Quantitation from Tafel Analysis in Solid-State Voltammetry

Anal. Chem. 2008, 80, 2704-2716

Quantitation from Tafel Analysis in Solid-State Voltammetry. Application to the Study of Cobalt and Copper Pigments in Severely Damaged Frescoes Antonio Domeńech,*,† Marıá Teresa Domeńech-Carbo´,‡ and Howell G. M. Edwards§

Departament de Quı´mica Analı´tica. Universitat de Vale` ncia. Dr. Moliner, 50, 46100 Burjassot, Vale` ncia, Spain, Institut de Restauracio´ del Patrimoni, Universitat Polite` cnica de Vale` ncia. Camı´ de Vera s/n. 46022 Vale` ncia, Spain, and University Analytical Centre, Chemical & Forensic Sciences, School of Life Sciences, University of Bradford, Bradford, BD7 1DP, UK

A novel method, using Tafel plots, for quantifying electroactive species in solid materials when their voltammetric signals are strongly overlapped is described. This is applied to the analysis of submicrosamples from the highly damaged frescoes painted by Palomino (1707) in the ceiling vault of the Sant Joan del Mercat church in Valencia, Spain. These paintings, which were fired in 1936, contained cobalt smalt plus azurite mixtures, this last being altered to tenorite (CuO). The reported method provides a quantitation of the cobalt smalt/azurite, tenorite/(azurite + smalt) relationships in samples, thus providing direct information on pigment dosage (smalt/ azurite ratio) in pristine paintings, extent of alteration, and temperature experienced by the frescoes during the gunfire episode. Distinction between Palomino paintings and other painters was clearly obtained due to the presence of malachite in these last.

In this methodology, relative quantitation can be obtained from coulometric data9,10 or via measurement of peak areas in voltammograms5,9-18 and peak potential shifts.7,12,13Absolute quantitation can be obtained, also using the above parameters, by means of addition of internal standards.19-22 All these procedures require that analytes (and eventually standards) yield separated voltammetric peaks, a requirement that does not hold in a number of cases. In the current report, a method is proposed for the relative quantitation of components in solid samples using solidstate voltammetry when such components produce highly overlapping signals, based on the Tafel analysis of the rising portion of the common voltammetric curve. The use of this kind of analysis for identifying individual components in mixtures has been previously described.23 The proposed method is applied to the determination of the composition of a series of 18 microsamples containing cobalt and copper pigments from the frescoes on the vaulted ceiling of the

Quantitation of components in samples is a general aim for analytical purposes. In the fields of archaeometry, conservation, and restoration, quantitation of species in solid microsamples is of interest for characterizing materials and techniques, thus obtaining information for authentication, geographical location, etc. In the last years, the scope of available techniques for analyzing solid materials has been increased by the voltammetry of microparticles (VMP), a general methodology developed by Scholz et al.1,2 This approach, which extends classical studies on carbon paste electrodes,3-6 can be used for identification, speciation and quantitation of electroactive components in sparingly soluble solids, as described in recent extensive reviews.7,8

(7) Scholz, F.; Meyer, B. In Electroanalytical Chemistry, A Series of Advances; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1998; Vol. 20, pp 1-87. (8) Grygar, T.; Marken, F.; Schro ¨der, U.; Scholz, F. Collect. Czech. Chem. Commun. 2002, 67, 163-208. (9) Scholz, F.; Nitschke, L.; Henrion, G. Electroanalysis 1990, 2, 85-87. (10) Scholz, F.; Lange, B. Fresenius’ J. Anal. Chem. 1990, 338, 293-294. (11) Scholz, F.; Rabi, F.; Mu ¨ ller, W.-D. Electroanalysis 1992, 4, 339-346. (12) Zhang, S.; Meyer, B.; Moh, G. H.; Scholz, F. Electroanalysis 1995, 7, 319328. (13) Meyer, B.; Zhang, S.; Scholz, F. Fresenius’ J. Anal. Chem. 1996, 356, 267270. (14) Grygar, T.; van Oorschot, I. H. M. Electroanalysis 2002, 14, 339-344. (15) Cepria´, G.; Garcıá-Gareta, E.; Pe´rez-Arantegui, J. Electroanalysis 2005, 17, 1078-1084. (16) Domeńech, A.; Domeńech, M. T.; Osete, L.; Gimeno, J. V.; Bosch, F.; Mateo, R. Talanta 2002, 56, 161-174. (17) Domeńech, A.; Domeńech, M. T.; Osete, L.; Gimeno, J. V.; Sańchez, S.; Bosch, F. Electroanalysis 2003, 15, 1465-1475. (18) Domeńech, A.; Sańchez, S.; Yusa´, D. J.; Moya´, M.; Gimeno, J. V.; Bosch, F. Electroanalysis 2004, 16, 1814-1822. (19) Domeńech, A.; Sańchez, S.; Yusa´, D. J.; Moya´, M.; Gimeno, J. V.; Bosch, F. Anal. Chim. Acta 2004, 501, 103-111. (20) Domeńech, A.; Moya´, M.; Domeńech, M. T. Anal. Bioanal. Chem. 2004, 380, 146-156. (21) Domeńech, A.; Domeńech, M. T.; Gimeno, J. V.; Bosch, F. Anal. Bioanal. Chem. 2006, 385, 1552-1561. (22) Bosch, F.; Domeńech, A; Domeńech, M T; Gimeno, J V. Electroanalysis 2007, 19, 1575-1584. (23) Domeńech, A.; Domeńech, M. T.; Gimeno, J. V.; Bosch, F.; Saurı´, M. C.; Casas, M. J. Fresenius’ J. Anal. Chem. 2001, 369, 576-581.

* To whom correspondence should be addressed. E-mail: [email protected]. † Universitat de Vale`ncia. ‡ Universitat Polite ` cnica de Vale`ncia. § University of Bradford. (1) Scholz, F.; Nitschke, L.; Henrion, G. Naturwissenschaften 1989, 76, 7172. (2) Scholz, F.; Nitschke, L.; Henrion, G.; Damaschun, F. Naturwissenschaften 1991, 76, 167-168. (3) Schultz, F. A.; Kuwana, T. J. Electroanal. Chem. 1965, 10, 95-103. (4) Kuwana, T.; French, W. G. Anal. Chem. 1964, 36, 241-242. (5) Lamache, M.; Bauer, D. Anal. Chem. 1979, 51, 1320-1322. (6) Brainina, K. Zh.; Vidrevich, M. B. J. Electroanal. Chem. 1981, 121, 1-28.

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10.1021/ac7024333 CCC: $40.75

© 2008 American Chemical Society Published on Web 03/07/2008

church of Sant Joan del Mercat in Valencia, Spain. The frescoes, painted by Antonio Palomino (1655-1726) in 1707, were severely damaged by fire in the Spanish Civil War in 1936. As a result, an important fraction of the wall paint was destroyed and the surviving areas underwent severe deterioration, including chromatic changes among other dramatic damage. The current process of restoration, initiated in 2001, required the development of new analytical tools for facing the problem of identifying pigments and their alteration products.24 Two additional samples from that vault, whose attribution to Palomino was uncertain, were also studied. Apart from the need to use as minimal amount of sample as possible, this analytical objective is made difficult, even for wellconserved paints, by the presence of interfering pigments and the coexistence of additives (binders, varnishes, compounds in ground layers). In the case of damaged paints, the appearance of efflorescences, debries, poultice and deposits, and alteration products complicates seriously the identification of pigments in the sample. Accordingly, a synergic collection of several techniques, namely, optical end electron microscopies, atomic force microscopy, Fourier transform infrared and Raman spectroscopies, and solid-state electrochemistry was used for obtaining information about the original pigments, binders, and substrate treatments employed by Palomino.24 Thermal alteration of earth pigments was studied by applying multivariate chemometric techniques to VMP data.25 Quantitation via VMP was tested using synthetic specimens of pigment or mineral mixtures and samples from Palomino’s frescoes. The copper pigments mainly used in this period were azurite and malachite, two basic copper carbonate minerals (2CuCO3‚Cu(OH)2 and CuCO3‚Cu(OH)2, respectively), and verdigris, a basic copper acetate (Cu(CH3COO).Cu(OH)2).26 The synthetic analogues of azurite and malachite, respectively, blue and green verditer, were in production since the earlier 19th century. Smalt, a cobalt-containing glass-type pigment, was used since the 17th century. In contrast, cobalt blue (Co3O4) was only used since 1774.26 Alteration of copper pigments leads to copper trihydroxychlorides, Cu(OH)3Cl, (different polymorphs, generically, minerals of the atacamite group) but, as occurs for bronze disease, nantokite (CuCl) and cuprite (Cu2O) may be formed.27,28 As reviewed by Scott,27 the atacamite group comprises atacamite, clinoatacamite, and botallackite, but as pointed out by Antonio and Tennent,28 even under laboratory conditions, the mode of production of copper trihydroxychlorides is critical. Apart from classical spectroscopy and microscopy techniques, identification of copper pigments and their alteration products by VMP23 and Raman spectroscopy29-35 have been recently reported. (24) Edwards, H. G. M.; Domeńech, M. T.; Hargraves, M. D.; Domeńech, A. J. Raman Spectrosc. In press. (25) Domeńech, A.; Domeńech, M. T.; Edwards, H. G. M. Electroanalysis 2007, 19, 1890-1900. (26) Gettens, R. J.; FitzHugh, E. W. In Artists’ Pigments. A Handbook of their History and Characteristics; Roy, A. Ed.; National Gallery of Art; Washington and Oxford University Press: Oxford, UK, 1993; Vol. 2, pp. 23-36. (27) Eastaugh, N.; Walsh, V.; Chaplin, T. D.; Siddall, R. The Pigment Compendium; Elsevier: New York, 2004. (28) Scott, D. A. Stud. Conserv. 2000, 45, 39-53. (29) Tennent, N. H.; Antonio, K. M. ICOM Committee for Conservation 6th Triennial Meeting, Ottawa, 1981.

In the current report, the VMP approach was used for identifying and quantifying cobalt and copper species existing in microsamples from the Palomino’s frescoes. Since the majority of involved cobalt and copper compounds produce almost coincident voltammetric responses, conventional methods, based on separated peak record, cannot be used. In particular, three problems arise: (i) the distinction between different pigments, (ii) determination of dosages in pigment mixtures, and (iii) identification and eventually quantitation of alteration products. The two first problems deal with the characterization of materials and techniques used by the artist whereas the later provides information on the extent of the alteration in paint layers. Linear potential scan, cyclic and square wave voltammetries (LSV, CV, and SQWV, respectively) have been used, this last technique being of particular interest because of its inherently high sensitivity and immunity to capacitive effects.36 It should be noted that application of VMP for quantitation suffers from the difficulty in controlling the amount of sample transferred to the electrode, thus causing problems of reproducibility. In the approach for data treatment presented here, quantitation is derived from shape-dependent parameters, which are independent of sample loadings, thus avoiding the main source of repeatability problems.Voltammetric data were crossed with Raman spectroscopy and scanning electron microscopy coupled with X-ray energy dispersive analysis (SEM/EDX) for obtaining information for conservation/restoration purposes. EXPERIMENTAL SECTION Materials and Chemicals. Reference materials were CoO (Aldrich), CuO (Baker), CuCl (De Hae¨n), and Cu2O (Carlo Erba) reagents, and copper trihydroxichlorides prepared by means of recommended procedures.28,29 Clinoatacamite was prepared by immersion of a sheet of copper (1 × 5 cm) into a slurry of CuCl in water (0.1 g/L). After 24 h, a crystalline green precipitate was developed in contact with the copper sheet. The crystals were separated and rinsed with water and ethanol. Atacamite was prepared following a similar procedure but using a CaCO3 suspension (0.1 g/L) in a 0.1 g/L solution of CuCl2·2H2O (Merck) in water and stirring the solution magnetically for 24 h in contact with the copper sheet. Botallackite was prepared by an identical procedure, but the suspension was left unstirred. To prevent recrystallization into atacamite, the resulting green crystalline precipitate was merely separated from the aqueous suspension and desiccated.28 Paratacamite, a similar compound where Ni, Co, or Zn replaces some of the Cu,28,29 was not considered here. Reference pigments were azurite natural (standard, K10200), azurite natural (fine, K10210), azurite natural (dark standard, K10250), azurite natural (dark fine, K10260), (30) Bell, I. M.; Clark, R. J. H.; Gibbs, P. Spectrochim. Acta, Part A 1997, 53, 2159-2179. (31) David, A. R.; Edwards, H. G. M.; Farwell, D. W.; De Faria, D. L. A. Archaeometry 2001, 43, 461-473. (32) Gilbert, B.; Denoe¨l, S.; Weber, G.; Allart, D. Analyst 2003, 128, 12131217. (33) Frost, R. L.; Martens, W.; Kloprogge, J. T.; Wiliams, P. A. J. Raman Spectrosc. 2002, 33, 801-806. (34) Frost, R. L. Spectrochim. Acta, Part A 2003, 59, 1195-1204. (35) Hayez, V.; Costa, V.; Guillaume, J.; Terryn, H.; Hubin, A. Analyst 2005, 130, 550-556. (36) Lovric, M. In Electroanalytical Methods; Scholz, F., Ed.; Springer: Berlin, 2002; p 111.

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azurite natural (greenish gray-blue, K10280), azurite MP reddish deep (63-120 µm, K10201), azurite MP, deep (K10203), azurite MP (cobalt blue-type, K10204), azurite MP (cerulean blue, K10206), azurite MP (greenish light (K10207), azurite MP (exclusive, K10208), malachite natural (standard grind, K10300), malachite natural (very fine, K10310), malachite MP (coarse, K10341), malachite MP (medium, 80-100 µm, K10343), malachite MP (fine, 63-80 µm, K10344), malachite MP (very fine, 0-63 µm, K10345), malachite arabian (K10370), verdigris (synthetic, K44450), smalt (standard grind, K10000), smalt (very fine grind, K10010), and dark cobalt blue (K45700), all supported by Kremer. Heated specimens of azurite and smalt were prepared by heating in furnace at 200, 400, and 600 °C. A second series of specimens consisting of azurite + malachite, azurite + atacamite, and azurite + smalt mixtures were prepared from K10200, K10300, and K10000 materials. Compositions were 70:30, 50:50, and 30:70 w/w. A third series was prepared from the above adding CaCO3 (50% w/w). These mixtures were accurately powdered and homogenized in mortar and pestle before electrochemical measurements. Electrode Modification and Conditioning. Paraffin-impregnated graphite electrodes were prepared as described in the literature1,2,7,8 and consisted of cylindrical rods of diameter, 5 mm. Prior to the series of runs for each material or sample, a conditioning protocol was used for increased repeatability. The electrode surface was polished with alumina, rinsed with water, and submitted to potential cycles between +0.85 and -0.85 V during 10 min in contact with phosphate buffer. An amount of ∼10-20 µg of reference materials and ∼1.0 µg of samples was powdered in an agate mortar and pestle and further extended on the agate mortar forming a spot of finely distributed material. Then the lower end of the graphite electrode was gently rubbed over that spot of sample and finally rinsed with water to remove illadhered particles. Instrumentation and Procedures. Electrochemical experiments were performed at 298 K in a three-electrode cell under argon atmosphere. SQWVs and complementary CVs were obtained with CH 420I equipment. Paraffin-impregnated graphite working electrodes were dipped into the electrochemical cell so that only the lower end of the electrode was in contact with the electrolyte solution. This procedure provides an almost constant electrode area and reproducible background currents.7 A AgCl (3 M NaCl)/Ag reference electrode and a platinum wire auxiliary electrode completed the conventional three-electrode arrangement. A 0.50 M phosphate buffer (Panreac) was used as the electrolyte solution. Hierarchical cluster analysis was performed using the Minitab14 software package. Raman spectra were acquired using a Renishaw InVia confocal Raman microscope, operating with diode and gas laser excitation at 785, 633, 514.5, and 488 nm wavelengths and CCD detection. Minimal laser powers of the order of microwatts were used to prevent damage to sensitive pigments with lens objectives of 20× and 50×, which provided spectral footprints between 2 and 5 µm. A spectral resolution of 2 cm-1 was used over the wavenumber range 1800-200 cm-1, with the accumulation of between 10 and 20 scans to improve the signal-to-noise ratios. Calibration was effected using a silicon wafer and wavenumbers of sharp bands are accurate to ∀1 cm-1. 2706


Morphology of the surface of paintings was characterized using a Jeol JSM 6300 scanning electron microscope operating with a Link-Oxford-Isis X-ray microanalysis system. The analytical conditions were accelerating voltage 20 kV, beam current 2 × 10-9 Å, and working distance 1.5 mm. In parallel to the morphological examination of microsamples, elemental analysis was performed by means of SEM/EDX. Samples were carbon-coated to eliminate charging effects. Qualoitative analysis was performed in punctual mode. Quantitative microanalysis was carried out using the ZAF method for correcting interelemental effects. The counting time was 100 s for major and minor elements. Concentrations were calculated by stoichiometry from element percentages generated by ZAF software on the Oxford-Link-Isis EDX. Samples. As previously noted, the Palomino’s paintings in the ceiling vault of the Sant Joan del Mercat church in Valencia, dating from 1707, were gunfired during the Spanish Civil War in 1936. As a result, only some 20% of the original frescoes remain and they are in a serious condition. Over an extensive part of the paintings, the outer ground layer (intonaco) has been destroyed, exposing the intermediate ground layer (arricio), which itself has been removed in several parts along with the inner ground layer (arenato) to reveal the underlying brickwork. Figure 1 shows an image illustrative of the damage suffered by the paintings. Sampling was exercised from a representative selection of remaining fresco fragments prior to their consolidation during the conservation tasks. Samples were undertaken with a scalpel using minimal intervention but including, wherever possible, pigment particles that were adhered to the substrate. Each sample was divided in three aliquots for analysis using SEM/EDX, Raman spectroscopy, and VMP. Samples were taken during 2002 and 2005 from different areas of the hemicylindrical-shaped vault and were initially classified into two groups: blackened samples (PVB7, PVB8, PVB9), exhibiting a gross black surface layer, all excised from the central axis of the vault (highest part), and “dark” samples (PV1, PV2, PV3, PV3b, PV4, PV5, PV7, PV8, PV3a, PA4b, PA5b, PA7, PV8b, PV10, PV11) taken in different locations external to the central axis of the vault. Two additional samples, U7 and U11, were taken from the lunettes placed at the lowest part of the vault. The attribution of these samples to Antonio Palomino was uncertain because it is documented that, at this level of the vault, the painter Vicente Guillo´ Barcelo´ (1645-1698) started to execute a prior frescoe, which was, partially, maintained despite Antonio Palomino finally being the painter in charge for the decoration of the complete vault. RESULTS AND DISCUSSION Analysis of Voltammetric Responses. Figure 2 shows the CV responses of (a) azurite, (b) malachite, and (c) smalt, attached to PIGEs and immersed into 0.50 M phosphate buffer (pH 7.4). In the initial cathodic scan voltammograms of copper pigments, two overlapping cathodic waves appear at ∼-0.10 and ∼-0.20 V versus AgCl (3M NaCl)/Ag, followed, in the subsequent anodic scan, by a stripping peak at +0.02 V eventually exhibiting certain peak splitting. In the second and following cathodic scans, a more intense reduction peak ∼-0.05 V was recorded. If the potential scan is switched at -0.15, the stripping peak vanishes. For smalt, the CV presents a main cathodic peak at -0.18 V, accompanied by a stripping oxidation peak at -0,08 V.

Figure 1. Image of a portion (area 1 m2) of the damaged Palomino’s frescoes in the vault of the Sant Joan del Mercat church in Valencia, Spain.

In Figure 3, the SQWV responses of the following are compared: (a) azurite, (b) cuprite, (c) verdigris, and (d) atacamite, all immersed into 0.50 M phosphate buffer. On initiating the potential scan at +0.45 V in the negative direction, reduction peaks at -0.10 and -0.25 V appear. SQWVs for all other azurite specimens as well as malachite ones were similar. In contrast, verdigris and cuprite exhibit a unique peak at ∼-0.15 V looking like two strongly overlapped signals, preceding a weak signal at -0.55 V. Atacamite and botallackite exhibit a similar profile, with peaks at -0,15 and -0,25 V, while clinoatacamite produces a unique peak at -0,16 V. The voltammetric response of all azurite and malachite pigments (see Supporting Information) exhibited a close similarity, with variations lower than 10-15 mV in the peak potential and peak width from one specimen to another. The voltammetry of CuO, however, was clearly different (vide infra), consisting of a prominent cathodic peak at -0.60 V, also differing from that of CuCl, for which a unique reduction peak at -0.35 V was recorded in phosphate buffers. Figure 4 shows the response of (a) smalt, (b) cobalt blue, and (c) a smalt specimen heated at 600 °C during 24 h. Smalt yields a main reduction peak at -0,14 V, whereas cobalt blue yields waves at +0.20 and -0.50 V. The heated smalt specimen produces the reduction peak at -0.14 V followed by a broad wave at -0.50 V. The voltammetry of cobalt and copper pigmenting species can be described in terms of the overall reduction of the parent compounds to the corresponding metal, followed by the oxidative dissolution of the metal deposit to metal ions (Co2+, Cu2+) in solution.

The reduction of copper pigments proceeds apparently via two successive one-electron steps. Interestingly, upon addition of NaCl (in concentrations between 0.05 and 0.10 M) to the electrolyte, the voltammetric pattern of the different copper pigments remains essentially unchanged. Since in the presence of chloride ions, Cu(I)-chloride complexes in solution should be formed, thus providing a marked two-peak response,37 the above feature clearly suggests that the reduction of copper pigments involves a solidstate Cu(II) to Cu(I) transformation followed by epitactic reduction to copper metal lightly accompanied by a dissolution-metal deposition mechanism involving intermediate species in solution phase. The reduction process is then governed by proton insertion and the advance of a hydrated layer along the grains of pigment, similarly to the electrochemical reduction of lead oxide to lead metal described by Hasse and Scholz.38 Consistently, on increasing the potential scan rate, the second reduction peak for azurite and malachite decreases with respect to the first one while both peaks are lightly shifted in the negative direction. The overall reaction of reduction for azurite can be described as

2CuCO3‚Cu(OH)2 (s) + 6H+ (aq) + 6e- f 3Cu (s) + 2CO2 + 4H2O (1)

(s) denoting solid phases. In the subsequent anodic scan, the deposit of Cu metal is oxidized to Cu2+ (aq) ions, which in turn (37) Va´zquez, J.; La´zaro, I.; Cruz, R. Electrochim. Acta 2006, 52, 6106-6117. (38) Hasse, U.; Scholz, F. Electrochem. Commun. 2001, 3, 429-434.


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Figure 2. CVs of PIGEs modified with (a) azurite (K10200), (b) malachite (K10300), and (c) smalt (K10000), immersed into 0.50 M phosphate buffer, pH 7.4. Potential scan rate 50 mV/s.

are reduced to Cu metal in the second and successive potential scans. In the case of cobalt pigments, the response appears to depend on the structural environment of cobalt ions in the material, and, in particular, on the presence of both octahedral and tetrahedral Co2+ ions, as described for cobalt cordierites.39 Thus, smalt produces a reduction peak at -0.14 V (Figure 4a). Upon heating there is certain tetrahedral/octahedral interconversion, as described in the literature,26 so that an additional signal at -0.50 V appears (Figure 4c). For cobalt blue, where both tetrahedral and octahedral Co2+ ions coexist, but in a spinel-type structure, far from the glass smalt environment, two reduction waves at +0.20 and -0.50 V are recorded (Figure 4b). These electrochemical processes can be described on the basis of the model developed by Lovric, Oldham, Scholz et al. for the (39) Domeńech, A.; Torres, F. J.; Alarcoń, J. J. Solid State Electrochem. 2004, 8, 127-137.

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Figure 3. SQWVs for (a) azurite (K10200), (b) cuprite, (c) verdigris (K44450), and (d) atacamite, in contact with 0.50 M phosphate buffer, pH 7.4. Potential scan initiated at +0.45 mV in the negative direction. Potential step increment 4 mV; square wave amplitude 15 mV; frequency 2 Hz.

electrochemistry of nonconducting solids attached to inert electrodes.40-43 Here, the redox reaction is initiated at the particle/ electrolyte/electrode three-phase junction and propagates through the solid particle via electron hopping and proton insertion into the solid lattice. It should be noted that, for the studied systems, the overall reduction process can be controlled not only by the kinetics of the proton insertion or electron-transfer process but also by the kinetics of the nucleation and nuclii growth involved in the formation of the metal. (40) Lovric, M.; Scholz, F. J. Solid State Electrochem. 1997, 1, 108-113. (41) Lovric, M.; Scholz, F. J. Solid State Electrochem. 1999, 3, 172-175. (42) Oldham, K. B. J. Solid State Electrochem. 1998, 2, 367-377. (43) Schro ¨der, U.; Oldham, K. B.; Myland, J. C.; Mahon, P. J.; Scholz, F. J. Solid State Electrochem. 2000, 4, 314-324.

metric curves was used in order to obtain more discriminating parameters and quantitative data for pigments. As originally studied by Reinmuth for irreversible electrontransfer processes involving species in solution phase,44,45 the rising portion of voltammetric curves can be approached, in several cases, to a exponential variation of the current with the applied potential. In particular, this assumption applies for linear scan voltammograms of reversible and irreversible electron-transfer processes involving species attached to the electrode surface.46 In this last case, the current satisfies

i ) nFAkoΓoe exp(-RnaF(E - Eo′)/RT) exp

[

]

RTko exp(-RnaF(E - Eo′)/RT) (2) RnaFv

where Γo represents the surface concentration of the electroactive species, Rna the product of the coefficient of electron transfer by the number of electrons involved in the rate-determining step, ko the electrochemical rate constant at the zero potential, and the other symbols have their usual meaning. Extension of this treatment to SQWV is complicated by the recognized influence of potential step increment and square wave amplitude in the shape of voltammetric curves obtained by this technique, so that numerical solutions of diffusion equations rather than analytical ones are in general used. In the case of reversible electron transfer between species in solution, as long as the square wave amplitude, ESW, is lower than 0.5RT/nF, a condition easily accomplished under the usual experimental conditions, the net current flowing during the anodic and cathodic half-cycles can be represented, following Ramaley and Krause by an expression of the type:47,48

exp(nF(E - Eo′)/RT)

RTπ1/2

[1 + exp(nF(E - Eo′)/RT)]2

idif ) C

n2F2AD1/2cESWf1/2

Figure 4. SQWVs for (a) smalt (K10000), (b) cobalt blue (K45700), and (c) a smalt specimen heated at 600 °C for 24 h in contact with 0.50 M phosphate buffer, pH 7.4. Potential scan initiated at +0.45 mV in the negative direction. Potential step increment 4 mV; square wave amplitude 15 mV; frequency 2 Hz.

For our purposes, the relevant point to emphasize is that the electrochemical response is phase-dependent, allowing for the characterization of solid compounds. In view of the close vicinity between the voltammetric curves for the different copper and cobalt species, multiparametric fitting, and multivariate regression procedures were tested. For these purposes, a series of shapedependent parameters, which can be easily measured for the main reduction peak, were taken: (i) peak potential, Ep, (ii) onset potential obtained from the intersection of the almost linear portion of the peak with the baseline for current measurement, Eon, and (iii) peak-to-half peak potential separation, Ep(I)-Ep/2, were used. Pertinent data are summarized in Table 1. Hierarchical cluster analysis, however, indicated that although such parameters should provide a distinction between the studied species, the percentages of difference were small (See Supporting Information). Tafel Analysis. In view of the close vicinity between the voltammetric curves for azurite, malachite, verdigris, smalt, and the specimens of the atacamite group, Tafel analysis of voltam-

(3)

f being the square wave frequency, C a numerical constant, and the other symbols having their customary meaning. For a reduction process, both eqs 2 and 3 can be reduced to a linear variation of lni on E when the applied potential is clearly larger than the formal electrode potential, Eo′; i.e., at the foot of the voltammetric peak. Using reported numerical solutions for the diffusion equations,49-53 a similar Tafel-type relationship can be approximated, under favorable conditions, in SQWVs for oxidative/reductive dissolution of species immobilized on the electrode surface,49-51quasi reversible surface processes,52 and surfaceconfined electrochemical reactions.53 (44) Reinmuth, W. H. Anal. Chem. 1960, 32, 1891-1892. (45) Buck, R. P. Anal. Chem. 1964, 36, 947-949. (46) Bard, A. J.; Faulkner, L. R. Electrochemical methods; John Wiley & Sons: New York, 1980; pp 521-525. (47) Ramaley, L.; Krause, M. S.; Jr. Anal. Chem. 1969, 41, 1362-1365. (48) Krause, M. S. Jr.; Ramaley, L. Anal. Chem. 1969, 41, 1365-1369. (49) Lovric, M.; Komorsky-Lovric, S. J. Electroanal. Chem. 1988, 248, 239253. (50) Lovric, M.; Komorsky-Lovric, S.; Bond, A. M. J. Electroanal. Chem. 1991, 319, 1-18. (51) Komorsky-Lovric, S.; Lovric, M.; Bond, A. M. Anal. Chim. Acta 1992, 258, 299-305. (52) O’Dea, J. J.; Osteryoung, J. G. Anal. Chem. 1993, 65, 3090-3097. (53) Komorsky-Lovric, S.; Lovric, M. Anal. Chim. Acta 1995, 305, 248255.


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Table 1. Electrochemical Data for Reference Pigmenting Materiala specimen

Eon (mV)

Ep (mV)

Ep-Ep/2 (mV)

Tafel SL (mV-1)

Tafel OO

r2

azuriteb malachiteb atacamitec botallackitec clinoatacamitec verdigrisc smaltb cobalt bluec Azurite (200 °C) smalt (600 °C)

+30 ( 5 +35 ( 5 +15 ( 5 +20 ( 5 -65 ( 5 +30 ( 5 +5 ( 5 -60 ( 5 +35 ( 5 +15 ( 5

-110 ( 5 -105 ( 5 -155 ( 5 -160 ( 5 -165 ( 5 -150 ( 5 -155 ( 5 -250 ( 10 -105 ( 5 -145 ( 5

90 ( 5 70 ( 5 90 ( 5 90 ( 5 60 ( 5 120 ( 5 95 ( 5 120 ( 5 90 ( 5 90 ( 5

-0.0115 ( 0.0004 -0.0160 ( 0.0005 -0.0154 ( 0.0005 -0.0196 ( 0.0005 -0.0203 ( 0.0005 -0.0195 ( 0.0005 -0.0089 ( 0.0004 -0.0140 ( 0.0005 -0.0112 ( 0.0004 -0.0137 ( 0.0005

-1.00 ( 0.02 -1.30 ( 0.02 -1.66 ( 0.03 -1.82 ( 0.04 -2.62 ( 0.04 -1.26 ( 0.02 -1.57 ( 0.02 -2.98 ( 0.08 -1.00 ( 0.02 -1.53 ( 0.02

0.9996 0.9997 0.9995 0.9994 0.9996 0.99998 0.9996 0.9993 0.9994 0.9995

a From SQWVs at specimen-modified PIGEs immersed into 0.50 M phosphate buffer, pH 7.4. Initiated at +0.65 V in the negative direction. Potential step increment 4 mV; square wave amplitude 20 mV; frequency 5 Hz. bMean value for specimens listed in the Experimental Section, c Mean values for five independent measurements on the same material.

Although there is no disposal of a detailed model for describing reduction processes such as represented by eq 1, the Grygar model54 for reductive dissolution of solids provides a possible approach. Assuming that both linear scan and square wave voltammograms behave similarly, the current at the beginning of the voltammetric peak can tentatively be represented as

(

RnaF E RT

i ≈ qoko exp -

)

(4)

where qo represents the total charge involved in the complete reaction of the electroactive solid. Equation 4 predicts a linear dependence of lni on E whose slope depends on the phasecharacteristic coefficient Rna, while the ordinate at the origin depends on the electrochemical rate constant and the net amount of depolarizer deposited on the electrode regardless of the granulometry of the solid.55,56 In order to eliminate the contribution of this last quantity, it is convenient to use normalized currents. This is possible because in both linear scan46,54 and square wave voltammetries49-51 the peak current for the reduction of surfaceimmobilized species can be approached by an expression of the type

ip ) H

( )

RnaF q RT o

teristic of the solid analyte regardless of the amount of sample deposited on the electrode. For a two-component system, one can write

(

)

(

)

RXnaXFE RYnaYFE + qoYkoY exp RT RT

i ≈ qoXkoX exp -

(7)

If RjnajFE/RT , 1 (j ) X,Y), one can use the approximation e-z ≈1- z, so that the above equation reduces to

i ≈ (qoXkoX + qoYkoY)

[

]

(qoXkoXRXnaX + qoYkoYRYnaY)(FE/RT) (8) qoXkoX + qoYkoY

exp -

If voltammetric peaks for X and Y are strongly overlapped, a unique peak will be recorded, the peak potential being approached by

(

ip ≈ H X

)

(

)

RXnaXF RYnaYF vqoX + HY vqoY RT RT

(9)

(5) Thus, the i/ip ratio will be given by the approximate expression:

H being an electrochemical coefficient of response characteristic of the electrochemical process and the electrode area and the potential scan rate (LSV) or the square wave frequency (SQWV). Combining eqs 4 and 5, one obtains

ln(i/ip) ) ln

( )

RnaF koRT E HRnaF RT

(6)

(54) Grygar, T. J. Electroanal. Chem. 1996, 405, 117-125. (55) Grygar, T. J. Solid State Electrochem. 1998, 2, 127-136. (56) Bakardjieva, S.; Bezdicka, P.; Grygar, T.; Vorm, P. J. Solid State Electrochem. 2000, 4, 306-333.


[

]

(qoXkoXRXnaX + qoYkoYRYnaY)(FE/RT) (10) qoXkoX + qoYkoY

exp -

Here, both the generalized Tafel slope (SL ) RnaF/RT) and the ordinate at the origin (OO ) ln(koRT/HRnaF)) become charac-

2710

(qoXkoX + qoYkoY)RT i ≈ ip (HXRXnaXqoX + HYRYnaYqoY)nFv

This equation fits to a linear dependence of ln(i/ip) on E so that the slope and the ordinate at the origin will be intermediate between those obtained for the X and Y components separately via eq 7. For quantitation of a mixture of X plus Y, one can combine the Tafel dependence predicted by eq 10 for that mixture, with the Tafel dependence described by eq 5, applied separately for

Figure 5. Generalized Tafel plots for azurite (rhombs), malachite (solid squares), atacamite (triangles), and verdigris (open squares) from SQWV data in phosphate buffer. Potential step increment, 4 mV; square wave amplitude, 25 mV; frequency, 5 Hz.

the individual components. As a result, the X to Y molar ratio, g ()qoX/qoY) can be expressed as

g)

(

)( )( )

SLY - SLM koY RYnaY SLM - SLX koX RXnaX

(11)

In this equation, SLM represents the Tafel slope for the mixture of X plus Y, and SLX, SLY, the Tafel slopes for the individual components. This equation enables the a determination of g from Tafel representations providing that the quotients between the individual electrochemical rate constants, koX and koY, and the electron-transfer coefficients, RXnaX, RYnaY, are known. Considering eq 7, these ratios can be directly obtained from the normalized Tafel ordinates at the origin, OOX, OOY, and the Tafel slopes for the individual components, so that, finally

g)

(

)(

)( )

SLY - SLM exp(OOY) SLY SLM - SLX exp(OOX) SLX

(12)

In view of the close similarity between the predictions for SQWV and LSV concerning the Tafel-type behavior to be expected in the initial portion of voltammetric peaks, it will be assumed that eq 10 also applies for SQWVs of sparingly soluble electroactive solids mechanically attached to inert electrodes. On the basis of that assumption, the above treatment can be taken as a semiempirical approach whose application should be confirmed by experimental data. Analysis of Reference Materials. Figure 5 shows generalized Tafel plots of ln(i/ip) versus E for azurite, malachite, atacamite, and verdigris. In all cases, an excellent linearity was obtained (see Supporting Information) for potentials between 200 and 100 mV before the corresponding voltammetric peak. The values of SL and OO determined for the reference materials are listed in Table 1. Confirming the suitability of the Tafel analysis previously described, current-potential curves in the rising portion of SQWV peaks for all the studied pigments fitted well to linear ln(i/ip) on E dependences, with correlation coefficients larger than 0.999 in all cases (see Table 1 and Supporting Information). Figure 6 presents a two-dimensional diagram in which SL and OO were used as variables. As can be seen in this figure, data

Figure 6. Two-dimensional Tafel slope vs Tafel ordinate at the origin diagram for pigmenting materials studied here (solid rhombs) and azurite plus malachite (squares) and azurite plus smalt (triangles) mixtures. From SQWVs at specimen-modified PIGEs immersed into 0.50 M phosphate buffer, pH 7.4. initiated at +0.65 V in the negative direction. Potential step increment 4 mV; square wave amplitude 20 mV; frequency 5 Hz.

points representative for the different species fall in localized and well-separated regions of the diagram. In order to test the validity of the proposed methodology for analysis of mixtures, different specimens consisting of azurite + malachite, azurite + atacamite, and azurite + smalt mixtures were prepared. In order to approach the conditions of paint samples, a second series was prepared incorporating CaCO3 as diluent (50% w/w). In all cases, the voltammetric responses of the specimens were similar to those of the reference materials. Tafel analysis of the rising portion of the main reduction peak provided linear ln(i/ ip) versus E plots (correlation coefficients larger than 0.999; see Supporting Information), the values of SL and OO being intermediate between those determined for the parent materials separately. The corresponding data points are also depicted in Figure 6. Interestingly, no significant differences were obtained between pigment mixtures and pigment + CaCO3 ones. For these systems, quantitation using Tafel parameters provided results in satisfactory agreement with the nominal composition of the azurite + malachite mixtures, with standard deviations lower than 5% for all compositions. For azurite + atacamite and azurite + smalt mixtures, however, some major deviations (1015%) were obtained from nominal compositions. A reason for this can be obtained on considering data in Table 1. Thus, while for azurite and malachite the main reduction peak possesses identical peak potential, the peak potentials for azurite and atacamite (and for azurite and smalt) differ in 50-100 mV; i.e., one of the conditions for quantitation using Tafel analysis does not apply strictly. This situation can be summarized on considering that the peak current in these mixtures will be lower than the sum of the peak currents for the separated components (eq 7), thus distorting the i/ip values with respect to those for exactly coincident voltammetric peaks. Apart from this, eventual interactions between the components during electrochemical turnovers may distort voltammetric responses, as reported for iron and manganese oxide Analytical Chemistry, Vol. 80, No. 8, April 15, 2008

2711

Figure 7. Theoretical variation of the Tafel ordinate at the origin for azurite + smalt mixtures taking peak potential separations (from upper to below) of 0, 50, 75, and 100 mV. Data points correspond to synthetic specimens containing pure azurite; pure smalt; and 70:30, 50:50, and 30:70 (%, w/w) azurite-smalt mixtures. From SQWVs at specimen-modified PIGEs immersed into 0.50 M phosphate buffer, pH 7.4 initiated at +0.65 V in the negative direction. Potential step increment 4 mV; square wave amplitude 20 mV; frequency 5 Hz.

materials.14 To account for this effect, theoretical working currentpotential curves for azurite + atacamite and azurite + smalt mixtures were obtained from experimental voltammograms of azurite, atacamite, and smalt. The resulting Tafel parameters were close to those experimentally determined for the corresponding mixtures. A representation of theoretical curves and experimental data points for azurite plus smalt mixtures is presented in Figure 7. Here, experimental data agree with theoretical ones taking a peak potential separation of 50 mV. Analysis of Real Samples. Chemical and morphological analysis by SEM/EDX of the studied samples informed on the pigment distribution in the different paint strata as well as on the elemental composition of the different grains and crystalline aggregates identified on the secondary and backscattered electron images of the cross-section of the studied samples. It should be noted that samples, in general, consisted of mixtures of several pigments, which often appeared applied in different strata. Most of the studied samples exhibited X-ray emission lines characteristic of smalt KR(Si), KR(K), KR(As), Kβ(As), KR(Co), and Kβ (Co) and copper pigments KR(Cu) and Kβ(Cu). Interestingly, black color was observed in the cross section of the samples when they were observed with the light microscope in some grains and crystalline aggregates Cu-rich suggesting the probable transformation of the original pigment in tenorite, a black CuO. Red earths, Naples yellow, green earth, and iron oxide red were others of the pigments appearing in the set of samples studied corresponding to the brownish-green and blue areas of the vault (See Supporting Information.). SQWVs of samples from the Sant Joan del Mercat church can be divided into three morphological groups, respectively represented in Figure 8 by samples: (a) PV8B, (b) PV7, and (c) PV1. 2712 Analytical Chemistry, Vol. 80, No. 8, April 15, 2008

Figure 8. SQWVs for samples: (a) sample PVB9, (b) PV7, and (c) PV1 immersed into 0.50 M phosphate buffer, pH 7.4. Potential scan initiated at +0.45 or +0.65 mV in the negative direction. Potential step increment 4 mV; square wave amplitude 15 mV; frequency 2 Hz.

For “blackened” samples PVB7, PVB8, and PVB9 (Figure 8a), a prominent reduction peak at -0.60 V appears, preceded by a less intense peak at -0.10 V. For samples PV3b, PV7, PV8, PA3, PA4b, PA5b, PA7, PV10, and PV11; a main reduction peak located between -0.10 and -0.16 V is accompanied by broad signal at -0.60 V, as shown in Figure 8b. Finally, samples PV1, PV2, PV3, PV4, PV5, and PV8b show (see Figure 8c) a main reduction peak at -0.12 V, followed by weak signals at -0.20 and -0.60 V. A similar response was obtained for samples U7 and U11. In several samples, an additional reduction peak at -0.55 V, accompanied by a stripping anodic peak at -0.48 V, representative of Naples yellow,57 was also recorded (see Supporting Information) in agreement with SEM/EDX data. SQWVs performed on scanning the potential from -0.85 V in the positive direction also provide relevant information for analytical purposes. This can be seen in Figure 9, where the voltammetric responses for (a) azurite, (b) sample PV8b, (c) smalt, and (d) sample PA5b are shown. Copper pigments yield a unique stripping peak at -0.05 V whereas cobalt pigments produce a main anodic peak at +0.02 V accompanied by overlapping peaks at -0.02 and +0.22 V. SQWV in Figure 9b is representative of the response obtained for samples PV8b, PA7, U7, and U11, consisting of only one single stripping peak near to 0.0 V, characteristic of copper. (57) Domeńech, A.; Domeńech, M. T.; Mas, X. Talanta 2007, 71, 1569-1579.

Figure 9. SQWVs for (a) azurite (K10200), (b) sample PV8b, (c) smalt (K10010), and (d) sample PA5b, in contact with 0.50 M phosphate buffer, pH 7.4. Potential scan initiated at -0.85 mV in the positive direction. Potential step increment 4 mV; square wave amplitude 15 mV; frequency 2 Hz.

Samples PV3b and PA4b should be composed by smalt while all other samples showed a voltammetric profile that can be described in terms of the cobalt stripping or as a superposition of the stripping processes for cobalt and copper, as can be seen in Figure 9d for sample PA5b. In this voltammogram, an additional stripping peak appears at -0.48 V, due to the presence of Naples yellow in the sample. The foregoing set of data indicates that copper pigment and copper + cobalt pigment mixtures exist in the samples. The prominent signal at -0.60 V in “blackened” samples can unambiguously be attributed to tenorite (CuO), as can be assumed from a comparison between SQWVs in Figure 8a with those for tenorite and tenorite plus azurite mixtures (see Supporting

Information). Formation of tenorite from copper pigments should occur during the gunfire episode suffered by the paintings, as clearly suggested by thermochemical data. Thus, upon heating, azurite and malachite undergo loss of CO2 and water at 345 °C to give CuO. Further heating yields Cu2O at 840 °C.58-60 In turn, copper acetate dehydrates at 190 °C with partial decomposition at 220 °C forming CuO accompanied by small amounts of Cu2O and Cu3O4, further oxidized in air at 400 °C. Tafel analysis of the rising portion of the reduction peak at ∼-0.10 V produced linear log(i/ip) versus E plots for all the studied samples, as indicated by statistical parameters (correlation coefficients larger than 0.999; see Supporting Information). The corresponding SL and OO values are listed in Table 2. Insertion of such parameters into a two-dimensional diagram is illustrated in Figure 10. Here, one can observe that (i) data points for samples U7 and U11 fall in the malachite region, (ii) data points for blackened PVB7, PVB8, and PVB9 samples are located in a central position in the diagram, distanced from smalt and copper pigments, and (iii) all other samples are located in a region between azurite and smalt. These results suggest that samples PV3b, PV7, PV8, PA3, PA4b, PA5b, PA7, PV1, PV2, PV3, PV4, PV5, PV8b, PV10, and PV11 are constituted by azurite, smalt, and azurite + smalt mixtures, while samples U7 and U11 are composed of malachite. These results were confirmed by Raman spectroscopy. Azurite and smalt were identified on the basis of their characteristic vibrations at 402, 1430/1459, and 1577 cm-1for azurite and 1086, 475, 430, 377, 358, and 1370 cm-1 for smalt, whereas malachite displays characteristic signal at 433 cm-1, all in agreement with the literature.30-35 Additionally, the majority of the studied samples showed carbon signatures, whether arising from the fire or from addition as a darkening agent to other pigments, all being assignable to vegetable- or plant-based origin.24 In order to test the possible influence of thermal stress in the voltammetric response of the pigments, two additional series of specimens were prepared upon heating azurite (K10200) and smalt (K10010) in furnace during 24 h at 200, 400, and 600 °C. As expected, up to 400 °C, azurite was converted into tenorite, as denoted by blackening of the sample. For the sample treated at 200 °C, the voltammogram was essentially indistinguishable from that of the parent azurite pigment, with coincident Tafel parameters. Pertinent data are summarized in Table 1. For smalt, only a light change in the hue of the sample was obtained after thermal treatments. Remarkably, although the general profile of the voltammogram remained unchanged, Tafel parameters for the reduction peak at -0.15 V changed significantly with the temperature. Insertion of the corresponding data points into the SL versus OO diagram (see Figure 10) reveals that data points for blackened samples become now intermediate between the regions of azurite and smalt heated at 600 °C. The smalt/azurite ratio was determined from Tafel parameters using the proposed procedure. Pertinent data are summarized in Table 3. Remarkably, data points for samples PVB7, PVB8, PVB9, PA3, PV3b, PV7, PV8, PA4b, PA5b, PV1, PV2, PV3, PV4, and PV5 (58) Frost, R. L.; Ding, Z.; Kloprogge, J. T.; Martens, W. V. Thermochim. Acta 2002, 390, 133-144. (59) Kiseleva, I. A.; Ogorodova, L. P.; Melchakova, L. V.; Bisengalieva, M. R.; Becturganov, N. S. Phys. Chem. Miner. 1992, 19, 322-333. (60) Mansour, S. A. A. J. Therm. Anal. 1996, 46, 263-274.


2713

Table 2. Electrochemical Data for Samples from the Sant Joan del Mercat Church, from SQWVs at Specimen-Modified PIGEs Immersed into 0.50 M Phosphate Buffer, pH 7.4a

a

sample

Eon (mV)

Ep (mV)

Ep-Ep/2 (mV)

Tafel SL(mV-1)

Tafel OO

PVB7 PVB8 PVB9 PA3 PA4b PA5 PA7 PV3b PV7 PV8 PV1 PV2 PV3 PV4 PV5 PV8b PV10 PV11 U7 U11

+50 ( 5 +40 ( 5 +45 ( 5 +45 ( 5 +35 ( 5 +65 ( 5 +40 ( 5 +35 ( 5 +20 ( 5 +20 ( 5 +30 ( 5 +40 ( 5 +35 ( 5 +30 ( 5 +40 ( 5 +35 ( 5 +40 ( 5 +40 ( 5 +50 ( 5 +45 ( 5

-160 ( 5 -155 ( 5 -150 ( 5 -155 ( 5 -160 ( 5 -155 ( 5 -115 ( 5 -165 ( 5 -155 ( 5 -155 ( 5 -145 ( 5 -150 ( 5 -145 ( 5 -150 ( 5 -155 ( 5 -110 ( 5 -150 ( 5 -150 ( 5 -105 ( 5 -110 ( 5

110 ( 5 100 ( 5 105 ( 5 110 ( 5 110 ( 5 120 ( 5 100 ( 5 115 ( 5 105 ( 5 95 ( 5 95 ( 5 105 ( 5 100 ( 5 110 ( 5 95 ( 5 90 ( 5 100 ( 5 100 ( 5 80 ( 5 70 ( 5

-0.0126 ( 0.0005 -0.0128 ( 0.0005 -0.0133 ( 0.0005 -0.0098 ( 0.0005 -0.0088 ( 0.0005 -0.0095 ( 0.0005 -0.0111 ( 0.0005 -0.0089 ( 0.0005 -0.0096 ( 0.0005 -0.0098 ( 0.0005 -0.0105 ( 0.0005 -0.0100 ( 0.0005 -0.0106 ( 0.0005 -0.0101 ( 0.0005 -0.0103 ( 0.0005 -0.0115 ( 0.0005 -0.0095 ( 0.0005 -0.0105 ( 0.0005 -0.0164 ( 0.0005 -0.0170 ( 0.0005

-1.30 ( 0.03 -1.24 ( 0.03 -1.27 ( 0.04 -1.03 ( 0.03 -1.02 ( 0.03 -0.87 ( 0.03 -1.11 ( 0.03 -1.12 ( 0.04 -1.41 ( 0.06 -1.24 ( 0.04 -1.03 ( 0.03 -1.33 ( 0.03 -1.08 ( 0.03 -1.26 ( 0.04 -1.16 ( 0.04 -1.01 ( 0.03 -1.06 ( 0.03 -1.18 ( 0.03 -1.32 ( 0.03 -1.34 ( 0.03

Initiated at +0.65 V in the negative direction. Potential step increment 4 mV; square wave amplitude 20 mV; frequency 5 Hz.

Table 3. Quantitative Data for Samples from the Sant Joan del Mercat Church Derived from Electrochemical Data

Figure 10. Two-dimensional Tafel slope vs Tafel ordinate at the origin diagram for samples from the Sant Joan del Mercat church. From SQWVs at specimen-modified PIGEs immersed into 0.50 M phosphate buffer, pH 7.4 initiated at +0.65 V in the negative direction. Potential step increment 4 mV; square wave amplitude 20 mV; frequency 5 Hz. Squares, dark samples; triangles, strongly blackened samples; rhombs, samples whose attribution to Palomino was uncertain.

cover a relatively wider region between azurite and smalt as depicted in Figure 11. Here, samples PV1-PV5, PV7, and PV8 can be assigned to azurite plus smalt mixtures because data points fall in the Tafel diagram close to the theoretical working SL versus OO curve for a peak potential separation of 50 mV. Quantitation using Tafel parameters (eq 10) provides smalt molar percentages relative to the azurite + smalt mixture grouped in few dosages: pure azurite, pure smalt, and azurite plus smalt mixtures concentrated in smalt molar percentages of 55, 72, and 85%. Consistently, application of this method to blackened samples using Tafel parameters for azurite and smalt heated at 600 °C (see data in 2714 Analytical Chemistry, Vol. 80, No. 8, April 15, 2008

sample

tenorite/(azurite + smalt) (w/w) ratio

% of smalt (mol/mol) from Tafel analysis

% of smalt (w/w) from peak potentials

PVB7 PVB8 PVB9 PA3 PA4b PA5b PA7 PV3b PV7 PV8 PV1 PV2 PV3 PV4 PV5 PV8b PV10 PV11

0.27 ( 0.04 0.37 ( 0.04 0.71 ( 0.06 0.10 ( 0.02 0.08 ( 0.02 0.10 ( 0.02 0.09 ( 0.02 0.14 ( 0.03 0.08 ( 0.02 0.12 ( 0.03 0.24 ( 0.04 0.12 ( 0.03

59 ( 4 67 ( 4 86 ( 2 81 ( 3 100 ( 1 88 ( 2 0(1 100 ( 1 86 ( 2 81 ( 3 58 ( 4 76 ( 3 54 ( 4 73 ( 2 75 ( 3 0(1 72 ( 3 53 ( 4

78 ( 11 78 ( 11 67 ( 9 78 ( 11 100 ( 12 78 ( 11 6(6 100 ( 12 78 ( 11 78 ( 11 50 ( 8 67 ( 9 50 ( 8 67 ( 9 78 ( 11 0(4 78 ( 11 50 ( 8

Table 1) provide smalt percentages just in the aforementioned dosages (see Table 3). These results suggest that the painter used several fixed azurite + smalt dosages in order to obtain the desired chromatic effect in different areas of the frescoes. In view of the consistent use of azurite + smalt mixtures by Palomino, one can conclude that samples U7 and U11, where malachite is the copper pigment, should be attributed to Guillo´. Interestingly, samples PV3b, PA4b, and PA5 fall in a region of the SL versus OO diagrams in Figures 10 and 11 clearly separated from the theoretical curve for a peak potential separation of 50 mV. This can mainly be attributed to the following: i) the use of different pigment sources by the painter and/or their alteration

Figure 11. Detail of the SL vs OO diagram in the azurite + smalt region for samples from the Sant Joan del Mercat church. From SQWVs at specimen-modified PIGEs immersed into 0.50 M phosphate buffer, pH 7.4 initiated at +0.65 V in the negative direction. Potential step increment 4 mV; square wave amplitude 20 mV; frequency 5 Hz.

by effect of thermal stress, and (ii) the presence of a disturbing matrix. The issue i appears to be in contravention with the above data, because points for samples PV3b, PA4b, and PA5 separate not only from the azurite + smalt region but also from the azurite + heated smalt one. With regard to the issue ii, it should be noted that experiments with CaCO3 plus pigment for both azurite and smalt produced Tafel responses essentially identical to those displayed by pure pigments. In view of this, a possible option should be the presence of any remaining binding media in such samples, just obtained from zones of the frescoes far from the central axis of the vault. Since in these zones the thermal stress during the gunfire was relatively smooth (vide infra), one can conjecture that the rest of binding media remain, thus modifying the voltammetric response of the pigments. This is consistent with prior observations on lead pigments.57 Analyses carried out by means of gas chromatography/mass spectrometry have evidenced the presence of amino acids in samples containing copper pigments in a few Palomino samples.24 This result suggests that Antonio Palomino could bind pigments with some proteinaceous medium in order to prevent their alteration from the strongly alkaline medium provided by the Ca(OH)2 formed in the fresco technique. These results are in agreement with the recommendations published by the artist in his treatise on pictorial techniques El Museo Picto´ rico y Escala Optica, published in 1769.61 Quantitation can be completed with the determination of the amount of tenorite relative to the azurite + smalt mixture in samples. This was estimated from the peak areas for the azurite + smalt peak at -0.10 V and the tenorite peak at -0.60 V. Since the specific response of all materials was not identical, a calibration (61) Palomino, A. El museo picto´ rico y escala o´ ptica; Translation from the original published in 1759. Aguilar: Madrid, 1947; p 745.

Figure 12. Calibration graph for estimating the tenorite/(azurite + smalt) ratio in thermally altered samples from the Sant Joan del Mercat church using the quotient between the peak currents for voltammetric signals at -0.10 and -0.60 V.

graph, constructed from electrochemical data for azurite + smalt + tenorite mixtures was used. The resulting graph is shown in Figure 12, while the calculated percentages of tenorite are shown in Table 3. Crossing all these data with the position of the samples in the nave provides a scene for the gunfire attack suffered by the paintings. Thus, blackened samples PVB7, PVB8, and PVB9, having high tenorite content, were placed along the central axis of the vault. Crossing the foregoing set of data with those derived from the analysis of earth pigments,25 one can conclude that the central part of the vault reached temperatures between 600 and 650 °C during the gunfire. Samples with minor amounts of tenorite provided from the lateral zones of the nave probably experienced temperatures in the 350-460 °C range. Samples from paintings near the lunettes, for which no significant tenorite signals were recorded, reached probably temperatures of ∼260 °C. CONCLUSIONS Tafel analysis of voltammetric curves can be used for quantifying components in solid micro- and submicrosamples, where strongly overlapping peaks for two electroactive components are recorded, taking a semiempirical approach based on the assumption that the involved electrochemical processes approach this kind of current-potential dependence in a reasonably wide range of conditions. Experimental SQWV data for the reduction of copper and cobalt pigments and samples from the Sant Joan del Mercat church in Valencia satisfied Tafel-type equations. Twodimensional diagrams, using Tafel slope and ordinate at the origin, calculated from the rising portion in current/potential curves, enable the identification of individual components in such samples. This methodology permits the following: (i) characterization of Palomino paintings, with distinction between azurite, smalt, or azurite + smalt compositions; (ii) a satisfactory discrimination between the paintings executed by Antonio Palomino from those others from Vicente Guillo´-Barcelo´, where, in contrast to that found for Palomino paintings, malachite was Analytical Chemistry, Vol. 80, No. 8, April 15, 2008

2715

the only copper pigment used; (iii) quantification of pigment mixtures and determination of the extent of alterations in paint specimens in highly damaged frescoes. This methodology is limited, however, by the confidence level of the aforementioned Tafel approximation. Voltammetric data, confirmed by SEM/EDX and Raman spectroscopy data, indicated that azurite, very frequently accompanied by smalt, was used by Palomino in the frescoes of the Sant Joan del Mercat church. The composition of azurite-smalt mixtures was relatively homogeneous, including applications of essentially pure azurite to mixtures containing smalt proportions ∼60% (w/w) of smalt until pure smalt. This result informs on the technique used by the artist: azurite and azurite + smalt mixtures were used preferentially in some dosages by the painter in order to obtain the desired chromatic effect. As a result of the gunfire attack suffered by the frescoes in the past, tenorite was formed, thus producing considerable chromatic changes in the paint. This study illustrates the capabilities of the voltammetry of microparticles for obtaining information potentially interesting for archaeometry, conservation, and restoration of cultural goods from solid samples in relatively complicated systems.

2716


ACKNOWLEDGMENT Financial support is gratefully acknowledged from the Generalitat Valenciana GVAE07/140 and ACOMP/2007/138 Projects and the MEC Projects CTQ2005-09339-C03-01, 02 and CTQ200615672-C05-05/BQU, which are also supported with ERDEF funds. The authors thank Dr. Pilar Roig Picazo and Dr. Ignacio Bosch Reig art conservator and architect in charge of the conservation project of the San Joan del Mercat church. Financial support of this conservation project is kindly acknowledged from Lubasa and Fundacioń Aguas de Valencia. The authors thank Mr. Manuel Planes Insausti and Dr. Jose´ Luis Moya Lo´pez for technical assistance. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review November 28, 2007. Accepted January 28, 2008. AC7024333

Quantitation from Tafel Analysis in Solid-State Voltammetry

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