1
Basic Electrical Principles

Electrochemical methods of analysis measure electrical quantities in order to yield chemical information. In some cases, the measurement is an electric current (the movement of charge). In other cases, the measurement is a voltage (the amount of energy available to move a charge). Both of these techniques are useful for quantitative analysis of a chemical species, but they can also be used to determine characteristic properties that are useful for qualitative analysis. Some types of qualitative information can be useful for evaluating new materials, such as catalysts.

In describing the fundamentals of electroanalytical methods, this book emphasizes conceptual models. An effort has been made to tie conceptual models of phenomena to basic mathematical relationships in order to provide a foundation to use in reasoning through new situations. Greater insight into electroanalytical phenomena is the intended result. As with other branches of science, new developments displace older techniques. A fundamental understanding of the phenomena upon which electroanalytical tools operate enables one to appreciate the basis for new techniques and related progress in the field. A conceptual understanding also provides a good starting point for learning about other areas of science and technology that involve electrochemical processes. Electrochemical principles play important roles in many natural phenomena and in modern technology [1]. Among these fields are the subjects of energy storage and conversion; biological processes such as cellular action potentials, tissue repair, and growth [2]; electrochemical synthesis; separation technology, nanoparticles, and materials processing in the electronics industry.

Electroanalytical techniques are among the oldest instrumental methods of chemical analysis. They are still widely used for important analyses and are likely to continue to be important for many more decades. Although electroanalytical chemistry is a mature field in many ways, new developments in the realm of selective sensors and the application of electrochemical methods to demanding tasks, such as in vivo monitoring of neurotransmitters and remote environmental analysis continue to make instrumental analysis based on electrochemistry relevant. Some attributes of electrochemical analysis that lead to special advantages are summarized in Table 1.1.

TABLE 1.1 Attributes of electroanalytical techniques

Improved detection limits and greater selectivity have led to a greater range of applications. Some methods are capable of quantifying specific analytes down to the picomolar level. Another appealing attribute of electrochemical sensors is that they are relatively easy to miniaturize making them adaptable to a variety of new situations such as in vivo monitoring [3]. The sensing element can be very small making it possible to measure quantities of chemical species in tiny volumes or in precise locations, such as at the terminus of a single neuron. Electrochemical methods usually require only very simple accessories. That makes them portable and, in some cases, it makes medical implantation of the sensor possible. Sensors can often be made of inexpensive materials that can be mass produced making them attractive for personal healthcare monitors, such as the handheld glucose monitor used by millions of people to manage their diabetes [4]. Other electroanalytical instruments are capable of a wide range of experiments making them well-suited to studying organic reaction mechanisms associated with electron transfer.

Before launching into the principles of electrochemistry, it is appropriate to say a word about the structure of this book. Chapter summaries appear at the beginning of each chapter in the form of an overview. Unlike reading a novel, here it is helpful for you to know the plot in advance. It helps you to know what to take away from the story. It is worthwhile to read the overview both before and after reading the other sections of the chapter. This book is aimed at students of instrumental analysis, but it is also intended to be a solid introduction to electroanalytical principles for any professional scientist. A lot of care has gone into explaining physical mechanisms and underlying concepts. Recent developments leading to new and interesting methods with better performance characteristics and a wide range of applications are described in most chapters. However, there is much more material than can be reasonably absorbed during a typical two-to-three-week unit of a college instrumental analysis course. Therefore, in addition to summarizing the major ideas, these chapter briefings tell you what sections to read, if time is short.

1.1 Overview

This first chapter is a bit different. It serves as an introduction to basic electrical phenomena and should be read in its entirety. Among the important ideas discussed in this chapter are a few definitions. The term "voltage" refers to the electrical potential energy of a charged particle. It is a measure of the force of attraction or repulsion on a single charged particle by the local density of charges in the neighborhood. The density of charges in the earth itself is thought to be reasonably constant and, as such, provides a local reference point for electrical energy. Instruments are often attached to a conductor in contact with the earth. This reference point is often referred to as "ground," and it is considered to be a point representing 0?V.

One volt equals one joule per coulomb of charge. The charge on a mole of electrons is 9.6485?x?104 C/mol. This number shows up in a lot of electrochemical relationships and is called the Faraday, F, after the nineteenth-century scientist, Michael Faraday. Faraday established the relationship between charge, Q, transferred in an electrochemical reaction, such as the reduction of silver ions to silver metal, and the number of moles of reactant, N. This is Faraday's law: Q = nFN, where n is the number of moles of electrons transferred per mole of reactant. Another important concept is the free energy, ?G, that drives an electrochemical reaction. The free energy of an electrochemical system is proportional to the voltage, E, and is a measurable quantity, ?G = -nFE.

Electrical current is the movement of charge and is analogous to current in a river. While a river's current is measured in the volume flow rate of the water, electrical current is measured in amperes. One ampere is equivalent to a coulomb of charge moving past a given point per second. Electrons carry charge in electrical circuits. Ions carry charge in solution. Although electrons are negatively charged, current is defined as though positive charges are moving in a circuit. The direction of the current, then, is defined as movement of charge from a higher potential to a lower potential.

Electrochemical experiments are performed in containers called cells in which two or more electrodes connect the cell to an outside electrical circuit that allows one to measure the voltage and/or the current during the experiment. Potentiometric methods measure the voltage (that is, potential) between electrodes without the passage of a significant amount of current. No significant chemical changes occur in a properly performed potentiometric experiment. In Chapter 2, the Nernst equation that relates potential in a potentiometric experiment to the activity of an analyte is discussed. An activity is the effective reactant concentration of a species. An activity of a species is proportional to its concentration, Ci. ai = ?i·Ci, where the proportionality constant, ?i, is known as an activity coefficient and is dependent upon the ionic strength of the solution. In contrast, voltammetric methods deliberately apply energy in the form of a voltage from an outside source to a cell in order to drive a chemical reaction at a working electrode. In these experiments, the current is related to the number of moles of reactant that is converted in the process. This current can be used to quantify the concentration of the original reactant.

In addition to the working electrode (or an indicator electrode in a potentiometric experiment), a second electrode is needed to transfer electrons into or out of the cell in order to counterbalance the charge going into or out of the solution at the working or indicator electrode. This second electrode is a reference electrode. It exploits a simple, reliable electron transfer process that occurs at a well-established voltage. The reference electrode is designed to maintain its potential (voltage) in the process. Consequently, all of the energy applied to the cell from the outside is focused...