Stereoregularity of poly(2-vinylpyridine) and poly-(4-vinylpyridine)

In order to determine the stereoregularity of poly(4-vinylpyridine), 4-vinylpyridine-β,β-d₂ was synthesized from 4-acetylpyridine. The ¹H-NMR spectra of the deuterated and nondeuterated polymers were measured and analyzed. From the ¹H-NMR spectra of poly(4-vinylpyridine-β,β-d₂), triad tacticity can be obtained, while the ¹H-NMR spectra of nondeuterated poly(4-vinylpyridine) give the fraction of isotactic triad. The ¹³C-NMR spectra of poly(4-vinylpyridine) were also observed, and the spectra of C₄ carbon of polymers were assigned by the pentad tacticities. The fraction of isotactic triad of poly(2-vinylpyridine) and poly(4-vinylpyridine) obtained under various polymerization conditions were determined. The radical polymerization and anionic polymerizations with phenylmagnesium bromide and n-butyllithium as catalysts of 4-vinylpyridine gave atactic polymers.     

Chronocoulometric determination of nitrate on silver electrode and sodium hydroxide electrolyte

An electrochemical system that consists of a silver electrode in 0.01 M sodium hydroxide electrolyte was investigated in an effort to develop a sensitive in situ analytical method for nitrate. Cyclic voltammetry demonstrated that the proposed system has a high normalized sensitivity (2.47 A s(1/2) V(-1/2) M(-1) cm(-2)), compared to more complex electroanalytical schemes. Double-potential-step chronocoulometry was used to maximize the signal-to-noise ratio (SNR), and minimize interference from dissolved oxygen in the electrolyte. The integration period for double-potential-step chronocoulometry was determined by optimizing the extended Cottrell equation. The integrated current is proportional to nitrate up to 10 mM and the average detection limit is approximately 1.7 microM. Dissolved oxygen does not degrade performance. To examine the potential interference of other ions when analyzing nitrate, we measured the electrode response to 1000 microM each of NO(2-), Cl(-), PO(4)(3-), SO(4)(2-), F(-), CO(3)(2-), BO(2-), K(+), Ca(2+), and Sr(2+) with and without 1000 microM nitrate. Interference is negligible for most of the ions when nitrate is absent (i.e. <1% of the response to equimolar nitrate). However, interference is substantial (>20% increase or decrease in the electrode response to nitrate) for PO(4)(3-), Ca(2+), and Sr(2+) when equimolar nitrate is present.     

Comparison of inorganic films and poly(4-vinylpyridine) coatings as electrode modifiers for flow-injection systems

A glassy-carbon electrode modified with a ruthenium-containing inorganic film was used for the flow-injection determination of As(III). The linear working range was 5-100 muM, and the detection limit was 300 pg. The response was reproducible for periods of several days. A glassy-carbon electrode modified by adsorption of a quaternized poly(4-vinylpyridine) film impregnated with hexachloroiridate(II and III) was used for the oxidation of nitrite. The calibration graphs were non-linear and varied from day to day, and the peak widths were broad. Nitrite determination at a platinum electrode modified by adsorption of iodine gave results analogous to the As(III) study; however, an overlayer of quaternized poly(4-vinylpyridine) decreased the sensitivity.     

Solvation effects and reactivity of free pyridine residues in macromolecules of poly-4-vinylpyridine derivatives

Water-soluble derivatives of poly-4-vinylpyridine (PVP) were prepared by means of quaternization of PVP with ethyl bromide, butyl bromide, hexyl-bromide and bromacetic acid. In addition, hydrolysis of p-nitrophenylacetate (NPA) and 3-nitro-4-acetoxybenzoic acid (NABA) catalyzed by these polymers and 4-ethylpyridine as analogue were studied spectrophotometrically both in water and alcohol-water mixtures at 25° and pH 7·8.The second-order rate of NPA hydrolysis (K_t) was independent of the length of alkyl groups and the salt concentration. While an increase of the degree of quaternization (β) for PVP derivatives resulted in a reduction of the value of K_i in water, in 37–50 vol per cent alcohol-water there was the opposite effect. At large values of β, K_i did not depend on the content of ethanol.The bell-shaped dependence of K_i on β was shown in the case of amionic NABA. The value of K_i proved to be very sensitive to both the nature of salt and the salt concentration in aqueous solution.The use of viscosity, turbidimetry and potentiometry for studying these polymers allowed the role of the quaternized pyridine residue to be elucidated. The peculiarities of the kinetic behaviour of the free basic group were explained by the hydrophilic nature of the quaternized PVP changing as the degree of quaternization is increased so causing an exclusion of the substrate molecules from the polyion.     

Comparison of properties of an oxime-bound partially quaternized poly-4-vinylpyridine and a monomer analogous oxime

Six copolymers of 4-vinylpyridine and 4-vinyl-N-phenacyloximepyridinium bromide (PPyOx-12, 23, 40, 50, 60 and 72 per cent) were prepared from poly-4-vinylpyridine partially quaternized by phenacylbromide (PPyKt) and hydroxylamine. The potentiometric data of the polymer acid groups (oxime, keto-enol tautomerism) in PPyOx and PPyKt were compared with those of 4-ethyl-N-phenacyloximepyridinium bromide (PyOx) and 4-ethyl-N-phenacylpyridinium bromide (PyKt) respectively. In addition, the second-order constants of the nucleophilic reaction for all the copolymers (PPyOx) and the analogue towards p-nitrophenylacetate (NPA) were determined at various pH values in water at 25°. In contrast to the known potentiometric behaviour of polyacrylic and polymethacrylic acids, the dissociation constants (pK_a) of acid groups in PPyKt and PPyOx were found to be independent of the degree of ionization. Moreover, the values of pK_a of these groups for all PPyKt and PPyOx samples were appreciably less than the pK_a of the corresponding analogues. As a consequence, the oxime of PPyOx-12 in the range of pH between 7·5 and 9·0 reacts about two order of magnitude faster than the monomer with the ester. These results indicate that, in contrast to the known correlation between reactivity (nucleophilicity) and basicity for monomer compounds, the activity of the polymer oxime does not diminish with a decrease of the basicity. On the contrary, the reactivity of polymer anion towards the ester is markedly higher than that of the monomer anion of PyOx. An explanation may be found in the effects of microenvironment on the property of the functional groups attached to the side pyridine residues of PVP.     

Sensitive voltammetric determination of paracetamol by poly (4-vinylpyridine)/multiwalled carbon nanotubes modified glassy carbon electrode

A novel glassy carbon electrode (GCE) modified with a composite film of poly (4-vinylpyridine) (P4VP) and multiwalled carbon nanotubes (P4VP/MWCNT GCE) was used for the voltammetric determination of paracetamol (PCT). This novel electrode displayed a combined effect of P4VP and MWCNT on the electro-oxidation of PCT in a solution of phosphate buffer at pH 7. Hence, conducting properties of P4VP along with the remarkable physical properties of MWCNTs might have combined effects in enhancing the kinetics of PCT oxidation. The P4VP/MWCNT GCE has also demonstrated excellent electrochemical activity toward PCT oxidation compared to that with bare GCE and MWCNT GCE. The anodic peak currents of PCT on the P4VP/MWCNT GCE were about 300 fold higher than that of the non-modified electrodes. By applying differential pulse voltammetry technique under optimized experimental conditions, a good linear ratio of oxidation peak currents and concentrations of PCT over the range of 0.02–450 μM with a limit of detection of 1.69 nM were achieved. This novel electrode was stable for more than 60 days and reproducible responses were obtained at 99% of the initial current of PCT without any influence of physiologically common interferences such as ascorbic acid and uric acid. The application of this electrode to determine PCT in tablets and urine samples was proposed.     

Preparation and properties of some water-soluble cobalt (III)–poly-4-vinylpyridine complexes

Water-soluble cobalt(III) chelates having a polymeric ligand such as cis-[Co(en)_2-PVPCl]Cl₂ and cis-[Co-(trien)PVPCl]Cl₂ (PVP = poly-4-vinylpyridine) were prepared by substitution reactions between cobalt(III) chelate and PVP in water–alcohol solution. PVP of different degrees of polymerization was used as the ligand in preparation of these complexes. The PVP complexes were identified and their properties ascertained by microanalysis and by a study of the infrared, ultraviolet, visible, and PMR spectra. Most of the characteristic properties of these complexes may be ascribed to the polymeric structure of the PVP ligand.     

Electroreduction of peroxodisulfate anion at platinum rotating disc electrode in the cyclic voltammetry mode

Electroreduction of peroxodisulfate anion at smooth polycrystalline and platinized (at different deposition potentials) platinum in perchloric acid and sulfuric acid solutions is studied by rotating disc electrode and cyclic voltammetry techniques. Dependences of the process rate on the electrode rotating velocity, the potential scan rate, the anodic limit of the scanning, the peroxodisulfate anion concentration in the solution and the platinizing conditions are found. The suggestion on the complications in the peroxodisulfate anion reduction caused by adsorbate formation is corroborated, at least, for certain potential region. The reaction structure sensitivity is evidenced, which makes it possible to use the reaction for characterization of the platinized Pt surface structure. The comparing of obtained results with literature data concerning smooth platinum and the single-crystal platinum basis faces allows concluding that the peroxodisulfate anion reduction maximal rate in sulfuric-acid solutions occurs at the potentials close to those observed for the (110) face. When the platinized Pt surface roughness factor exceeds ~30, the peroxodisulfate anion reduction reaction proceeds under the inner-diffusion limitation control. The platinized-Pt rotating disc electrode can serve as model tool in the studying of properties of disperse material microdeposits.     

Photogalvanic cells: Part VIII. The theory of the transparent rotating disc electrode

The theory of the transparent rotating disc electrode is first extended to cover the case where two photogenerated species react with second-order kinetics and where one of the species is electroactive and the other is not. Secondly the theory is developed to describe the case where the solution is bleached close to the electrode. Thirdly theory is presented for experiments where the light source is modulated and the phase shift and amplitude of the resulting modulated photocurrent are measured.     

A rotating disc electrode study of oxygen reduction at platinised nickel and cobalt coatings

Platinized nickel and cobalt coatings, Pt(Ni) and Pt(Co), have been prepared on glassy carbon, GC, rotating disc electrode substrates by a two-step room temperature procedure that involved the electrodeposition of nickel and cobalt layers and their spontaneous partial replacement by platinum (“transmetalation”) when immersed into a chloroplatinic acid solution. By tuning the quantity of initially deposited nickel and cobalt, Pt(Ni) and Pt(Co) bimetallic coatings having a 26% atom Ni and 30% atom Co composition have been prepared. For both materials typical Pt surface electrochemistry was recorded during fast voltammetry in deaerated acid, pointing to the existence of a continuous Pt skin over a Pt–Ni and Pt–Co core. Oxygen reduction at the Pt(Ni)/GC and Pt(Co)/GC electrodes was studied by means of steady-state voltammetry at a rotating disc electrode and the construction of Tafel plots from corresponding voltammetric data. It was found that, when the initial potential of the voltammetric sweep allowed the formation of a complete Pt oxide monolayer, then oxygen reduction was hindered for low overpotentials at Pt(Ni) and Pt(Co), compared to pure bulk Pt. On the other hand, when the initial potential was less positive (thus leading to the formation of a fraction of surface oxide monolayer) the presence of Ni and Co enhanced the kinetics of oxygen reduction. The former behaviour is attributed to a decrease in oxide reduction ability of Pt in the presence of Ni and Co, while the latter to an increase in dissociative oxygen chemisorption due to Ni and Co.     

The effect of electrocatalytic nanoparticle injection on the electrochemical response at a rotating disc electrode

A new approach to study electrocatalytic oxidation of glucose is proposed. As opposed to numerous studies on electrodes modified with gold nanoparticles this reaction was studied in their suspension of gold nanoparticles under hydrodynamic conditions on a noncatalytic glassy carbon rotating disc electrode. It has been shown that addition of nanogram amount of positively charged Au nanoparticles results in a clear current response, whereas no clear response is seen for negatively charged ones. This effect results from the electrocatalytic oxidation of glucose on Au nanoparticles mainly adsorbed on glassy carbon electrode. The role of electrode preparation method on reproducibility of the results is emphasized.     

A study of the cathodic reduction of ozone at the rotating disc electrode in acid electrolyte

The cathodic reduction of ozone according to O₃+2H⁺+2e^?→H₂O+O₂ was studied at a rotating disc electrode consisting of bright platinum in 1 M HClO₄ at 20°C. The current-potential curves obtained potentiostatically are determined by slow charge transfer in the range of low polarization (Tafel region) and diffusion as rate controlling step at high overvoltage. From the exchange current densities (i₀), obtained by extrapolation of Tafel lines to the reversible potentials, the heterogeneous rate constant k was evaluated giving an average value of 2.5×10^?7 cm s^?1. Likewise the diffusion coefficient (D_O3=1.7×10^?5 cm² s^?1) was calculated from the limiting currents measured at various partial pressures of ozone and different values of rotation rate. An equation for the total current-polarization curve was developed. The computed curves are in good agreement with the data which were obtained experimentally.     

Theoretical study of ion desorption from poly-(methyl methacrylate) and poly-(isopropenyl acetate) thin films through core excitation

Site-specific chemical reactions following core excitation of poly-(methyl methacrylate) (PMMA) and poly-(isopropenyl acetate) (PiPAc) thin films were investigated. New x-ray absorption spectra of PMMA and PiPAc at the C and O K edges and theoretical spectra within the framework of density functional theory using model molecules were reported, and some new peak assignments were proposed for these spectra. Core-hole excited state molecular dynamics simulations were performed to discuss dissociation dynamics for the target systems, and some specific reaction mechanisms were discussed and explained theoretically; for example, the amount of CH3 ion fragments for PMMA was enhanced at the C and O K edges through the existence of the repulsive sigma*(O-CH3) excited state.     

Radiation-grafting of 4-vinylpyridine and N-isopropylacrylamide onto polypropylene to give novel pH and thermo-sensitive films

Here 4-vinylpyridine (4VP) was grafted onto polypropylene films (PP) by mutual irradiation method to give PP-g-4VP; N-isopropylacrylamide (NIPAAm) was then grafted onto the PP-g-4VP films to give (PP-g-4VP)-g-NIPAAm by pre-irradiation method, using a ⁶⁰Co γ-source. The dependence of grafting percentage on radiation dose, temperature, reaction time, and monomer concentration was studied. (PP-g-4VP)-g-NIPAAm films were characterized by infrared spectroscopy (FTIR-ATR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The critical pH point and lower critical solution temperature (LCST) were determined by swelling and water contact angle measurements. The LCST also was determined by DSC. The binary graft copolymer films are shown to be thermo-pH sensitive.     

Modeling of mass and charge transfer in an inverted rotating disk electrode (IRDE) reactor

In this work, the validation of a newly constructed inverted rotating disk electrode (IRDE) reactor is reported. Compared to the rotating disk electrode (RDE) reactor, the working electrode is changed in position from the top to the bottom of the electrochemical cell. The IRDE reactor is designed to facilitate the actual study of gas evolution reactions. It is studied whether the first-order analytical expression for the velocity field in an RDE reactor is also acceptable for an IRDE configuration. To that purpose, the kinetic parameters of the well-known ferri/ferro cyanide redox system are determined in both configurations and compared. This is done qualitatively by comparing the polarization curves obtained in the inverted and the conventional RDE configuration. Additionally, a statistically founded fitting algorithm is used to quantitatively determine the model parameters of the oxidation and reduction reaction. Not only the diffusion coefficients of Fe²⁺ and Fe³⁺ are calculated, but also the rate constants (k_ox and k_red) and the transfer coefficients (α_ox and α_red) are quantified and compared together with their respective standard deviation. It is found that the parameters of mass and charge transfer in both configurations agree well. So it is concluded that the same analytical equations of mass and charge transfer can be used in both the RDE and the IRDE reactor.     

Selective and sensitive detection of nitrite based on NO sensing on a polymer-coated rotating disc electrode

A new effective way of nitrite detection in complex samples is presented. It is based on chemical conversion of nitrite to nitric monoxide (NO) in acidic aqueous solution containing hexacyanoferrate(II) as a reductor. NO is then detected on a poly-eugenol coated platinum electrode. When the electrode is rotating and the reduction medium is continuously purged with nitrogen, the addition of a nitrite-containing sample produces narrowed current spikes. The peak current is proportional to nitrite content in the sample over the range of 5.0–100 μM and detection limit is 0.6 μM. The method is simple and highly reproducible. Relative standard deviation of 10 repetitions is less than 4%. Practical utility of the proposed approach is demonstrated by nitrite determination in human saliva.     

Electrochemical determination of NADH using a glassy carbon electrode modified with Fe3O4 nanoparticles and poly-2,6-pyridinedicarboxylic acid, and its application to the determination of antioxidant capacity

We have prepared a glassy carbon electrode modified with poly-2,6-pyridinedicarboxylic acid and with magnetic Fe₃O₄ nanoparticles. This modification enhances the effective surface area and the electrocatalytic oxidation of nicotinamide adenine dinucleotide (NADH) in addition to providing positively charged groups for electrostatic assembly of the phosphate group of NADH. The modified electrode responds linearly to NADH in the range from 5?×?10^?8 to 2.5?×?10^?5?M and gives a lower detection limit of 1?×?10^?8?M. It displays satisfactory selectivity and reproducibility. The sensor was applied to rapid screening of plant extracts for their antioxidant properties.

Electroreduction of bromate anion in acidic solutions at the inactive rotating disc electrode under steady-state conditions: Numerical modeling of the process with bromate anions being in excess compared to protons

The process of electroreduction of bromate anion BrO₃ in acidic solutions at catalytically inactive electrodes when bromate anions are in excess compared to protons has been studied by numerical integration of transport equations. Under these conditions, the maximum possible current is limited by the limiting diffusion current of protons (rather than bromate), since both ions are consumed in the comproportionation reaction. It has been demonstrated that the curve of maximum current versus diffusion layer thickness has an anomalous segment where the current increases with an increase of the latter.