2017-11-01
194
Lecture 6
Electrochemistry. Potentiometry. Faraday’s Laws.
Introduction to electroanalytical chemistry: basic ideas
Potentiometric methods of analysis.
Faraday’s Laws.
Introduction to electroanalytical chemistry
Electrochemistry can be broadly defined as the study of charge-transfer phenomena. As such, the field of electrochemistry includes a wide range of different chemical and physical phenomena. These areas include (but are not limited to): battery chemistry, photosynthesis, ion-selective electrodes, coulometry, and many biochemical processes. Although wide ranging, electrochemistry has found many practical applications in analytical measurements. Electroanalytical techniques are concerned with the interplay between electricity & chemistry, namely the measurement of electrical quantities such as current, potential or charge and their relationship to chemical parameters such as concentration. The use of electrical measurements for analytical purposes has found large range of applications including environmental monitoring, industrial quality control & biomedical analysis.
Five Important Concepts. To understand electrochemistry we need to appreciate five important and interrelated concepts: (1) the electrode’s potential determines the analyte’s form at the electrode’s surface; (2) the concentration of analyte at the electrode’s surface may not be the same as its concentration in bulk solution; (3) in addition to an oxidation–reduction reaction, the analyte may participate in other reactions; (4) current is a measure of the rate of the analyte’s oxidation or reduction; and (5) we cannot simultaneously control current and potential.
Electroanalytical chemistry
A good working definition of the field of electroanalytical chemistry would be that it is the field of electrochemistry that utilizes the relationship between chemical phenomena which involve charge transfer (e.g. redox reactions, ion separation, etc.) and the electrical properties that accompany these phenomena for some analytical determination. This relationship is further broken down into fields based on the type of measurement that is made.
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Electroanalytical methods have certain advantages over other analytical methods. Electrochemical analysis allows for the determination of different oxidation states of an element in a solution, not just the total concentration of the element. Electroanalytical techniques are capable of producing exceptionally low detection limits and an abundance of characterization information including chemical kinetics information. The other important advantage of this method is its low cost.
Electroanalytical methods.
• Electrochemical reactions involve electron transfer (ET) processes at electrode solution interfaces. These ET reactions may be kinetically sluggish or kinetically facile depending on the details of the ET reaction and the nature of the electrode surface.
• Provided an analyte species exhibits electroactivity (can be oxidised or reduced) then it may be detected using the tools of electrochemistry.
• Thus, electrochemical methods may be split up into two major classes: Potentiometric and Amperometric.
• In potentiometry the ET reaction is kinetically facile and we measure the potential of a Galvanic cell under conditions of zero current flow. The cell potential responds to changes in the activity of the analyte species present in the solution in a well defined manner described by the Nernst equation. Indeed the cell potential varies in a linear manner with the logarithm of the analyte activity.
• In amperometry the kinetics of the ET reaction will have to be driven by an applied potential and so we measure the diffusion controlled current flowing across the electrode/solution interface. This current is directly proportional to the bulk concentration of the analyte present in the solution.
