The high analytical sensitivity and high spatial resolution of synchrotron radiation-based techniques, in particular SR-XRD and SR-FT-IR, allows the identification of complex micrometric mixtures of compounds that constitute the different layers of ancient paintings. The reliability of the measurements even with an extremely small amount of sampled material is very high, and this is particularly important when analyzing art works.

The resinous materials from the interior surfaces of two Roman and one Iberian amphora were studied with Fourier transform infrared (FTIR) spectroscopy. The results were then compared with those obtained by synchrotron radiation-FTIR, scanning electron microscopy (SEM), and gas chromatography-mass spectrometry (GC-MS). The FTIR spectra obtained by the technique of KBr micropellets, prepared directly with the materials scraped from the amphora without any further sample preparation, provided enough information to establish their diterpenoic nature, and even to differentiate between the two main materials employed for waterproofing purposes, pitch and wood tar. Methyl dehydroabietic acid (DHAM) is the main chemical marker that allows a distinction to be made between these two materials. Pitch and wood tar were prepared in the laboratory heating pine resin and resinous pine wood, respectively. These resinous waterproofing materials were compared with the coatings extracted from the amphorae. The samples whose direct FTIR spectra showed a signal at approximately 1740 cm(-1), attributed to a carbonyl group of methyl ester, presented as well a peak of DHAM in the GC-MS chromatogram of the neutral fraction of their extract. Samples without this signal in their spectra did not present DHAM in their chromatogram. This work studies, for the first time, waterproofing of an amphora attributed to the Iberian culture.

Synchrotron radiation Fourier transform infrared (SR-FT-IR) microspectroscopy represents an advance over conventional FT-IR spectroscopy because it gives a higher signal/noise ratio at the highest spatial resolution due to the high brightness and collimation of synchrotron radiation. It has been successfully applied to the study of ancient paintings, alteration and corrosion layers which are heterogeneous microlayered materials made by complex mixtures of organic and inorganic compounds. Moreover, the high brightness attribute allows FT-IR spectra to be routinely obtained directly on the surfaces of the objects and opens the possibility for nondestructive testing of museum objects. We present in this paper a selection of applications of SR-FT-IR to the analysis of ancient paintings, alteration and corrosion layers where the technique has proven to be especially useful: first, the separation and identification of pigment microparticles from ancient Roman wall paintings; second, the determination of the binding media and the byproducts resulting from the interaction between binders and pigments from medieval altarpieces; and third, the study of the surface corrosion layers of a bronze helmet by means of direct analysis of the surface.

Neutron and synchrotron X-ray diffraction, X-ray fluorescence and FTIR were used to examine a Corinthian-type bronze helmet which is now on display at The Manchester Museum, UK. This type of helmet was manufactured out of a single piece of bronze, probably on a rod-anvil, and like all body-armour it was made to measure. Neutron diffraction sampling of the bronze volume in different areas was used to study the composition, microstructure and crystallographic texture of the alloy in order to draw conclusions about the manufacturing processes. The neutron data revealed the presence of microstrains and non-random distributions of bronze grains hinting at annealing-hammering working cycles in order to harden and shape the alloy. X-ray fluorescence showed that the main body of the helmet is a copper–tin alloy, while the noseguard contains zinc in high abundance. This key compositional difference confirms that the noseguard is not the original but is a modern substitute fabricated for restoration purposes. SR XRD and FTIR from several spots on the head and noseguard identified several surface corrosion products and showed a variation of the Cu–Sn or Cu–Zn percentage compositions, and of the mineral phases. Small samples of corrosion flakes extracted from the outside and inside of the helmet were used to obtain powder XRD patterns.

The dissociation constant of picric acid in nitrobenzene, pKa=6.63, was established spectrophotometrically, and the standardization of potentiometric systems in this medium using picric acid titration is proposed as a standard method. The acid–base equilibria of a series of bases in nitrobenzene were also studied. The pKHB+ values of the protonated bases of various structure were determined and compared with the pKHB+ values in water and acetonitrile in order to evaluate the resolution of acid strength in these media.

Dissociation constants of acid components of pH reference materials in tetrahydrofuran–water mixtures containing 0, 10, 20, 30, 40, 50, 60, 70 and 100% (w/w) tetrahydrofuran, THF, were obtained. Dissociation constant values determined were: pK1 and pK2 for tartaric and phthalic acid; pK1, pK2 and pK3 for citric acid; pKa for acetic and boric acid and pK2 for phosphoric acid. These values are essential to the determination of the reference pHS values of the standard buffer solutions, which are needed for accurate pH measurements in THF–water mixtures. In order to explain the variation of the pKa values obtained over the whole composition range studied, the quasi-lattice quasi chemical (QLQC) theory of preferential solvation was applied. To identify the solvent characteristics affecting the pKa values, the results are discussed in terms of average macroscopic properties of the mixed solvent and in terms of the solvation shells around the solute. Thus, the methodology of linear solvation energy relationships (LSER) was used to relate pKa values with parameters of THF–water mixtures. The equations obtained allow calculation of the pKa values of substances in THF–water mixtures.

The solvent characteristics of tetrahydrofuran (THF)–water mixtures that have an influence on the chromatographic behaviour of ionizable compounds in liquid chromatography (LC) were investigated (Debye–Hückel parameters, pH scale, LC-useful range, solvatochromic parameters). Solute properties, such as the pKa values of the acids used to prepare reference standard buffer solutions used in LC, depend on the composition of the mixture. The pKa values in several THF–water mixtures were subjected to factor analysis in order to determine how many factors affect the variation in the data sets, and then to target factor analysis to identify these factors. The influence of solvatochromic parameters on the interactions between acidic groups and the solvents studied was identified and quantified. Standard pH values, pHS, for the seven primary reference buffer solutions of the NIST scale and for acetate buffer in THF–water mixtures containing 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70% and 100% (w/w) THF were determined using IUPAC standardization rules. The quasi-lattice–quasi-chemical (QLQC) theory of preferential solvation was applied to obtain the degree of preferential solvation of hydrogen ions in these media in order to explain the variation in pHS values obtained over the range of mixtures studied. The relationships obtained allow calculation of the pHS values of the standard buffers in THF–water mixtures. Thus pH measurements in these media can be performed as in water. The chromatographic behaviour of the solute can then be related to the real pH value of the mobile phase used.

Standard emf for the cell Pt/Ag/AgCl/HCl in THF-water/GE (glass electrode) in tetrahydrofuran-water mixtures containing 0-70% (w/w) of tetrahydrofuran and values of the autoprotolysis constants, K-ap, of these media were obtained. The influence of variations in the solvent composition on pK(ap) values was evaluated. The resulting equations enables us to determine pK(ap) values in any tetrahydrofuran-water mixture, and thus permit one to establish the pH scale in these mixtures. The quasilattice quasichemical (QLQC) theory of preferential solvation was applied to obtain the degree of preferential solvation of hydrogen ions in these media, in order to explain the variation of the pK(ap) values obtained in the tetrahydrofuran-water mixtures studied.

Dissociation constant values for reference materials were determined in tetrahydrofuran–water (THF–water) mixtures containing 0, 10, 20, 30, 40, 50, 60, 70 and 100% (w/w) THF. The values obtained were: pK1 and pK2 for phthalic acid; pK1, pK2 and pK3 for citric acid; pKa for acetic and boric acids. In order to explain the variation of the pK values obtained over the whole composition range studied, the quasi-lattice quasi chemical (QLQC) theory of preferential solvation was applied. The results are discussed in terms of average macroscopic properties of the mixed solvent and in terms of the solvation shells around the solute in order to identify the solvent characteristics affecting the pKa values. Thus, the methodology of linear solvation energy relationships (LSER) was used to relate pKa values with solvatochromic parameters of THF–water mixtures. The equations obtained allow calculation of the pKa values of substances in the widely used THF–water mixtures.

Standard pH(S) values for five reference buffer solutions of the NIST series: KH(2)citrate, KH-phthalate, acetate, tetraborate and carbonate buffers in tetrahydrofuran-water, THF-water, mixtures containing 0, 10, 20, 30, 40, 50, 60 and 70% (w/w) THF at 298.15 K, have been determined using IUPAC standardization rules. The variation in the pH(S) values over the whole composition range of THF-water mixtures studied has been explained by preferential solvation effects, and quantified by application of the quasi-lattice quasi-chemical theory (QLQC). In order to identify the solvent features affecting the variation of pH(S) values with the percentage of THF, the pH(S) values obtained were subjected to factor analysis (FA) and were correlated with macroscopic and solvatochromic parameters, pi*, alpha, beta and E-T, of the solvent mixtures. The equations obtained permit the standardization of potentiometric sensors in THF-water mixtures and thus allow pH measurements in the usual way. The correct pH values measured permit the study of acid-base behaviour of substances in methodologies using THF-water media. (C) 1999 Elsevier Science Ltd. All rights reserved.

Dissociation constant values of the acid components of the standard NIST scale buffer solutions and of acetate buffer in several hydroorganic mixtures (acetonitrile–water, tetrahydrofuran–water and methanol–water) were first subjected to factor analysis in order to obtain the number of factors that affect the variation of the whole data sets, and then to target factor analysis to identify these factors. The pKa values were correlated with the Kamlet and Taft solvatochromic parameters (α, β and π*) of the solvent mixtures according to the results obtained with these chemometric techniques. Two or three factors emerged, depending on the range of percentage of organic cosolvent in the mixture. Target factor analysis results showed that Kamlet–Taft equations were reduced in these mixtures: the independent term and π* solvatochromic parameter of polarity-polarizability appear up to 50% w/w of organic solvent, as does the hydrogen bond basicity β solvatochromic parameter when data up to 70% w/w were analyzed. Further, the quasi-lattice quasi-chemical theory has been applied to quantify the preferential solvation by water of electrolytes in acetonitrile–water, tetrahydrofuran–water and methanol–water mixtures. An overview of the effect of this preferential solvation on the values of the protonation constants in these media was also obtained.

The dissociation pK values of a series of diuretics in 10, 30, 40, 50 and 70% (w/w) acetonitrile–water mixed solvents at 25°C were determined according to the criteria endorsed by IUPAC. The series of diuretics chosen includes compounds with differences in molecular structures and physico-chemical properties. Acidic compounds (loop diuretics, such as furosemide, bumetanide and ethacrynic acid), weakly acidic (thiazides, such as chlorthiazide and trichlormethiazide), neutral (aldosterone antagonists, such as canrenone) and basic compounds (potassium-sparing diuretics, such as amiloride and triamterene) were all considered. The variation of the pK values obtained over the whole composition range studied can be explained by taking into account the preferential solvation of ionizable compounds in acetonitrile–water mixtures. Moreover, in order to obtain pK values in any of the unlimited number of possible binary solvent acetonitrile–water mixtures, relationships between pK values and different bulk properties were examined, and the linear solvation energy relationships method (LSER) was applied to study the correlations of pK values with the solvatochromic parameters π*, α and β of acetonitrile–water mixtures. The equations obtained allowed calculation of the pK values of diuretics in any acetonitrile–water mixtures up to 70% (w/w) and thus permitted the acid–base behaviour of these important substances in the widely used acetonitrile–water media to be known.

The dissociation constants of representative loop (furosemide), thiazide (chlorthiazide and trichlormethiazide) and potassium sparing (amiloride) diuretics in 10, 30, 40, 50 and 70% (w/w) acetonitrile–water mixtures at 298.15 K were determined, in accordance with IUPAC procedures. The variation in pKa values over the whole composition range can be explained by preferential solvation and the structural features in acetonitrile–water mixtures. Correlations between pKa values and various bulk and solvatochromic properties of the solvents were calculated. The linear solvation energy relationship (LSER) method was applied. The resulting equations allowed us to calculate pKa values for the diuretics studied in any acetonitrile–water mixture up to 70% (w/w) and should help to clarify the acid–base behaviour of diuretics in the widely-used acetonitrile–water mixed solvents.

The dissociation constants in several acetonitrile-water mixtures of the carboxylic acid groups of 19 substances, included into four different sets (diuretics, quinolones, buffers and peptides) were first submitted to factor analysis (FA) in order to find how many factors affect the variations in the data sets. Then target factor analysis (TFA) was performed in order to identify these factors. The influence of solvatochromic parameters in the interactions between carboxylic acid groups and the solvents studied was identified and quantified. The model Kamlet-Taft equation for each substance was established. The general effect of the preferential solvation of electrolytes in acetonitrile-water mixtures on the values of the protonation constants in these media is also described.

The dissociation pK values of the representative loop diuretics furosemide, bumetanide and ethacrynic acid in 10, 30, 40, 50 and 70% (w/w) acetonitrile-water mixtures at 298.15 K were determined, according to the rules and procedures endorsed by IUPAC. The variation in pK values over the whole composition range studied can be explained by tacking into account the preferential solvation of ionizable substances in acetonitrile-water mixtures. With a view to determining the pK values of the loop diuretics studied in any of the binary solvent acetonitrile-water mixtures, correlations of pK values and different bulk properties of the solvent were examined, and the linear solvation energy relationships method, LSER, has been applied. The pK values were then correlated with the pi*, alpha and beta solvatochromic parameters of acetonitrile-water mixtures. The resulting equations allowed us to calculate pK values for the loop diuretics in any acetonitrile-water mixture up to 70% (w/w) acetonitrile.

Reference value standards, pH(s) in 10% (w/w) acetonitrile-water solvent mixtures for 11 reference buffer solutions have been determined from reversible emf measurements of the cell Pt/Ag/AgCl/standard buffer + KCl, in acetonitrile-water/glass electrode, at 298.15 K. Values of ionization constants, required for above calculations, have been determined from reversible emf measurements of the cell Pt/Ag/AgCl/HA + A + KCl, in acetonitrile-water/glass electrode, in the same solvent composition and temperature.