Coulometry fundamentals

Before in situ external reflectance FTIR can be employed quantitatively to the study of near-electrode processes, one final experimental problem must be overcome the determination of the thickness of the thin layer between electrode and window. This is a fundamental aspect of the application of this increasingly important technique, marking an obstacle that must be overcome if it is to attain its true potential, due to the dearth of extinction coefficients in the IR available in the literature. In the study of adsorbed species this determination is unimportant, as the extinction coefficients of the absorption bands of the surface species can be determined via coulometry. 

It should also be recalled that a full electrochemical, as well as spectroscopic and photophysical, characterization of complex systems such as rotaxanes and catenanes requires the comparison with the behavior of the separated molecular components (ring and thread for rotaxanes and constituting rings in the case of catenanes), or suitable model compounds. As it will appear clearly from the examples reported in the following, this comparison is of fundamental importance to evidence how and to which extent the molecular and supramolecular architecture influences the electronic properties of the component units. An appropriate experimental and theoretical approach comprises the use of several techniques that, as far as electrochemistry is concerned, include cyclic voltammetry, steady-state voltammetry, chronoampero-metry, coulometry, impedance spectroscopy, and spectra- and photoelectrochemistry. 

Determination of the amount of substance is thus in direct relation to basic units of the SI system and does not need a RM for comparison. The Faraday constant is one of the fundamental constants (it can be expressed as the product of the electron charge and the Avogadro constant). It enables the attainment of high precision and accuracy and is independent of the atomic weights of the elements in the sample. Its drawback is lower selectivity, a feature common to titration methods. This makes coulometry especially suitable for determination of relatively pure substances used as standards by other (relative) methods. The Faraday constant has been proposed as an ultimate standard in chemistry [3],... 

Coulometry — In 1834 - Faraday described two fundamental laws of - electrolysis. According to Faraday the amount of material deposited or evolved (m) during electrolysis is directly proportional to the current (I) and the time (t), i.e., on the quantity of electricity (amount of charge) (Q) that passes through the solution (first law). The amount of the product depends on the equivalent mass of the substance electrolyzed (second law). 

In this chapter, the fundamental electrochemical principles of potentiometry, voltammetry and/or amperometry, conductance, and coulometry will be summarized and clinical apphcations presented. Next, optodes and biosensors will be discussed. The chapter concludes with a discussion of in vivo and minimally invasive sensors. 

W e now turn our attention to several analytical methods that are based on oxidation/reduction reactions. These methods, which are described in Chapters 18 through 23, include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry, and voltammetry. Fundamentals of electrochemistry that are necessary for understanding the principles of these procedures are presented in this chapter. 

A fundamental requirement for all coulometric methods is 100% current efficiency that is, each faraday of electricity must bring about chemical change in the analyte equivalent to one mole of electrons. Note that 100% current efficiency can be achieved without direct participation of the analyte in electron transfer at an electrode. For example, chloride ion may be determined quite easily using poten-tiostatic coulometry or using coulometric titrations with silver ion at a silver anode. Silver ion then reacts with chloride to form a precipitate or deposit of silver chloride. The quantity of electricity required to complete the silver chloride formation serves as the analytical variable. In this instance, 100% current efficiency is realized because the number of moles of electrons is essentially equal to the number of moles of chloride ion in the sample despite the fact that these ions do not react directly at the electrode surface. 

Type I or calculable method Direct through fundamental laws (1) Mass of reagent (volume) towards mass of sample Titrimetry Coulometry Gravimetry Small uncertainty limited to mass determination and purity of reagent and reaction product... 

Analytical Applications of Coulometry. The major advantage of coulometry is its high accuracy, because the reagent is electrical current, which can be well controlled and accurately measured. Coulometry is used for analysis, for generation of both unstable and stable titrants on demand , and for studies of redox reactions and evaluation of fundamental constants. With careful experimental technique, it is possible to evaluate the Faraday constant to seven significant figures, for example. 

This relationship involves only fundamental quantities there are no empirically determined calibration factors, so standardization and calibration are not required. This makes coulometry an absolute method it does not require calibration with external standards. The current efQciency must be 100%. For a variable current, the charge is given by... 

The fundamental principle of coulometry originates in Faraday s laws, which relate the quantity of a substance transformed in an electrolysis to the... 

Lewis TD (1961) Columetric methods in analysis a review. Analyst 86 494—506 Bard AJ (2001) Electrochemical methods, fundamentals and applications. Wiley, New York Harrar JE (1987) Analytical controlled-potential coulometry. TrAC Trend Anal (Them 6(6) 152-157... 

The fundamentals of gas titration with solid electrolyte cells are described in detail within the entries titration and coulometry, and special aspects are also treated within solid electrolyte, potentiometry, and amperometry. Therefore, the focus is set here to the most important errors of gas titration with SE cells. These errors are related mainly to the peripheral parameters as... 

Coulogravimetry — A combination of coulometry and -> electrogravimetry, in which the weight of the deposited analyte and the passed charge are measured [i]. R. [i] Skoog D, West D, Holler F (1996) Fundamentals of analytical chemistry. Saunders College Publishing New York... 

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