Analytical Electrochemistry : The Basic Concepts

1. Analytical Electrochemistry : The Basic Concepts • Module Description • What’s 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? • References and Links • Tell me more! 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 • Tell me more! 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 • What’s 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 • Tell me more! • 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 • What’s 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 • Tell me more! • 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-wavelength 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 • What’s 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 • Tell me more! • 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 • What’s 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 • Tell me more! • 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 amplitudepotential 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), andanodic 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 electrodeand measuring the current that passes as a result of electron transfer to solution species of interest. The working electrode, along with a reference electrodeand an auxiliary electrodeare 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