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1. Basic Concepts in Electrochemistry

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  • Electrochemical processes are commonly used for analytical measurements. There are a variety of electrochemical methods with different degrees of utility for quantitative and qualitative analysis that are included in this unit. The coverage herein is not exhaustive and methods that are most important or demonstrate different aspects of electrochemical measurements are included. Also, in most cases the coverage is designed to provide a broad overview of how the method works and not delve deeply into all of the associated subtleties. There are two free sources of information available through the Analytical Sciences Digital Library for those desiring a more in-depth coverage of particular methods. One is a general textbook on analytical chemistry written by David Harvey.

    The other is a module on electrochemistry written by Richard Kelly.

    The methods we will explore in this unit rely on one of two different electrochemical phenomena. The first is that many chemical species have the ability to transfer electrons through an oxidation-reduction process. With appropriate design of an electrochemical system, this transfer of electrons can be measured as a current. Since we also know that different species have different oxidation or reduction abilities, electrochemical measurements relying on electron transfer can often be used for the purpose of species identification.

    The second method of using electrochemical processes for measurement purposes relies on the measurement of a potential. In particular, we will focus on some methods that rely on something called a junction potential. You likely have some familiarity with electrochemical cells. Such a device consists of electrodes and the design of electrodes creates interfaces or junctions (e.g., a metal electrode in contact with a solution represents a junction). Any junction in an electrochemical system will have a potential associated with it and in certain cases, the magnitude of this junction potential can be related to the concentration of a species in solution. For example, a pH electrode is the best known example of the use of a junction potential for determining the concentration of a species. The key feature of a pH electrode is a thin glass membrane. When placed into an aqueous solution, a junction potential occurs at the glass-solution interface and the magnitude of this potential is determined by the concentration of H+ in solution.

    Define what is meant by oxidation and reduction.

    In a chemical reaction involving a transfer of electrons, one species gains one or more electrons while another species loses one or more electrons. Oxidation refers to the species that loses electrons and reduction to the species that gains electrons. If you have trouble remembering which is which, using the pneumonic “LEO the lion goes GER” can be helpful (LEO = Loss of Electrons is an Oxidation; GER = Gain of Electrons is a Reduction). It is important to remember that both processes must occur simultaneously.

    Define what is meant by an oxidizing and reducing agent. Give a good example of each.

    An oxidizing reagent promotes the oxidation of another substance so is reduced in the overall electrochemical reaction. Good or strong oxidizing agents are species that really want to be reduced. Fluorine and chlorine are strong oxidizing agents because they very much want to be the fluoride or chloride ion.

    A reducing agent promotes the reduction of another substance so is oxidized in the overall electrochemical reaction. Good or strong reducing agents are species that really want to be oxidized. Since alkali metals such as lithium, sodium or potassium want to be oxidized into their cationic forms, they would be good reducing agents.

    Define what is meant by a half-reaction.

    Overall electrochemical reactions consist of both a reduction and oxidation. Each half of this overall process is represented by an appropriate half reaction. A half-reaction shows the reduced and oxidized form of the species and these two forms are referred to as a redox couple.

    Give an example of a half-reaction and determine whether a half-reaction can be an equilibrium expression. Why or why not?

    One of many possible half reactions is shown below for the reduction of cadmium ion to cadmium metal.

    \[\mathrm{Cd^{2+}(aq) + 2e^- = Cd(s)}\]

    If we were to try to write an equilibrium constant expression for this reaction, it would need to have the concentration of free electrons in the expression. Since we really cannot weigh out a mass of electrons to use in a reaction and cannot dissolve free electrons into a solution, we cannot write a true equilibrium constant expression for a half reaction.

    However, an interesting aspect of electrochemical reactions is that we can design a device known as an electrochemical cell that has each of the half reactions isolated from each other in separate halves of the cell. As we begin to examine electrochemical cells and processes in more detail, we will often focus our attention on only one half of the overall process and will write expressions for half reactions that are essentially an equilibrium constant expression. The expression will not have a term for electrons in it. For the half reaction shown above the expression would be as follows:


    Just like in equilibrium constant expressions, there is no term for the Cd(s) because a solid will not have a concentration.

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