Voltammetry of microparticles

The application of solution phase electrochemistry for studying the work of art samples and archaeological artifacts requires, as previously indicated, sample treatment via extraction or chemical attack. This is an obvious drawback, because these operations, apart from the requirement of relatively high amounts of sample, usually lead to a loss of information since the solid is dissolved and all signals that are specific for the solid compound or material are not available anymore for measurements. 

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. 

As in solution phase electrochemistry, selection of solvent and supporting electrolytes, electrode material, and method of electrode modification, electrochemical technique, parameters and data treatment, is required. In general, long-time voltam-metric experiments will be preferred because solid state electrochemical processes involve diffusion and surface reactions whose typical rates are lower than those involved in solution phase electrochemistry. 

A paradigmatic example is provided by an extensively used pigment Pmssian blue— potassium-iron(III) hexacyanoferrate(II), also called potassium-iron(III) fer-rocyanide. This is constituted by potassium-iron(III) ferrocyanide, KFe[Fe(CN)g], whose electrochemistry has been extensively studied [74-77], 

As shown in Fig. 2.5, the cyclic voltammograms for Prussian blue attached to paraffin-impregnated graphite electrodes (PIGEs) in contact with aqueous electrolytes exhibit two well-defined one-electron couples. Prussian blue crystals possess a cubic structure, with carbon-coordinated Fe + ions and nitrogen-coordinated Fe + ions, in which potassium ions, and eventually some Fe + ions, are placed in the holes of the cubes as interstitial ions. The redox couple at more positive potentials can be described as a solid-state process involving the oxidation of Fe + ions. Charge conservation requires the parallel expulsion of K+ ions [77]  

---

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. 

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. 

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. 

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

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. 

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. 

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). 

For our purposes, the relevant point to emphasize is that voltammetry of microparticles permits us to discriminate between electron transfer processes for the different iron species. This can be seen in Figure 8.22, which shows the square-wave... 

Domenech, A., Domenech, M.T., and Vazquez, M.L. 2007h. Chemometric study of Maya blue from the voltammetry of microparticles approach. Analytical Chemistry 19. 2812-2821. 

Grygar, T., Marken, F., Schroder, U., and Scholz, F. 2002. Voltammetry of microparticles A review. Collection Czechoslovack Chemical Communications (Jl, 163-208. 

Others [73] have also used the voltammetry of microparticles of common sulfide... 

Likewise, Komorsky-Lovric et al. investigated the behavior of lutetium bisphtha-locyanine with the voltammetry of microparticles [108]. This solid-state reaction (which may be studied with either square-wave or cyclic voltammetry) was shown to proceed via the simultaneous insertion/expulsion of anion ions. The oxidation was found to have quasi-reversible characteristics in electrolyte solutions containing perchlorate, nitrate, and chloride, whereas bromide and thiocyanate... 

---