Anilinepropylsilica xerogel used as a selective Cu (II) adsorbent in aqueous solution

The metal ion adsorption properties of the microporous hybrid anilinepropylsilica xerogel were studied using divalent copper, zinc, and cadmium ions in aqueous solutions in concentrations ranging from 10(-4) up to 5x10(-3) moll(-1). At low concentrations the surface of the solid phase presents selectivity for Cu (II), even in competitive conditions. This preferential sorption ability for copper in relation to zinc and cadmium ions was interpreted by considering the xerogel morphology.     

A new procedure for bovine milk digestion in a focused microwave oven: gradual sample addition to pre-heated acid

Milk samples can be efficiently digested using a focused microwave oven, however the conventional procedure of addition of concentrated acids to the liquid sample leads to digestates with elevated acidity and residual carbon concentrations. In this work a focused microwave oven was applied for acid digestion of bovine milk samples using a conventional and an alternative procedure based on gradual sample addition to hot and concentrated acids. A two-level 2³ full factorial design experiment with eight runs was carried out to evaluate the optimum experimental conditions for reducing both the residual carbon and the final acidity of digestates. The three studied parameters were: temperature of the digestion medium for sample addition, addition of sulfuric acid before the sample or during the first step, and number of aliquots of the sample gradually added. The best conditions were attained by adding small aliquots of milk (ten-fold a volume of 0.5 ml added during 5.0 min) to a digestion mixture containing 3.0 ml nitric acid plus 1.0 ml sulfuric acid heated at 105 °C. It was demonstrated that the digestion efficiency of the alternative procedure was better than the conventional procedure, i.e. 98 and 80%, respectively. The alternative procedure was applied for determination of Ba, Ca, Cu, K, Mg, Na, P, and Zn in whole and non-fat bovine milk. The accuracy was proved using two certified reference materials (whole and non-fat milk powder).     

Metal furnace heated by flame as a hydride atomizer for atomic absorption spectrometry: Sb determination in environmental and pharmaceutical samples

The present work describes a metallic hydride atomizer for atomic absorption spectrometry, by evaluating the performance of the Inconel 600^® tube. For this purpose, stibine was used as the model volatile compound and antimony determination in river and lake sediments and in pharmaceutical samples was carried out to assess the metal furnace performance. Some parameters are evaluated such as those referring to the generation and transport of the hydride (such as KBH₄ and acid concentrations, carrier gas flow rate, injected volume, etc.), as well as those referring to the metal furnace (such as tube hole area, flame composition, long-term stability, etc.). The method presents linear Sb concentration from 2 to 80 μg L⁻¹ range (r > 0.998; n = 3) and the analytical frequency of ca. 140 h⁻¹. The limit of detection (LOD) is 0.23 μg L⁻¹ and the precision, expressed as R.S.D., is less than 5% (40 μg L⁻¹; n = 10). The accuracy is evaluated through the reference materials, and the results are similar at 95% confidence level according to the t-test.     

Electronic and Steric Influence of Axial and Equatorial Ligands in Vitamin B12 Model Complexes Derived from Cobaloxime: Electrochemical and 59Co-NMR Studies

In four series of strictly related organocobalt complexes, derived from cobaloximes by replacement of the O…H…O with O…-BF₂…O and/or (CH₂)₃ groups, the trends of ⁵⁹Co-NMR shielding and electrochemical data are discussed. A largely parallel behaviour of the plots of E_1/2(I) values for the first Co(III)/Co(II) electron transfer vs. the ₅₉Co chemical shifts reflects the similar sensitivity of the two parameters to a change in electron affinity of the central metal ion due to a variation of the organic group R. E_1/2(II) values for the second Co(II)/CO(I) electron transfer are less sensitive to the change of R, but the trend of the plot vs. δ(₅₉Co) is still parallel in the four series. Consistent deviations from a roughly linear dependence of E_1/2(I) on pK_a of the hydrocarbon acid corresponding to R, on Taft constant s?_* and on ⁵⁹Co shielding are noticed for the isopropyl derivatives and attributed to a steric effect. This was confirmed in a series of R? Co(DMG)pyridine complexes in which ⁵⁹Co shielding decreases steadily with increasing steric parameter E_s (Taft) of the alkyl group. There is experimental evidence from X-ray data that δ(⁵⁹Co) decreases with an increase of the Co? C bond length, illustrating steric hindrance in alkyl coordination to be responsible for the decreased shielding of the ⁵⁹Co nucleus. The relative displacements of the graphic displays for the different series reflect the effect of changes in electron affinity of the redox center, due to the equatorial ligand, which, in turn, is caused by variations in the electron-withdrawing power due to the introduction of the BF₂ group and by the change from ?2 to ?1 valence of the (CH₂)₃-capped ligands.     

Operational comparison of various thermodynamic treatments of organic substance adsorption at the electrode/solution interface

Three isotherms are considered: Frumkin's Bennes' and Mohilner's. The relationship between the interaction parameters in the first two and the adsorbate activity coefficient in the last is discussed. The significance of various standard states for the definition of ΔG° _ads is analysed. The operational derivation of ΔG°_ads in the three cases and the relationship between the resulting three quantities are illustrated. The isotherms have been used to describe the adsorption of 1,4-dioxane on a polarized Hg electrode. The analysis has been carried out both at constant potential and constant charge. The picture emerging in each of the three cases is discussed in the light of the results obtained in the other two. It is concluded that the three approaches give the same qualitative information about the interfacial behaviour of dioxane, especially as far as the effect of the electric field and the particle-particle lateral interaction are concerned. The usefulness of the Frumkin isotherm for analysing promptly experimental adsorption data is thus maintained. Limitations of the present approach are discussed.     

Chemical and thermal stability of alkanethiol and sulfur passivated InP(100)

InP(100) surfaces treated with Na2Sx9H20 and CnH(2n+1)SH are examined by contact angle measurement, X-ray photoelectron spectroscopy, and atomic force microscopy to determine the chemical and thermal behavior of these passivated surfaces. The surfaces coated by octadecanethiol (n = 18) self-assembled monolayers (SAMs) are found to be more stable toward oxidation than the S-passivated surface. The chemical stability of octadecanethiol SAMs in various environments is examined. The thiol monolayer is found to be stable in 0.1 M HCl but degrades in 0.1 M NaOH, boiling chloroform, and water. The behavior of these surfaces at elevated temperatures under a vacuum is also investigated. The octadecanethiol-coated InP(100) is stable up to 473 K, above which the films begin to degrade. Unlike other substrates on which the entire molecule including the sulfur headgroup desorbs together, on InP, the sulfur headgroup remains on the surface even after annealing to 673 K. These observations suggest that the desorption occurs by S-C bond cleavage as well as In-S bond cleavage. The sulfur of S-passivated InP is found to be more thermally stable than that of the octadecanethiol monolayer, perhaps due to their different bonding geometries and hence energies.     

Kinetics and Mechanism of Aminolysis of Phenyl Acetates in Aqueous Solutions of Poly(ethylenimine)

Second-order rate constants (k(n)) for the aminolysis of some phenyl acetates with poly(ethylenimine) (PEI) were obtained in a pH range 4.36-11.20 at 25 degrees C in 1 M KCl. Linear Bronsted-type plots (log k(n) vs pK(N) of PEI) were found for less reactive esters 2-nitrophenyl acetate, 4-acetoxy-3-chlorobenzoic acid, and 4-acetoxybenzenesulfonate with slopes of 0.92, 0.99, and 0.82, respectively. Curved plots were obtained for 3-acetoxy-2,6-dinitrobenzoic acid and 4-acetoxy-3-nitrobenzenesulfonate, which are consistent with a stepwise reaction. The most likely mechanism involves the existence of a tetrahedral intermediate (T(+/-)) and a change in the rate-determining step from its breakdown to its formation when the basicity of the polyamine increases. A semiempirical equation was used to calculate the values of limiting slopes of the plots (0.9 and 0.1 for both esters) and pK(N) at the center of the curvature of the plots (pK(N degrees ) = 7.94 and 9.02, respectively). The values of pK(N degrees ) are lower than those estimated for the aminolysis of the same esters with simple monomeric amines (pK(n degrees ) > 11) because of a better leaving ability of the aryl oxide ion from the tetrahedral intermediate when amino groups of PEI instead of simple amines are involved. Estimation of the pK's of the reactive intermediates and of the microscopic rate constants for the proton transfer from T(+/-) to PEI or from PEIH(+) to T(+/-) indicates that either base or acid catalysis is unimportant in the aminolysis of these esters by PEI.     

Synthesis and X-Ray Analysis of a Nickel Triphenyl-Cyclopropenyl Complex. A mo-interpretation of the geometry and bonding in the (C3H3)ML3-type of complexes

The reaction of [(C₃Ph₃)Ni(PPh₃)₂]ClO₄ with P(CH₂CH₂PPh₂)₃(pp₃) and NaBPh₄ yields the [(C₃Ph₃)Ni(pp₃)]BPh₄-complex. After long exposure of the solution of this compound in acetone/butanol to the air a new derivative [(C₃Ph₃)-Ni(pp₂po)]BPh₄· 0.5 C₄H₉OH, where pp₂po is (Ph₂PCH₂CH₂)₂P(CH₂ CH₂POPh₂), is obtained. Complete X-ray analysis has been carried out for the latter complex: a=18.303 (5); b=29.445 (6), c=13.305 (5) Å, β=112.70 (9)°; space group monoclinic, P2₁/a, Z=4. Disorder problems were encountered in the refinement of the structure. The best R is 0.093. One of the arms of the parent pp₃-molecule, not coordinated to the metal, undergoes oxidation. The Ni-atom, coordinated by the three remaining P-atoms of the ligand, is also linked in a roughly η³-mode to the cyclopropenium ligand. The geometry of the molecule is examined in detail. Extended HMO-calculations were performed to interpret how the variation of P? Ni? P angles affects the bonding between the NiP₃- and C₃H₃-fragments. The conclusion is that the overall energy of the complex may be lowered in spite of a weakening of the Ni-cyclopropenium linkage. Extensions are made to other systems containing a linkage between a metal and a X₃-ring (X=P,As).     

Electron impact mass spectrometric investigation of pyrrole mono-, di- and tri-carboxylic acids

The electron impact-induced fragmentation processes of pyrrole-2-, -3-, -2,3-, -3,4-, -2,5- and -2,3,4-carboxylic acids were investigated with the aid of metastable ion, collisional and deuterium labelling experiments. The general behaviour of these compounds is discussed in detail, together with the reasonable structures of the more relevant fragment ions.