cyclic voltammetry
is one of the most widely used of
electroanalytical techniques. Its popularity
results from the modest cost of the required
instrumentation, the abundance of mechanistic
information that it can deliver, and the
conceptual simplicity of the data display. We
saw in a previous section that the
current-potential profile for LSV was peak shaped
for an electroactive species undergoing electron
transfer. In cyclic voltammetry, the potential
scan is reversed at some point beyond the peak,
and scanned back in the direction of the initial
potential. In the simplest CV experiment, the
potential waveform is a triangle, with the
initial and final potentials being the same.
12
13
Analytical Electrochemistry
The Basic Concepts

• III. Experimental Hardware
• A. Electrochemical cells
• 1. Defining the cell
• 2. Positioning the electrodes
• 3. Other considerations
• B. Reference and auxiliary electrodes
• 1. Reference electrodes
• a) Saturated calomel
• b) Silver/Silver chloride
• c) Non-aqueous reference
• 2. Auxiliary electrodes
• C. Working electrodes
• 1. Electrode types
• 2. Advantages and limitations
• D. Potentiostats
• Back to Introduction

III. Experimental Hardware Most
voltammetric measurements make use of a device
called a potentiostat, which is capable of
applying a controlled potential to a working
electrode and measuring the current that passes
as a result of electron transfer to solution
species of interest. The working electrode,
along with a reference electrode and an auxiliary
electrode are commonly placed in an
electrochemical cell, with a fill solution
containing an inert electrolyte and the analyte.
Cell designs can be quite simple to quite
complex, with solution volumes ranging from a few
microliters to many milliliters.
13