Strong surface adsorption of aqueous sodium nitrite as an ion pair

We describe the first detailed experimental characterization of surface adsorption of an aqueous ion pair and quantify the unusual surface behavior of sodium nitrite, a ubiquitous component of natural waters. The onset of unusually strong adsorption at concentrations as low as ca. 0.1 M resembles the controversial ‘Jones–Ray Effect,’ wherein 13 salts exhibited surface tension minima in the millimolar region before resuming the normal linear increase with bulk concentrations. Given the compelling analogy recently found between adsorption of ions to the air–water interface, and the Hofmeister Effects of biochemistry, these results have important implications for the mechanism underlying these phenomena.     

The hydrolysis of phenylhydroxamic acid in concentrated aqueous sodium hydroxide

Phenylhydroxamic acid undergoes hydrolysis in concentrated NaOH via a complex mechanism: a maximum is observed in the plot of K_o versus [NaOH) which is attributed to the existence of one or more pre-equilibria influencing the rate-limiting step.     

Dynamic control of gold nanoparticle morphology in a microchannel flow reactor by glucose reduction in aqueous sodium hydroxide solution

Continuous flow synthesis of gold nanoparticles was demonstrated using a microchannel reactor with glucose reduction in aqueous alkaline medium. Particle size, morphology, and visual/optical properties of the dispersion liquid were controlled dynamically by tuning of the rate of NaOH addition. Characteristic star-like nanoparticles formed spontaneously as a quasi-stable state, but they changed the morphology to round shape and showed spectral change over time.     

Electrical conductivity of aqueous sodium hydroxide solutions at high temperatures

Electrical conductivities of dilute sodium hydroxide aqueous solutions have been determined at 75, 100 and 150°C at 1.6 MPa using a recently developed DC-measuring technique especially suited for the study of aqueous solutions above room temperature. The data were analyzed with modern theories to obtain the infinite dilution conductivity and the association constant at the three temperatures.     

Synthesis of palladium nanoparticles by sonochemical reduction of palladium(II) nitrate in aqueous solution

The sonochemical synthesis of stable palladium nanoparticles has been achieved by ultrasonic irradiation of palladium(II) nitrate solution. The starting solutions were prepared by the addition of different concentrations of palladium(II) nitrate in ethylene glycol and poly(vinylpyrrolidone) (PVP). The resulting mixtures were irradiated with ultrasonic 50 kHz waves in a glass vessel for 180 min. The UV-visible absorption spectroscopy and pH measurements revealed that the reduction of Pd(II) to metallic Pd has been successfully achieved and that the obtained suspensions have a long shelf life. The protective effect of PVP was studied using Fourier transform infrared (FT-IR) spectroscopy. It has been found that, in the presence of ethylene glycol, the stabilization of the nanoparticles results from the adsorption of the PVP chain on the palladium particle surface via the coordination of the PVP carbonyl group to the palladium atoms. The effect of the initial Pd(II) concentration on the Pd nanoparticle morphology has been investigated by transmission electron microscopy. It has been shown that the increase of the Pd(II)/PVP molar ratio from 0.13 x 10(-3) to 0.53 x 10(-3) decreases the number of palladium nanoparticles with a slight increase in particle size. For the highest Pd(II)/PVP value, 0.53 x 10(-3), the reduction reaction leads to the unexpected smallest nanoparticles in the form of aggregates.     

Electrocatalytic reduction of nitrite on the surface of a glassy carbon electrode modified by palladium(II)-substituted Dawson type heptadecatungstodiphosphate

A palladium(II)-substituted Dawson type heptadecatungstodiphosphate, K₈[P₂W₁₇O₆₁Pd(H₂O)](abbreviated as P₂W₁₇Pd in the following), was used to construct a new kind of chemically modified electrode (CME) by electrodeposition on the surface of a glassy carbon electrode for the first time. This modified electrode was characterized by Fourier transform infrared reflection absorption spectroscopy (FTIRRAS), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). FTIRRAS and XPS suggested that the adsorbed species on the electrode surface has the same Dawson type structure as the heteropolyanion in buffer solution. A couple of waves was observed on the P₂W₁₇Pd CME, which was ascribed to the redox process of the palladium center in the heteropolytungstate. After continuous potential scanning for 30 min in a pH 4.0 buffer, 94% of the original coverage remains for the CME. The P₂W₁₇Pd CME had high electrocatalytic activity for nitrite reduction and exhibited good reproducibility and stability. The catalytic peak current was found to be linear with the nitrite concentration in the range of 1 ∼ 50 mmol/L, with a correlation coefficient of 0.994. The effect of solution pH on the catalytic activity of the CME for nitrite reduction was also investigated. Received: 20 May 1998 / Revised: 7 September 1998 / Accepted: 15 September 1998     

Electrocatalytic reduction of nitrite on the surface of a glassy carbon electrode modified by palladium(II)-substituted Dawson type heptadecatungstodiphosphate

A palladium(II)-substituted Dawson type heptadecatungstodiphosphate, K₈[P₂W₁₇O₆₁Pd(H₂O)](abbreviated as P₂W₁₇Pd in the following), was used to construct a new kind of chemically modified electrode (CME) by electrodeposition on the surface of a glassy carbon electrode for the first time. This modified electrode was characterized by Fourier transform infrared reflection absorption spectroscopy (FTIRRAS), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). FTIRRAS and XPS suggested that the adsorbed species on the electrode surface has the same Dawson type structure as the heteropolyanion in buffer solution. A couple of waves was observed on the P₂W₁₇Pd CME, which was ascribed to the redox process of the palladium center in the heteropolytungstate. After continuous potential scanning for 30 min in a pH 4.0 buffer, 94% of the original coverage remains for the CME. The P₂W₁₇Pd CME had high electrocatalytic activity for nitrite reduction and exhibited good reproducibility and stability. The catalytic peak current was found to be linear with the nitrite concentration in the range of 1 ∼ 50 mmol/L, with a correlation coefficient of 0.994. The effect of solution pH on the catalytic activity of the CME for nitrite reduction was also investigated. Received: 20 May 1998 / Revised: 7 September 1998 / Accepted: 15 September 1998     

Kinetics of hydrogen absorption by porous nickel in an aqueous sodium hydroxide solution

Kinetic studies of hydrogen absorption by porous nickel catalyst establish that there are three regions on kinetic curves of the process differing in their regularities of adsorption. The dependence of the observed rate of hydrogen absorption on the adsorbate amount is formally described by a first order kinetic equation, and the absorption rate constants corresponding to distinct parts of the kinetic curve differ by 1–2 orders of magnitude. The obtained data are explained using the concept that hydrogen adsorption on nickel active centers proceeds with three forms of adsorption.     

Tailoring the electrode surface structure by cathodic corrosion in alkali metal hydroxide solution: Nanostructuring and faceting of Au

Nanostructured metals are vital materials in several (electro)chemical applications. Despite the substantial progress in this field, still many limitations are associated with traditional synthetic procedures, including the availability of stable nanoparticles on appropriate supports by avoiding migration and aggregation. On that front, cathodic corrosion has emerged as a powerful technique to tailor the surface structure of metal surfaces on the nanoscale. Cathodic corrosion crucially depends on the electrode potential, the nature and the concentration of cations, as well as the electrode material. Controlling these parameters is essential for applying cathodic corrosion in materials design. In this short review, we discuss the most critical parameters controlling cathodic corrosion and highlight the importance of the nature and the concentration of alkali metal hydroxides in aqueous solution.     

Selective reduction of nitro-heterocycles with sodium sulfide in aqueous p-dioxane

Nitro-heterocycles, particularly those containing halogen, were selectively reduced with sodium sulfide in aqueous p-dioxane to give the corresponding amino-heterocycles in 70-89% yields.     

Physisorption of hydroxide ions from aqueous solution to a hydrophobic surface

We present results from detailed molecular dynamics simulations revealing a counterintuitive spontaneous physical adsorption of hydroxide ions at a water/hydrophobic interface. The driving force for the migration of the hydroxide ions from the aqueous phase is the preferential orientation of the water molecules in the first two water layers away from the hydrophobic surface. This ordering of the water molecules generates an electrical potential gradient that strongly and favorably interacts with the dipole moment of the hydroxide ion. These findings offer a physical mechanism that explains intriguing experimental reports indicating that the interface between water and a nonionic surface is negatively charged.     

Kinetics and mechanism of the cobalt phthalocyanine catalyzed reduction of nitrite and nitrate by dithionite in aqueous solution

The reaction of sodium nitrite with sodium dithionite was studied in the presence of cobalt(II) tetrasulfophthalocyanine, COII(TSPc)4-, in aqueous alkaline solution. The overall mechanism comprises the reduction of CoII(TSPc)4- by dithionite, followed by the formation of an intermediate complex between COI(TSPc)5- and nitrite, which undergoes two parallel subsequent reactions with and without nitrite as a reagent. Kinetic parameters for the different reaction steps of the catalytic process were determined. The final product of the reaction was found to be ammonia. Contrary to those found for the catalytic reduction of nitrite, the products of the catalytic reduction of nitrate were found to be dinitrogen and nitrous oxide. The possible catalytic reduction of nitrous oxide was confirmed by independent experiments. The striking differences in the reduction products of nitrite and nitrate are explained in terms of different structures of the intermediate complex between CoI(TSPc)5- and substrate, in which nitrite and nitrate are suggested to coordinate via nitrogen and oxygen, respectively.     

Kinetics and mechanism of the iron phthalocyanine catalyzed reduction of nitrite by dithionite and sulfoxylate in aqueous solution

The reactions of sodium nitrite with sodium dithionite and sulfoxylate ion were studied in the presence of iron(III) tetrasulfophthalocyanine, Fe(III)(TSPc)3-, in aqueous alkaline solution. Kinetic parameters for the different reaction steps in the catalytic reduction by dithionite were determined. The final product of the reaction was found to be nitrous oxide. Contrary to this, the product of the catalytic reduction of nitrite by sulfoxylate was found to be ammonia. The striking difference in the reaction products is accounted for in terms of different structures of the intermediate complexes formed during the reduction by dithionite and sulfoxylate, in which nitrite is suggested to coordinate to the iron complex via nitrogen and oxygen, respectively. Sulfoxylate is shown to be a convenient reductant for the synthesis of the highly reduced iron phthalocyanine species Fe(I)(TSPc*)6- in aqueous solution. The kinetics of the reduction of Fe(I)(TSPc)5- to Fe(I)(TSPc*)6-, as well as the oxidation of the latter species by nitrite, was studied in detail.     

Concentration dependence of thermoelectric power of aqueous sodium hydroxide solutions using hydrogen electrodes

Using a hydrogen-electrode thermocell with a temperature difference of 8 K, the initial thermoelectric power ⁱⁿ has been determined for aqueous NaOH solutions from 0.003 to 0.2 molal at mean temperatures of 25 and 15°C. Graphs of a function of ⁱⁿ vs. a function of m^1/2 yield infinitedilution limiting slopes as m^1/2 0 which are shown to be in agreement with the five-term theory of this laboratory. Extrapolation of these curves to infinite dilution yields the value for the standard transported entropy of the hydroxide ion .     

Determination of lead in paints by electro-oxidation in cationic surfactant—aqueous sodium hydroxide suspensions

It was shown that anodic voltammetry can be used to determine lead in paint samples by solubilizing the sample in an aqueous solution using a cationic surfactant (Hyamine 2389, prdominantly methyldodecylbenzyltrimethylammonium chloride). In addition to solubilizing the solid paint particles, the cationic surfactant increased the oxidation potential of water to the point where one could observe, in concentrated sodium hydroxide solutions, an anodic peak for the oxidation of lead monoxide to lead dioxide. If the surfactant was below the critical micelle concentration, the height of the peak was linearly proportional to the concentration of lead oxide in the paint. The technique was experimentally verified by comparing the concentration of lead in a commercial paint sample, as determined by x-ray fluorescence, with the concentration determined voltammetrically.     

Direct electrochemistry and electrocatalysis of nitrite based on nano-alumina-modified electrode

Simple and sensitive electrochemical method for the determination of nitrite, based on a nano-alumina-modified glassy carbon electrode (GCE), is described. Nitrite yields a well-defined oxidation peak whose potential is 0.74 V at the nano-alumina-coated GCE in 0.1 mol L⁻¹ phosphate buffer (pH 5.0). Compared with bare GCE, the nano-alumina-modified GCE has evident catalytic effect towards the oxidation of nitrite, and its peak current can be significantly enhanced. Some of the experimental parameters were optimized for the determination of nitrite. The oxidation peak current was proportional to nitrite concentration in the range of 5.0 × 10⁻⁸–1.1 × 10⁻³ mol L⁻¹, and a detection limit of 1.0 × 10⁻⁸ mol L⁻¹ was obtained. This method has been successfully used to the determination of nitrite in sausage sample. Furthermore, results obtained by the method have been compared with spectrophotometric method.     

Indirect complexometric determination of palladium(II) using sodium nitrite as a selective masking reagent

A selective complexometric method is described for the determination of palladium, sodium nitrite being used as masking reagent. Palladium(II) in a given sample solution is initially cornplexed with an excess of EDTA and the surplus EDTA is titrated with zinc sulfate solution at pH 4.5–5.5 (acetic acid-sodium acetate buffer), using xylenol orange as indicator. An excess of sodium nitrite is then added, the mixture is shaken well and the EDTA released from the Pd-EDTA complex is titrated with a standard zinc sulfate solution. Results are obtained for 2.5–27.5 mg of Pd with relative errors 0.5% and standard deviations 0.05 mg. The interferences of various ions are studied. The method is applied for the determination of palladium(II) in alloys and complexes.