A reliable source of very small amounts of hydrogen chloride for analytical purposes

Sources of small quantities of hydrogen chloride (nmol s^-1 and pmol s^-1), based on molecular diffusion through a membrane from a reservoir of strong hydrochloric acid, are described. Accurate procedures of standardizing and checking the sources are reported. Extension to other reagents is possible. This standardized source provides a reliable means for the preparation of gaseous reference mixtures as well as standard solutions of hydrochloric acid.     

Uncertainty of measurement: approaches and open problems

Despite of exhaustive literature information, analysts are still fronting serious problems in estimating the uncertainty of measurement (UOM), mainly because of the intrinsic difficulties in choosing an appropriate approach and/or of an inadequate background in statistics and metrology fundamentals. Sometimes, additional problems, at present under debate, make their efforts even more stressful. In this paper, attention is mainly paid to intra-laboratory estimation of UOM, the most frequent situation. After a short re-examination of the existing UOM approaches, some applicative problems are discussed in the light of the most representative literature information.     

Nature of the chemical bond between metal atoms and oxide surfaces: new evidences from spin density studies of K atoms on alkaline earth oxides

We have studied the interaction of K atoms with the surface of polycrystalline alkaline-earth metal oxides (MgO, CaO, SrO) by means of CW- and Pulsed-EPR, UV-Vis-NIR spectroscopies and DFT cluster model calculations. The K adsorption site is proposed to be an anionic reverse corner formed at the intersection of two steps, where K binds by more than 1 eV, resulting in thermally stable species up to about 400 K. The bonding has small covalent and large polarization contributions, and the K atom remains neutral, with one unpaired electron in the valence shell. The interaction results in strong modifications of the K electronic wave function which are directly reflected by the hyperfine coupling constant, (K)a(iso). This is found to be a very efficient "probe" to measure the degree of metal-oxide interaction which directly depends on the substrate basicity. These results provide an original and general model of the early stages of the metal-support interaction in the case of ionic oxides.     

ESR study of N-acyl-N-alkyl nitroxide radicals from photolysis of N-nitrosoamides

A new method of generation of N-acyl-N-alkyl nitroxide radicals by photolysis of N-nitrosoamides is described. ESR parameters are reported for 17 radicals of the general formula R? CO? N(R′)O^.; the high resolution obtained allows conformational analysis which shows a preferred trans conformation around the CO? N bond and hindered rotation around the N? R′ σ-bond.     

EPR study of the surface basicity of calcium oxide. 3. Surface reactivity and nonstoichiometry

High surface area polycrystalline calcium oxide forms ozonide O3- ions upon O2 adsorption and NO3(2-) anions under low pressures of NO. Both radical anions, detected by electron paramagnetic resonance (EPR), are not observed in the case of the homologous magnesium oxide. This behavior reveals the presence, in CaO, of anomalies with respect to the ideal composition of an ionic oxide which are identified in terms of two main types of defects. The first type consists of positive holes dispersed in the bulk and originated by the unavoidable presence of Na+ ions in the composition of the solid. The decomposition of the surface ozonide shows the formation of a transient surface stabilized O- (the chemical notation of a positive hole associated to an oxide ion) which is for the first time reported at the surface of CaO. The second type of defect consists of surface peroxide groups (present at particular surface sites where they are formed by pairing of two distinct O-) which react with nitric oxide (NO) yielding NO3(2-) radical anions. The presence of peroxide is not related to the presence of impurities but, rather, to a certain propensity of the solid to form such ions at the surface along the dehydration process.     

Properties of alkali metal atoms deposited on a MgO surface: a systematic experimental and theoretical study

The adsorption of small amounts of alkali metal atoms (Li, Na, K, Rb, and Cs) on the surface of MgO powders and thin films has been studied by means of EPR spectroscopy and DFT calculations. From a comparison of the measured and computed g values and hyperfine coupling constants (hfccs), a tentative assignment of the preferred adsorption sites is proposed. All atoms bind preferentially to surface oxide anions, but the location of these anions differs as a function of the deposition temperature and alkali metal. Lithium forms relatively strong bonds with MgO and can be stabilized at low temperatures on terrace sites. Potassium interacts very weakly with MgO and is stabilized only at specific sites, such as at reverse corners where it can interact simultaneously with three surface oxygen atoms (rubidium and cesium presumably behave in the same way). Sodium forms bonds of intermediate strength and could, in principle, populate more than a single site when deposited at room temperature. In all cases, large deviations of the hfccs from the gas-phase values are observed. These reductions in the hfccs are due to polarization effects and are not connected to ionization of the alkali metal, which would lead to the formation of an adsorbed cation and a trapped electron. In this respect, hydrogen atoms behave completely differently. Under similar conditions, they form (H(+))(e(-)) pairs. The reasons for this different behavior are discussed.     

Use of an Octapeptide–Guanidiniocarbonylpyrrole Conjugate for the Formation of a Supramolecular β‐Helix that Self‐Assembles into pH‐Responsive Fibers

Peptides that adopt β‐helix structures are predominantly found in transmembrane protein domains or in the lipid bilayer of vesicles. Constructing a β‐helix structure in pure water has been considered difficult without the addition of membrane mimics. Herein, we report such an example; peptide 1 self‐assembles into a supramolecular β‐helix in pure water based on charge interactions between the individual peptides. Peptide 1 further showed intriguing transitions from small particles to helical fibers in a time‐dependent process. The fibers can be switched to vesicles by changing the pH value.     

Zeolites as model solids for investigations on the role of iron at the solid-liquid interface in particulate toxicity

In order to investigate the molecular mechanism whereby iron in asbestos and in other iron containing fibres plays a crucial role in fibre induced carcinogenicity, two Fe²⁺-exchanged zeolites (Y and H-ZSM-5) have been prepared and used as model solids. Fe-Y was active both in free radical generation via hydrogen abstraction and in single strand DNA damage, but lost both these properties when oxidized by hydrogen peroxide. Fe-ZSM-5, on the other hand, was nearly inactive. Iron chelators and reductants (ascorbate, citrate and EDTA) enhance the activity in DNA single strand breaks. Moreover, they reactivate oxidized Fe-Y and activate Fe-ZSM-5. Iron is mobilized and Fe³⁺ is partially reduced to Fe²⁺. The most pronounced effect was found when citrate was present together with ascorbate. H-abstraction turns out to be a rather general reaction, occurring also with peptides as target molecules. In this case EPR spectra show that more than one type of radical moiety is generated.     

Preparation and spectroscopic characterization of visible light sensitized N doped TiO2 (rutile)

Nitrogen doped TiO₂ represents one of the most promising material for photocatalitic degradation of environmental pollutants with visible light. However, at present, a great deal of activity is devoted to the anatase polymorph while few data about rutile are available. In the present paper we report an experimental characterization of N doped polycrystalline rutile TiO₂ prepared via sol-gel synthesis. Nitrogen doping does not affect the valence band to conduction band separation but, generates intra band gap localized states which are responsible of the on set of visible light absorption. The intra band gap states correspond to a nitrogen containing defect similar but not coincident with that recently reported for N doped anatase.