Host-guest-like thiamine-anion complexation in thiaminium bis(tetrafluoroborate)

In the title compound, 3-[(4-amino-2-methyl-5-pyrimidin-1-io)methyl]-5-(2-hydroxyethyl)-4-methylthiazolium(2+) bis(tetrafluoroborate), C12H18N4OS2+. 2BF(4)(-), the divalent thiamine cation (in the F conformation) is associated with BF4- anions via two characteristic bridging interactions between the thiazolium and pyrimidinium rings, i.e. C-H . . . BF4- . . . pyrimidinium and N-H . . . BF4- . . . thiazolium interactions. Thiamine molecules are linked by N-H . . .O hydrogen bonds to form a helical chain structure.

Liquid chromatography amperometric detection of nitrite using a polypyrrole modified glassy carbon electrode doped with tungstodiphosphate anion

Chromatography-amperometric detection of nitrite with a polypyrrole modified glassy carbon electrode doped with tungstodiphosphate anion (Dawson-type P2W18O626-/PPy/GC electrode) based on its electrocatalytic reduction of nitrite is described. The cyclic and hydrodynamic voltammetry of nitrite at the P2W18O626-/PPy/GC electrode was studied. The factors affecting the detection of nitrite and the analytical performance of the modified electrode in flowing stream were investigated. The results show that the modified electrode has a good sensitivity (the limit of detection is 1 mu mol dm(-3)) and a satisfactory reproducibility (RSD = 3.78%, N = 21). The modified electrode was used in the chromatographic detection of nitrite spiked in the liquid from a tin of mushrooms and the mineralized spring water. It was found that the modified electrode exhibited good selectivity for nitrite.

The transfer of chloride ion across an anion exchange membrane

The transfer of chloride ions into a low resistance anion exchange membrane (AEM) was investigated by cyclic voltammetry (CV) and electrochemical impedance spectra. In all cases, concentration polarization of Cl- ions is exterior to the membrane. It controls the flux and produces the limiting currents: either steady state or transient (peak type) current. In CV experiments, when the size of the holes in the membrane was much smaller than the distance between membrane holes, the Cl- anion transfer showed steady state voltammetric behavior. Each hole in the membrane can be regarded as a microelectrode and the membrane was equivalent to a microelectrode array in this condition. When the hole in the membrane was large or the distance between membrane holes was small, the CV curve of the Cl- anion transfer across the membrane showed a peak shape, which was attributed to linear diffusion. In AC impedance measurement, the impedance spectrum of the membrane system was composed of two semicircles at low DC bias, corresponding to the bulk characteristics of the membrane and the kinetic process of ion transfer, respectively. The bulk membrane resistance increases with increasing DC bias and only one semicircle was observed at higher DC bias. The parameters related to kinetic and membrane properties were discussed.

PREPARATION OF MODIFIED ELECTRODE WITH MOLYBDOPHOSPHATE ANION AND ITS ELECTROCATALYSIS FOR BROMATE REDUCTION

A molybdophosphate anion modified electrode has been prepared in 2 M sulfuric acid solution containing PMo12O403- by electrochemical cycling scan or simple adsorption on a glassy carbon electrode anodized before modification. The film electrode obtained is very stable upon potential cycling in acid solution. The catalytic effect of the film for reduction of bromate was investigated in detail.

ELECTROCHEMICAL STUDY OF ISOPOLY AND HETEROPOLY ANION TRANSFER ACROSS THE WATER NITROBENZENE INTERFACE .2. VANADIUM-CONTAINING HETEROPOLYTUNGSTATE ANIONS

The electrochemical transfer behaviour of vanadium-containing heteropolytungstate anions [PW12-xVxO40]((3+r)-) (x = 1-4) across the water \nitrobenzene interface has been investigated by cyclic voltammetry and chronopotentiometry with cyclic linear current scanning. The transfer of PW11V1O404-, HPW10V2O404-, H2PW10V2O403-, H3PW9V3O403- and H4PW8V4O(40)(3-) across the water \nitrobenzene interface can be observed within the potential window. The effects were observed of pH in the water phase on the transfer behaviour and the formation of vanadium-containing heteropolytungstate anions in solution. Heteropolytungstate anions become more stable due to their involving the vanadium atom. The degree of protonation and the dissociation constant of the trivalent vanadium-containing heteropolytungstate anion of protonation increase with increasing vanadium content. The transfer processes are diffusion-controlled The standard transfer potential, the standard Gibbs energy and the dissociation constant for vanadium-containing heteropolytungstate anions have been obtained and the transfer mechanisms are discussed.

AMPEROMETRIC DETECTION OF CATECHOLAMINES WITH LIQUID-CHROMATOGRAPHY AT A POLYPYRROLE-PHOSPHOMOLYBDIC ANION-MODIFIED ELECTRODE

A conducting polypyrrole film immobilized with PMo12O403- anion on a glassy carbon electrode was prepared by an electrochemical method. This kind of chemically modified electrode (CME) was prepared successfully by doping the polypyrrole film electrode wit

ELECTROCHEMICAL STUDY OF ISOPOLYMOLYBDATE(VI) ANION TRANSFER ACROSS THE WATER/NITROBENZENE INTERFACE

Isopolymolybdate (VI) anion transfer across the water/nitrobenzene (w/n) interface was studied by cyclic voltammetry. The effect of pH and responsed kinetics of isopolymolybdate anion's formation in the water phase on the transfer behavior have been studi

ELECTROCHEMICAL AND ELECTROCATALYTIC PROPERTIES OF IRON(III)-SUBSTITUTED DAWSON-TYPE TUNGSTOPHOSPHATE ANION

The electrochemical and electrocatalytic properties of iron(III)-substituted Dawson-type tungstophosphate anion are described. The anion exhibits a one-electron couple associated with the Fe(III) center and two two-electron waves attributed to redox proce

STUDY OF A 1/12 PHOSPHOMOLYBDIC ANION DOPED POLYPYRROLE FILM ELECTRODE AND ITS CATALYSIS

A phosphomolybdic anion doped polypyrrole (PMo12O403- + PPy) film electrode has been prepared by electrochemical polymerization of pyrrole in an aqueous solution of 0.5 mol l-1 H2SO4 or 0.5 mol l-1 KNO3 containing PMo12O403- anions, and characterized by scanning electron microscopy and in-situ UV-visible spectroelectrochemical methods. The film electrode obtained is very stable upon potential cycling in acidic solution, but not in neutral solution. The catalytic effect of the film electrode on the reduction of ClO3- and BrO3- was studied.

ELECTROCHEMICAL STUDY OF ISOPOLY-OXOMETALLATES AND HETEROPOLY-OXOMETALLATES FILM MODIFIED MICROELECTRODES .3. FURTHER STUDY OF ELECTRODE PROCESS OF ISOPOLYMOLYBDIC ANION DOPED IN POLYANILINE FILM

The isopolymolybdic anion-polyaniline film modified carbon fiber (CF) microelectrode with high stability and electroactivity in aqueous acid solution has been successfully prepared by cycling the potential between -0.15 V and +0.85 V vs. sce at 100 mV s-1 or applying constant potential (+0.85 V) for electropolymerization in a 0.5 M H2SO4 solution containing 5.0 x 10(-2) M aniline and 5.0 x 10(-3) M H4Mo8O26. The electrochemical behaviour of the isopolymolybdic anion entrapped in the polyaniline film is strongly influenced by the sweep-potential range besides the acidity of electrolyte solution. In some acidic electrolyte solution (eg 0.5 M H2SO4), the change of the sweep-potential range causes the structure alternation of the isopolymolybdic anion and resulting in a new electrode process. The cyclic voltammogram of Mo8O264- in 0.5 M H2SO4 solution exhibits three two-electron reversible waves between +0.70 and -0.20 V. However, when the potential sweeps to the lower-limit of -0.3 V, where the fourth four-electron cathodic wave appears, the redoxidation process of the reduction product of Mo8O264- becomes relatively complicated. The 10-electron reduction product seems to change into other isopolyanion (this unknown structure isopolyanions are simply called [Mo-O]), which can be reoxidized to Mo8O264- by five successive two-electron oxidation steps from -0.30 to +0.70 V. However, when the lower-limit of the cycling potential is maintained at -0.30 V and the upper-limit reduces to +0.40 V from +0.70 V, the [Mo-O] in the film exhibits four two-electron reversible waves. We have presented a novel explanation about its electrode reaction mechanism on the basis of our experimental results.

ELECTROCHEMICAL STUDY OF ISOPOLY AND HETEROPOLY OXOMETALLATE FILM MODIFIED MICROELECTRODES .5. PREPARATION AND ELECTROCHEMICAL-BEHAVIOR OF A 2/18-MOLYBDODIPHOSPHATE ANION MONOLAYER MODIFIED ELECTRODE

In this paper a carbon fibre (CF) microelectrode modified with the 2:18-molybdodiphosphate anion by simple adsorption is described and its electrochemical behaviour is reported. The 2:18-molybdodiphosphate anion (alpha-P2Mo18O626-), which is a Dawson structure, undergoes five successive multielectron reductions in acidic solution. The first three redox waves correspond to the two-electron process, and the last two waves are four-electron and six-electron processes respectively. On the basis of the experimental results it is shown that the electrode process of alpha-P2Mo18O626- on the CF electrode in acidic solution is simultaneously controlled by the diffusion and adsorption of alpha-P2Mo18O626- anions. When the concentration of the alpha-P2Mo18O626- in the solution is reduced, the electrode process mainly exhibits non-diffusion-controlled behaviour, and the diffusion-limited process takes over as the concentration of alpha-P2Mo18O626- becomes higher. The CF electrode modified with a thin film of alpha-P2Mo18O626- exhibits very good stability and redox behaviour in aqueous acidic solution. The alpha-P2Mo18O626- is reduced to heteropoly blue, with an accompanying protonation process. The addition of more than six electrons to the alpha-P2Mo18O626- anion in an aqueous solution does not result in its decomposition. The result obtained is not the same as that reported previously.

RESEARCHES ON CHEMICALLY MODIFIED ELECTRODES - ELECTROCHEMISTRY OF 1-12 PHOSPHOMOLYBDIC HETEROPOLY ANION FILM MODIFIED ELECTRODE

A monolayer of 1:12 phosphomolybdic anion (PMo_(12)) was modified electrochemically on a glassy carbon electrode and its electrochemical behavior was studied with cyclic voltammetry (CV). It is shown that PMo_(12) film is adsoibed strongly on the surface of glassy carbon electrodes, H~+ ions in the solution plays an important role in the electrochemical processes of PMo_(12) film modified electrodes, whereas other anions, such as Cl~-, NO_3~-, SO_4~(2-), ClO_4~- and PO_4~(3-) etc., do not take par...

ELECTROCHEMICAL TRANSFER OF 12-SILICOTUNGSTATE ANION ACROSS THE WATER NITROBENZENE INTERFACE

It has been firstly found by means of cyclic voltammetry (CV) and chronopotentiometry with linear current-scanning (CLC)that 12-silicotungstate anion (SiW_(12)O_(40)~(4-)) with high charge numbers, large molecular volume and symmetric structure can cross

Separation of acidic compounds by strong anion-exchange capillary electrochromatography

Separation of the acidic compounds in the ion-exchange capillary electrochromatograph (IE-CEC) with strong anion-exchange packing as the stationary phase was studied. It was observed that the electroosmotic flow (EOF) in strong anion-exchange CEC moderately changed with increase of the eluent ionic strength and decrease of the eluent pH, but the acetonitrile concentration in the eluent had almost no effect on the EOF. The EOF in Strong anion-exchange CEC with eluent of low pH value was much larger than that in RP-CEC with Spherisorb-ODS as the stationary phase. The retention of acidic compounds on the strong anion-exchange packing was relatively weak due to only partial ionization of them, and both chromatographic and electrophoretic processes contributed to separation. It was observed that the retention values of acidic compounds decreased with the increase of phosphate buffer and acetonitrile concentration in the eluent as well as the decrease of the applied voltage, and even the acidic compounds could elute before the void time. These factors also made an important contribution to the separation selectivity for tested acidic compounds, which could be separated rapidly with high column efficiency of more than 220 000 plates/m under the optimized separation conditions. (C) 2000 Elsevier Science BN. All rights reserved.