Gold ultra-microelectrode arrays: application to the steady-state voltammetry of hydroxide ion in aqueous solution.

Gold ultra-microelectrode arrays are used to explore the electrochemical oxidation of hydroxide ions and are shown to be analytical useful. Two types of ultra-microelectrode arrays are used; the first consist of 256 individual electrodes of 5 microm in radius, 170 of which are electrochemically active in a cubic arrangement which are separated from their nearest neighbour by a distance of 100 microm. The second array compromises 2597 electrodes of 2.5 microm in radius and of which 1550 of which are electrochemically active in a hexagonal arrangement separated by the nearest neighbour by 55 microm. Well defined voltammetric waves are found with peak currents proportional to the concentration of hydroxide ions in the range 50 microM to 1 mM. Detection limits of 20 microM using the 170 ultra-microelectrode and 10 microM with the 1550 ultra-microelectrode array are shown to be possible but with a higher sensitivity of 4 mA M(-1) observed using the 1550 ultra-microelectrode array compared to 1.2 mA M(-1) with the 170 ultra-microelectrode array.     

Density estimation and adaptive control of markov processes: Average and discounted criteria

We consider a class of discrete-time Markov control processes with Borel state and action spaces, and ^d i.i.d. disturbances with unknown distribution . Under mild semi-continuity and compactness conditions, and assuming that is absolutely continuous with respect to Lebesgue measure, we establish the existence of adaptive control policies which are (1) optimal for the average-reward criterion, and (2) asymptotically optimal in the discounted case. Our results are obtained by taking advantage of some well-known facts in the theory of density estimation. This approach allows us to avoid restrictive conditions on the state space and/or on the system's transition law imposed in recent works, and on the other hand, it clearly shows the way to other applications of nonparametric (density) estimation to adaptive control.Research partially supported by The Third World Academy of Sciences under Research Grant No. MP 898-152.     

Preparation and electrochemical behavior of ordered rh adlayers on Pt(100) electrodes

Rhodium adlayers on Pt(100) substrates have been prepared by electrodeposition from dilute Rh(III) acidic solutions. The initially disordered layer is electrochemically annealed by applying a polarization program consisting of high-sweep-rate multicycle sequences between 0.05 and 0.78 V(RHE) in 0.1 M H(2)SO(4). In this way, a pseudomorphic Rh monolayer can be prepared on Pt(100) substrates. The degree of order of the electrochemically annealed layer has been evidenced not only through voltammetric experiments but also by means of scanning tunneling microscopy with atomic resolution for iodine-protected adlayers, which show a c(2 x 2) structure. The electrochemically induced ordering of the Rh adlayer appears to be a consequence of the repeated cycles of adsorption/desorption of H and, especially, oxygenated species. Voltammetry in sulfuric acid solutions permits examination of the energetics of H/anions and OH/O adsorption as a function of the Rh coverage. The first monolayer adsorbs both hydrogen and oxygenated species more strongly than the second one. This can be explained through an electronic effect caused by the underlying Pt(100) substrate.     

Defect concentration in Ti-substituted YIG from TG curves

The article discusses theory and experiment about the measurement of defect concentration in Ti-substituted yttrium iron garnet by means of thermogravimetry techniques. The two possible cases arising from oxygen interchange with atmosphere, oxygen vacancies and interstitial cations, may be analyzed quantitatively from the derived expressions. The possibility of another type of defects being present in the samples, not associated to oxygen evolution, is not excluded. Measurements were carried out in air and CO₂ for different Ti contents. The substitution tends to increase slightly the defect parameter θ in YIG, while a CO₂ reducing atmosphere is much more effective than the Ti substitution for increasing θ. Calculations for a given single-phased sample sintered in air show two possibilities: a deficit of one oxygen atom for every 2.5 unit cells of 96 atoms, or one excess cation for every 3.5 cells. Other samples show very similar results. The accuracy involved in the measurements is about 2–3%.     

Proton nuclear magnetic resonance characterisation of glycosaminoglycans using chemometric techniques

Various types of glycosaminoglycans (GAGs) including heparins, chondroitin sulfates, dermatan sulfate and hyaluronic acid were studied from their proton nuclear magnetic resonance (1H NMR) spectra using chemometric techniques. Despite the complexity of the 1H NMR signals, data analysis using principal component analysis enabled the different GAG classes to be distinguished and permitted their classification according to their chemical structure. The analysis of the composition of the major disaccharide unit and other relevant chemical structures in the heparin samples was performed using partial least squares regression.     

Losartan potassium,a non‐peptide agent for the treatment of arterial hypertension

In the title compound, potassium 2‐butyl‐4‐chloro‐1‐{[2′‐(5‐tetrazolido)­bi­phenyl‐4‐yl]­methyl}‐1H‐imidazol‐5‐yl­methanol, K⁺·C₂₂H₂₂ClN₆O^?, the imidazole and tetrazole rings are at angles of 85.0 (2) and 51.8 (1)°, respectively, to the phenyl rings to which they are attached, while the dihedral angle between the latter two rings is 46.7 (1)°. The coordination sphere of the metal cation consists of six tetrazoyl N atoms, the methanol O atom and the π cloud of one of the phenyl rings. These interactions determine the formation of columns of molecular anions that lie parallel to the b axis, while hydrogen bonding contributes to intercolumnar cohesion. Far from the centre of the columns, the hydro­carbon chain is immersed in a hydro­phobic environment.