Surface coulometric methods

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. 

PUgrimm, H. and Sonntag, H., Determination of surface charge density of dispersed oxidic particles in aqueous solutions by a coulometric method. Colloid Polym. Sci., 258,471, 1980. 

Electrochemical coulometric method was used [13] for the determination of active metal surface. In this method, metals in catalysts were transformed to their oxides by oxidation in air at 473 K before the surface determination. The method is able to determine only a relative surface of metals since it is not possible to verify the values of measured surfaces by another conventional method. 

Coulometry, milli- and microcouIometry< ) are also used for the determination of the number of electrons transferred. In these methods the quantity of electricity necessary to reduce a distinct amount of the substance is measured at the potential of the limiting current, controlled by a potentiostat. Coulometric methods are usually not very accurate, and sometimes side reactions occur when electrodes of constant surface are used instead of a dropping electrode. An insufficient separation of cathodic and anodic spacing can also cause complications. Coulometric methods are thus best suited for systems, where n = 1 or 2, but for higher numbers of electrons transferred, the decision is often difficult. 

Detection techniques. Detection techniques for surface-based measurements of ozone include (1) UV absorption at 254 nm (2) chemiluminescence on reaction with NO (or ethene) (3) DOAS (4) TDLS and (5) wet chemical methods, mainly those involving the oxidation of I to 12 and measurement of the I2 colori-metrically or coulometrically. The wet chemical method and the principles behind DOAS and TDLS were discussed earlier and are not treated further here. 

Phenol and the three dihydroxybenzenes (20, 42, 66) in water were determined by LLE with a hydrophilic solvent followed by amperometric titration. LOD was in the ppm range . A dual electrode in a FIA system has been used as detector for total phenols in wastewater. The upstream coulometric electrode has a large surface area and is used to eliminate compounds that cause interference and the second one is an amperometric electrode for oxidative detection of all phenols. Optimal results were found working with a phosphate buffer at pH 6.8, at potentials of +0.35 V and +0.78 V for the coulometric and amperometric electrodes, respectively. A high sample throughput of 60 per hour can be attained with RSD of 0.1-4%. This method is more reliable than the colorimetric method . The concentration of fenobucarb (142) in drinking water was determined after a short alkaline hydrolysis, and oxidation of the resulting 2-s-butylphenol with a GCE at 750 mV, pH 3.5 LOD was 3.6 x 1Q- M, RSD 3.74% for 1 x IQ- M (n = 11, p = 0.05)37 . 

The current-potential curve for n- and p-type electrodes look similar to those given in Fig. 7.10, i.e. the anodic current increases exponentially with potential for a p-type electrode and it saturates at a low value for an n-type electrode in the dark. A quantitative evaluation showed that the slope of the current-potential at a p-type electrode exhibits an ideal slope of 60 mV/decade as illustrated by a semilogarithmic plot of the current potential curve (Fig. 8.5) [8]. This is an ideal situation insofar as the current is proportional to the hole density at the surface, as already discussed in detail in Chapter 7. Using the thin slice method, it was shown that the oxidation of the Si electrode occurred entirely via the valence band and that there was no injection of electrons into the conduction band. In addition it was found by coulometric analysis that two and not four charges were required for the dissolution of one Si atom [8, 9]. Whereas about... 

Indoor specimens may be evaluated by several techniques including mass gain, mass loss, electrochemical reduction, (coulometric measurement), and electrical resistance increase. Microbalance measurements on mass gain after removal of particulates is probably the most popular method because it requires the least specimen handling. However, it will detect dust and particulates on the surface that adhere strongly and may overestimate the corrosion damage in cases where this condition occurs. 

ASTM B 825 Test Method for Coulometric Reduction of Surface Films on Metallic Test Samples... 

Coulometric analysis is an electrochemical method, in which an analyte of interest is exhaustively electrolysis adjacent to the surface of... 

The electrochemical detector is one of the most sensitive and the most specific LC detectors available. It will respond to substances that are either oxidizable or reducible. The output from the detector results from the electron flow caused by an electrochemical reaction that takes place at the surface of an electrode. The reaction can be either oxidation or reduction and if the reaction proceeds to completion, exhausting all the reactant, then the current flow becomes zero and the total charge passed will be proportional to the total mass of material that has been reacted. This procedure is, for obvious reasons, termed coulometric detection. If, however, the electrolyte is flowing past the electrodes, e.g. the electrodes are situated in the eluent from a column, the solute, which constitutes the reactant, will be continuously replaced throughout the elution of a peak. Thus, while there is solute present between the electrodes a current will be maintained, albeit, varying in magnitude. This method of electrochemical detection, is termed amperometric detection and is, at present, virtually the only method employed in LC. It is used almost to the complete exclusion of coulometric detection. 

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