Voltammetry microparticles

Fig. 3.8 Square-wave voltammetry of simvastatin microparticles in 0.09 M NaC104, pH 7. Net response (/net) and its forward (If) and backward (I, ) components. Frequency is 150 Hz, amplitude is 50 mV and potential increment is 2 mV (reprinted from [188] with permission)...

F. Scholz, B. Meyer, Voltammetry of solid microparticles immobilized on electrode surfaces in Electroanalytical Chemistry, A Series of Advances (Eds. A. J. Bard, I. Rubinstein), Marcel Dekker, New York, 1998, p. 1, Vol. 20. 

Solid state voltammetric methods can be used to obtain information on the composition of the materials used in works of art. Here, the methodology of the voltammetry of microparticles, developed by Scholz et al. [72, 73], will be presented. This methodology provides qualitative, quantitative, and structural information on sparingly soluble solid materials, as described in extensive reviews [74-77] and a precedent monograph [78], just requiring sample amounts in the ng-pg level. 

These drawbacks can be avoided to a large extent, using the voltammetry of microparticles—a technique involving solid state electrochemistry where down to about 10 to 10 mol of sample [74-78] can be transferred by abrasion into the surface of an inert electrode, usually paraffin-impregnated graphite electrodes, and the electrode is later immersed in a suitable electrolyte for recording its voltam-metric response. The response of this sample-modified electrode, consisting of the reduction or oxidation of the solid materials, becomes phase-characteristic. 

A number of electrochemical processes involving solid materials can be described in terms of the reductive or oxidative dissolution of such materials. Within this type of processes, one can include the stripping of metal deposits previously mentioned. In the context of archaeometry, conservation, and restoration sciences, the reductive dissolution of iron oxide-type materials is of particular interest. Thus, application of the voltammetry of the microparticles approach for identifying iron pigments has been described [108, 137-139]. 

Table 3.1 Diagnostic criteria for characterizing lead pigments via voltammetry of microparticles using deposits of the pristine pigments on parafiSn-impregnated graphite electrodes. Data from square-wave voltammograms at a potential step increment of 4 mV, square-wave amplitude of 25 mV, and frequency of 15 Hz. All potentials refer to AgQ (3M NaCl)/Ag. Electrolyte, 0.50 M acetate buffer, pH 4.85...

Assuming that solid-state electrochemical processes involved in our voltammetry of microparticles analysis satisfy Tafel dependence between current and potential at the rising portion of voltammetric curves, the current can be approached by the expression... 

In the voltammetry of microparticles, relative quantitations can easily be obtained as follows. Let us first consider a sample containing two electroactive species, A and B. Assuming that under the selected electrochemical conditions (technique, parameters, electrolyte), separated signals are obtained for A and B, the voltanunetric peak currents (or peak areas), IaJe, can be taken as proportional to the number of mols of A and B, xa,Xb, respectively transferred to the electrode. Then, one can write... 

The main drawback for determining the absolute composition of solid samples using the voltammetry of microparticles approach is that there is no possibility of controlling the exact amount of sample deposited on the electrode surface. To solve... 

In the context of the voltammetry of microparticles methodology, the H-point standard addition method has been adapted for determining organic dyes [241] as well as lead and tin in ceramics [242]. Let us consider a mixture of material containing unknown amounts of two electroactive compounds, A and B, and a reference compound, R. It is assumed that weighted amounts of both materials are accurately powdered and thoroughly mixed so that the mass ratio between the A,B-containing material and the reference compound, m/mR, is known. 

If all A, B, and R compounds are electroactive in a suitable electrolyte, voltam-mograms of the mixture must provide peaks corresponding to their respective redox processes. In solid-state voltammetry, in which separate microparticles of each one of the electroactive compounds are mechanically transferred to the surface of an inert electrode, independent electrochemical processes must occur. Accordingly, peak currents, ip j) j =A,B,R), recorded for A-, B-, and R-centered voltammetric processes, can generally be taken as proportional to the amount of each one of the compounds deposited on the electrode [131, 243-245]. If separate peaks are recorded for A, B, and R, the respective peak currents must satisfy the relationship... 

These results illustrate the inherent capabilities of the voltammetry of microparticles for determining the absolute concentration of analytes in samples from works of art. Here, the most serious limitations are associated with (i) the need for well-defined electrochemical responses, and (ii) the need for relatively high amounts of sample. The second limitation, however, does not apply when relative quantitation procedures are used. As a result, a judicious use of such methodologies can provide valuable information for archaeometry, conservation, and restoration. 

Scholz F, Meyer B (1998) Voltammetry of sohd microparticles immobdized on electrode surfaces. Electroanal Chem, A Series of Advances. 20 1-86. 

Grygar T, Marken F, Schroder U, Scholz F (2002) Voltammetry of microparticles a review. CoU Czech Chem Commun 67 163-208. 

Komorsky-Lovric S, Mirceski V, Scholz F (1999) Voltammetry of organic microparticles. Mikrochimica Acta 132 67-77. 

Domenech A, Domenech-Carbo MT, Sauri MC, Gimeno JV, Bosch F (2005) Identification of curcuma and safflower dyes by voltammetry of microparticles using paraffin-impregnated graphite electrodes. Microchim Acta 152 75-84. 

Domenech A, Domenech-Carbo MT, Osete L (2004) Electrochemistry of archaeological metals an approach from the voltammetry of microparticles, In Trends in Electrochemistry and Corrosion at the Beginning of the 21st Century, Biillas E, Cabot PL (Eds), Universitat de Barcelona, Barcelona, pp. 857-871. 

Domenech A, Domenech-Carbo MT, Mas X, Ciarrocci J (2007) Simultaneous identification of lead pigments and binding media in paint samples using voltammetry of microparticles. 

Domenech A, Domenech-Carbo MT, Vazquez ML (2006) Dehydroindigo a new piece into the maya blue puzzle from the voltammetry of microparticles approach. J Phys Chem B 110 6027-6039. 

Cepria G, Garcia-Gareta E, Perez-Arantegui J (2005) Cadmium yellow detection and quantification by voltammetry of immobilized microparticles, Electroanalysis 17 1078-1084. 

Bosch F, Domenech A, Domenech-Carbo MT, Gimeno JV (2007) H-point standard addition method applied to voltammetry of microparticles. Quantitation of dyes in pictorial samples. Electroanalysis 19 1575-1584. 

Scientific examination of archaeological pieces and works of art is undoubtedly a necessary task for archaeometry, conservation and preservation/restoration sciences. Although essentially focused on metal corrosion problems, electrochemistry was one of the early applied scientific methodologies in such fields, in both its analytical and conservative/restorative aspects. Over the last few decades, the scope of electrochemical methods ability to interact with archaeometry, conservation and restoration has been significantly extended, by virtue of the application of new approaches—in particular, the voltammetry of microparticles. 

Activation (of noble metal electrodes) — Noble metal electrodes never work well without appropriate pretreatment. Polycrystalline electrodes are polished with diamond or alumina particles of size from 10 pm to a fraction of 1 pm to obtain the mirror-like surface. The suspensions of polishing microparticles are available in aqueous and oil media. The medium employed determines the final hydrophobicity of the electrode. The mechanical treatment is often followed by electrochemical cleaning. There is no common electrochemical procedure and hundreds of papers on the electrochemical activation of -> gold and platinum (- electrode materials) aimed at a particular problem have been published in the literature. Most often, -> cyclic and - square-wave voltammetry and a sequence of potential - pulses are used. For platinum electrodes, it is important that during this prepolarization step the electrode is covered consecutively by a layer of platinum oxide and a layer of adsorbed hydrogen. In the work with single-crystal (- monocrystal) electrodes the preliminary polishing of the surface can not be done. 

Paraffin impregnated graphite electrode (PIGE) - electrode prepared from graphite rods by impregnation with melted paraffin under vacuum. These electrodes are not permeated by aqueous solutions and can be used for solution studies, as well as for immobilizing microparticles and microdroplets to study their electrochemistry. See also - carbon, - voltammetry of immobilized microparticles. 

Solid-state electrochemistry — is traditionally seen as that branch of electrochemistry which concerns (a) the -> charge transport processes in -> solid electrolytes, and (b) the electrode processes in - insertion electrodes (see also -> insertion electrochemistry). More recently, also any other electrochemical reactions of solid compounds and materials are considered as part of solid state electrochemistry. Solid-state electrochemical systems are of great importance in many fields of science and technology including -> batteries, - fuel cells, - electrocatalysis, -> photoelectrochemistry, - sensors, and - corrosion. There are many different experimental approaches and types of applicable compounds. In general, solid-state electrochemical studies can be performed on thin solid films (- surface-modified electrodes), microparticles (-> voltammetry of immobilized microparticles), and even with millimeter-size bulk materials immobilized on electrode surfaces or investigated with use of ultramicroelectrodes. The actual measurements can be performed with liquid or solid electrolytes. 

Refs. [i] Scholz F, Nitschke L, Henrion G (1989) Naturwissenschaften 76 71 [ii] ScholzF, Meyer B (1998) Voltammetry of solid microparticles immobilized on electrode surfaces. In Bard A], Rubinstein I (eds) Elec-troanalytical chemistry, vol. 20. Marcel Dekker, New York [iii] Scholz F, Schroder U, GulaboskiR (2005) Electrochemistry of immobilized particles and droplets. Springer, Berlin... 

Electrochemical methods can also be used for obtaining analytical information on porous materials. Voltammetric methods and related techniques have been largely used to acquire information on reaction mechanisms for species in solution phase, whereas impedance techniques have been extensively used in corrosion and metal surface studies. In the past decades, the scope of available methods has been increased by the development of the voltammetry of microparticles (Scholz et al., 1989a,b). This methodology, conceived as the recording of the voltammetric response of a solid material mechanically transferred to the surface of an inert electrode, provides information on the chemical composition, mineralogical composition, and speciation of solids (Scholz and Lange, 1992 Scholz and Meyer, 1994, 1998 ... 

Description of electrocatalytic processes in such modified electrodes can be derived from the intersection between the theory of Andrieux and Saveant (1980, 1988) for mediated electrocatalysis in redox polymers and those for metal oxide electrocatalysis (Lyons et al., 1992,1994 Attard, 2001 Pleus and Schulte, 2001) and the recent models for the voltammetry of microparticles given by Lovric and Scholz (1997, 1999) and Oldham (1998) and combined by Schroder et al. (2000). 

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