Electroanalytical Chemistry. Gary A. MabbottЧитать онлайн книгу.

Figure 1.3 Electronic circuits form a continuous loop including all components. Usually some point in the circuit is linked to a conductor that has direct contact with the earth. That point becomes a reference point and is treated as though its potential is 0 V.

      In electrochemical experiments, the reference potential is established by the use of a reference electrode. It is common practice to refer to a potential or voltage at an electrode when in actuality the value being discussed is the electric potential difference between the electrode in question and the reference electrode being used in that experiment. In older literature, this potential is also called the electromotive force or EMF as it is the energy available to drive charges from one point to the other and do work. Properties of reference electrodes are discussed in chapter 2 (section 2.3.4.3).

      1.2.1 Volt Defined

In the thought experiment described earlier, a single particle was used as a test charge. The standard definition for electric potential is the energy required to move a unit of positive charge to the position in question. The standard unit of charge in physics is the coulomb and the standard unit of energy is the joule. Electrical energy is measured in volts. One volt represents one joule per coulomb of charge moved.

      (1.1)

      The volt is the unit of electric potential energy per unit charge that one normally uses with simple meters in the laboratory. So, whenever someone refers to a voltage at some part of their system, they are describing the electric potential difference between that point and some reference point (usually the ground). The voltage is the number of joules released or spent in moving a coulomb of charge from the reference point to the point in question.

      It is important to remember that the potential is the electrical work done per unit charge. However, a coulomb is a rather large amount of charge compared to the charge on a single electron. If moving a coulomb of charge from point A to point B costs 1.00 J of energy, then how much energy is required to move a single positively charged particle between the same points? First, one can calculate charge in coulombs/electron using Faraday's constant, F, the number of coulombs per mole of electrons, 9.6485 x 10⁴ C/mol, together with Avogadro's number.

      (1.2)

      This value is the charge on an electron and is known as the elementary charge. One can calculate the energy that would be required to move a single electron through a voltage difference of 1 V by multiplying the elementary charge by 1 V in the units of 1 J/C:

      (1.3)

      The result of this calculation is frequently useful and provides the definition for a separate unit of electric potential energy per elementary charge, namely, the electron‐volt, eV.

      (1.4)

      1.2.2 Current Defined

      Current describes the movement of charge. It is the measure of the rate of change in charge moving past a specific observation point.

      (1.5)

where Q is the charge in coulombs and time, t, is in seconds. Current has the units of coulombs per second or amperes.

      (1.6)

      If one were to make the analogy of an electrical circuit with a river, then the current in amperes or coulombs per second parallels the volume flow rate of the river in gallons per second. The potential energy difference or voltage that is associated with any given component (such as an electrode in an electrochemical experiment) in the electrical circuit, is analogous to the energy available to do work per gallon of water as it drops over a waterfall. Current describes the rate of charge moving (amount per unit of time) and potential is a measure of the energy per unit charge in moving between two points.

      1.2.3 Oxidation and Reduction

      The exchange of electrons between two chemical species is generally known as an oxidation/reduction process or redox reaction. In a redox reaction occurring in a homogeneous solution, one reactant gains electrons while the other reactant loses. It is often useful to consider a redox reaction from the perspective of one of the reactants. Consider a redox reaction between cerium and iron ions in aqueous solution:

      (1.7)

      The net reaction equation does not show any electrons as either reactants or products. However, it is useful to separate the two reactants into “half reactions” where electrons do appear.

      (1.8)

      (1.9)

      A process in which a chemical species accepts one or more electrons is known as a reduction reaction; a process in which a species loses electrons is an oxidation reaction. Writing the half reactions indicates that the Fe²⁺ ion is being oxidized and the Ce⁴⁺ ion is being reduced. It also clearly states how many electrons are being transferred per mole of a given reactant.

      1.2.4 Current and Faraday's Law

In many instrumental electrochemical methods, an electrode surface – usually a metal or carbon conductor – exchanges electrons with the analyte. In those cases, the electrode is treated as an inert source or sink for the electron exchange. The electrode does not appear in the reaction equation. Instead, the electrode reaction appears to be the same as the half reaction for the species being converted. A major benefit of using electrodes in place of a reagent in solution is the fact that the current passing through the electrode can be measured unlike the situation when two chemical species in solution exchange