Potentiometry measures the potential of a solution between two electrodes passively, affecting the solution very little in the process. The potential is then related to the concentration of one or more analytes. The cell structure used is often referred to as an electrode even though it contains two electrodes an indicator electrode and reference electrode (distinct form the reference electrode used in the three electrode system). Potentiometry is usually conducted in an ion selective way with a different electrode for each ion. The most common potentiometric electrode is the glass pH meter.
Coulometry uses applied current or potential to completely convert an analyte from one oxidation state to another. The in these experiments the total current passed is measured directly or indirectly to determine the number of electrons passed. Knowing the number electrons passed can indicate the concentration of the analyte or, when the concentration is known, the number of electrons involved with a redox couple. The bulk electrolysis also known as controlled potential coulometry or some hybrid of the two names is perhaps the most common form of coulometry.
Voltammetry applies a constant and/or varying potential at an electrodes surface and measures the resulting current with a three electrode system. This method can reveal the reduction potential of an analyte and electrochemical reactivity among other things. This method in practical terms is nondestructive since only a very very small amount of the analyte is consumed at the two-dimensional surface of the working and auxiliary electrode. In practice the analyte solutions is usually disposed of since it is difficult to separate the anaylte from the bulk electrolyte and the experiment requires a small amount of analyte. A normal experiment involving 1-10 ml solution with an analyte concentration between 1-10 mM.
Polarometry is a subclass of voltammetry that employees a dropping mercury electrode as the working electrode and often uses the resulting mercury pool as the auxiliary electrode. Concern over the toxicity of mercury combined with the development of affordable, inert, easily cleaned, high quality electrodes made of materials such as noble metals and glass carbon has caused a great reduction of mercury electrodes.
Amperometry a subclass of voltammetry in which the electrode is held at constant potentials for various lengths of time. Mostly a historic distinction that still results in some confusion for example differential pulse voltammetry is also referred to as and differential pulse amperometry. This experiment can be seen as the combination of linear sweep voltammetry and chronoamperometry thus the confusion in what it should be called. One thing that distinguishes amperometry form other forms of voltammetry is that its common to sum the currents over a given time period rather than considering them at individual potentials. This summing can result in larger data sets and reduced error. Amperometric titration is a technique that would be considered amperometry since it measures the current, but would not be considered voltammetry since the entire solution is transformed during the experiment.
The various techniques are each suited to investigating chemical phenomenon over different time scale window.
The various elctrochemical methods can operate under different anaylte concentrations. Notably potentiometry may measure the widest range of analyte concentrations of any analytic method with fluorometry one of the next major competitor.
- Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications. New York: John Wiley & Sons, 2nd Edition, 2000.
- Skoog, D.A.; West, D.M.; Holler, F.J. Fundamentals of Analytical Chemistry New York: Saunders College Publishing, 5th Edition, 1988.