Influence of Two-step Mechanism of Dissociation of H2SO4 on the Limiting Cathodic Current of Copper Deposition from Sulfate Solutions in Conditions of Natural Convection

The limiting cathodic current of copper deposition at a flat vertical electrode from solutions containing a mixture of copper sulfate and sulfuric acid (in excess) in conditions of natural convection is calculated. The possibility of the two-step dissociation of sulfuric acid is accounted for. The obtained formulas yield the limiting cathodic current at any ratio of bisulfate and sulfate anions in solution.     

Surface Layers on Chromium in Sulfuric Acid Solutions of Organic Substances: An X-ray Photoelectron Spectroscopy Study

Surface layers formed at a contact of chromium with sulfuric acid solutions containing some organic substances (sodium oxalate, dimethylformamide, methanol) are analyzed by the X-ray photoelectron spectroscopy method. The layers are shown to contain products of deep destruction of organic molecules, i.e. carbon species and those containing functional groups. The data obtained point to the electrocatalytic activity of chromium in cathodic processes. The results of this study may provide a deeper insight into the mechanism of carbon incorporation (in the form of carbide compounds as well) into amorphous chromium deposits formed in sulfuric acid baths containing some organic substances.     

Corrosion–Electrochemical Behavior of Chromium Deposits Obtained from Sulfuric Acid Solutions Containing Oxalates

The corrosion–electrochemical behavior of chromium electrodeposits, which are formed in sulfuric acid solutions of Cr(III) containing oxalates, is studied by taking steady-state polarization measurements in a 0.5 M H₂SO₄ solution. No region of the metal's active dissolution is observed for such coatings, and the open-circuit potential is localized in the passivity region, i.e. it is substantially displaced in the positive direction as compared with that for metallurgic chromium or the coatings deposited from standard chromium-plating electrolytes. According to XPS data, the surface layer of the passive metal a few nanometers thick includes oxide compounds of chromium and also carbides formed during the coating electrodeposition. Specific features of the corrosion–electrochemical behavior of the deposits are attributed to the presence of carbide compounds of chromium in them, with the compounds playing the role of a cathodic alloying agent.     

Tribological behaviors of DLC films in sulfuric acid and sodium hydroxide solutions

Tribological behaviors of three typical kinds of diamond-like carbon (DLC) films (a-C, a-C:Cr, and a-C:H) in sulfuric acid and sodium hydroxide solutions were investigated. The a-C film showed the lowest stable coefficients of friction (COF) in both sulfuric acid and sodium hydroxide solutions but the worst wear resistance in sulfuric acid solution. The a-C:H film showed the highest COF in sulfuric acid solution and the best wear resistance in both sulfuric acid and sodium hydroxide solutions. The a-C:Cr film exhibited superior comprehensive tribological performance in sulfuric acid solution, while in sodium hydroxide solution, high COF and very poor wear resistance was observed. What is more, friction and wear mechanism was revealed by investigating the friction-induced material evolutions on the sliding surface.     

Partial processes during cobalt and molybdenum codeposition from solutions containing oxyacids

Distribution of citrate complexes of Co(II) and Mo(VI) in weakly acidic solutions is analyzed. It is found that the primary cathodic process is hydrogen evolution. When cathodic polarization is increased, Co(II) reduction is possible, while more negative potentials may result in full reduction of Mo(VI) to the metallic state. Reduction of H₃O⁺ ions in the solutions of oxyacids occurs with participation of a ligand playing the role of proton donor. Hydrogen evolution also remains the main electrochemical process in the molybdate solutions. It is assumed that Mo(VI) reduction occurs with participation of atomic hydrogen, but not all molybdate is reduced to metallic state. Co(II) reduction in solutions containing Mg(II) occurs to a higher extent but the degree of Mo(VI) reduction depends on the Co(II) concentration: increase in the latter results in a decrease in the content of metallic molybdenum in coatings.     

Measurement of inorganic acids with rotating ball inlet mass spectrometry

Using a new acid resistant inlet we have evaluated the use of rotating ball inlet mass spectrometry (ROBIN-MS) as a tool for the measurement of inorganic acids in aqueous solution. EI-spectra of sulfuric, sulfurous, nitric, perchloric, bromic, iodic and boric acids were recorded and interpreted with respect to preionization decomposition and electron impact fragmentation. With the exception of sulfuric acid the detection limits for these acids were in the order of 1 mM and the response times are in the order of 1 s. Sulfuric acid had a significantly higher detection limit and response time than the other acids. No mass spectrum of phosphoric acid could be detected. High concentrations of phosphoric acid reduced the signals of other acids. HCl formed by decomposition of perchloric acid caused a strong and slowly recovering decrease of the sensitivity of the mass spectrometer. Sodium sulfate or sodium nitrate in neutral solution did not yield any mass spectrum. The intensity of the sulfuric acid peak of acid solutions of sodium sulfate was measured at different molar ratios of sodium and sulfate. The results indicate that the evaporation of such samples leaves a remnant containing three molecules of sulfuric acid per sodium ion. Similar measurements on acid solutions of sodium nitrate indicate that the remnant contains one molecule of nitric acid per sodium ion.     

Voltammetric Behavior of Uranium(VI) in Sulfuric Acid Solutions Containing Pyridine

It was shown that 8.8 × 10^–5 to 4.4 × 10^–3 M uranium(VI) in sulfuric acid solutions containing pyridine give a clearly defined cathodic peak with a height proportional to uranium(VI) concentration. Under the specified conditions, the electroreduction of uranium(VI) is irreversible and involves one electron, the limiting current of uranium(VI) is diffusion- and kinetically controlled. The kinetic parameters of the process under study were determined, and the effect of concomitant elements on the voltammetric behavior of uranium(VI) in pyridine-containing sulfuric acid supporting electrolytes was estimated.     

Utilization of Cr(VI) solutions with steel shavings

New single-stage method was developed for chemical utilization of wastewater containing 0.25 to 400 g L^–1 of chromium in terms of chromium anhydride with the us of sulfuric acid and steel cuttings. The method makes it possible to convert hexavalent chromium into easily used ferrochromium precipitates. It was found that there occur periodic synchronous concentration fluctuations of Cr(VI) and Cr(III) up to, respectively, 400 g L^–1 and 150 mg L^–1 in the course of reduction of hexavalent chromium with the use of steel cuttings in sulfuric acid solutions.     

Boron-capped tris(glyoximato) cobalt clathrochelate as a precursor for the electrodeposition of nanoparticles catalyzing H2 evolution in water

Electrochemical investigation of a boron-capped tris(glyoximato)cobalt clathrochelate complex in the presence of acid reveals that the catalytic activity toward hydrogen evolution results from an electrodeposition of cobalt-containing nanoparticles on the electrode surface at a modest cathodic potential. The deposited particles act as remarkably active catalysts for H(2) production in water at pH 7.     

Cathodic Hydrogen Evolution from Aqueous Solutions of Acetic Acid

In continuation of the work on establishing hydrogen donors in the hydrogen evolution reaction from different acid molecules, cathodic evolution of hydrogen on silver is investigated from solutions of monobasic acetic acid with the aim to establish the origin of reduced hydrogen. Solutions of 0.2 M acetic acid with 0.2 M perchloric acid, neutralized to different pH values by NaOH, are used. The earlier established criterion is used for discerning between two possible hydrogen evolution mechanisms: (1) from dissociated hydrogen ions and (2) from undissociated hydrogen atoms in the molecule. At medium pH values, the undissociated acid molecules participate as hydrogen donors. Rate constants for reactions 1 and 2, evaluated at a potential of –800 mV (SCE), at which the entire pH range can be scanned, are 2.9 × 10^–6 and 1.9 × 10^–8.     

Supercapacitor carbon electrodes with high capacitance

Electrochemical behavior of electrodes on the basis of CH900-20 activated carbon (AC) cloth has been studied in concentrated sulfuric acid solution. Cyclic voltammetric curves have been studied in the reversibility range (from 0.1 to 0.9 V RHE) and in the deep cathodic charging potential range (from –0.8 to 1 V RHE). It has been shown that electric double layer (EDL) charging occurs in the reversibility range, while faradaic processes of hydrogen intercalation into AC carbon take place in the range of negative potentials (←0.1 V). The intercalation process is governed by slow solid-phase hydrogen diffusion. The specific charge value grows at an increase in concentrated sulfuric acid solution. The mechanism of double intercalation of sulfuric acid and hydrogen into the AC material is suggested. On the basis of the reached specific discharge capacitance of 1,560 C/g (or 1,110 F/g) and Faraday's law, it has been concluded that the compound of C₆H is formed in the limiting case of deepest cathodic charging. The obtained data have been used in a mathematical charge–discharge model for an AC electrode taking into account the EDL charging and the hydrogen intercalation. The galvanostatic recharge curves have been calculated in the diapason of currents by the developed model.     

Electrodialytic recovery of cobalt(II) with liquid membranes based on di(2-ethylhexyl)phosphoric acid

Electrodialytic transfer of cobalt(II) from sulfuric acid solutions across liquid membranes containing di(2-ethylhexyl)phosphoric acid with addition of tri-n-octylamine in 1,2-dichloroethane was studied, with the metal electrodeposited from dilute solutions of various acids. The influence exerted by the electrodialysis current density and by the composition of aqueous solutions and liquid membranes on the rate of cobalt(II) transport and electrodeposition was examined.     

Effect of Cathodic Polarization on the Hydrogen Diffusion through a Steel Membrane from Ethylene Glycol Solutions of HCl Containing As(V)

The connection of the hydrogen diffusion flux through a steel membrane and the fraction of adsorbed hydrogen removed due to the absorption in the metal with the cathodic polarization of the working membrane side and the hydrogen evolution rate in ethylene glycol solutions of HCl (0.05–0.99 M) with a constant ionic force containing 0.5–10 mM of As(V) in the form of H₃AsO₄ is studied.     

Kinetics and Mechanism of Cathodic Reduction of Nitrate Ions in a Sulfuric Acid Solution

Russian Journal of Physical Chemistry A - The cathodic reduction of nitrate ions in aqueous solutions of sulfuric acid in the presence of chromium(III) ions is studied via potentiometry. It is...     

Acid Effects on the Measurement of Mercury by Cold Vapor Atomic Absorption Spectrometry

Abstract

The influence of nitric, hydrochloric and sulfuric acids on the measurement of mercury by cold vapor atomic absorption spectrometry has been investigated. Small pre-reduction peaks associated with the instability of mercury were observed in solutions containing ≤ 12.5, < 2 and ≥ 12.5% v/v of each acid, respectively. Mercury was found to be most stable in ≥, 2% v/v hydrochloric acid and the measured absorbance was not greatly influenced by varying concentration of the acid. The mercury absorbance measurements were more sensitive in solutions containing ≤ 6.3% v/v hydrochloric acid than in similar concentrations of nitric and sulfuric acids. The use of the three acids as a digestion mixture result in serious interference from nitrogen oxides. The interferant was removed by use of expelling agents such as urea and sulfamic acid or overcome by use of excess stannous chloride, prior to the reduction of mercury(II) ions. The determination of mercury in NBS albacore tuna using both of these approaches to overcome the interference problem proved to be successful.     

Mechanism of nitrate electroreduction on Pt(100)

Kinetics and mechanism of nitrate anion reduction on the Pt(100) electrode in perchloric and sulfuric acid solutions are studied. Analysis of the results of electrochemical measurements (combination of potentiostatic treatment and cyclic voltammetry) and the data of in situ IR spectroscopy allow suggesting the following scheme of the nitrate reduction process on Pt(100) differing from that in the literature. If the potential of 0.85 V is chosen as the starting potential for a clean flame-annealed electrode surface and negativegoing (cathodic) potential sweep is applied, then an NO adlayer with the coverage of about 0.5 monolayer is formed on Pt(100) in the nitrate solution already at 0.6 V. The further decrease in the potential results in NO reduction to hydroxylamine or/and ammonia, desorbing products vacate the adsorption sites for nitrate and hydrogen adatoms. At E < 0.1 V, adsorbed hydrogen is mostly present on the surface. During positive-going (anodic) potential sweep, the process of nitrate reduction starts after partial hydrogen desorption, the cathodic peak of nitrate reduction to hydroxylamine or ammonia is observed at 0.32 V on cyclic voltammograms. The process of nitrate anion reduction continues up to 0.7 V; at higher potentials, the surface redox process with participation of hydroxylamine or ammonia (the anodic peak at 0.78 V) and nitrate (the cathodic peak at 0.74 V is due to nitrate reduction to NO on the vacant adsorption sites) occurs.     

Influence of cations on the proton leakage through anion-exchange membranes

In the recovery of acids from wastewaters or the regeneration of acids and bases from salts by electromembrane processes, the most important phenomenon which limits the current efficiency is the transport of protons through the anion-exchange membrane (AEM). In this work, the proton leakage through an AEM is studied with a system containing hydrochloric acid or sulfuric acid on the cathodic side and the mixture of acid with one homoanionic salt (Li⁺, Na⁺, K⁺, Cr³⁺, NH₄⁺, (CH₃)₄N⁺ and (C₂H₅)₄N⁺) on the anodic side. The proton leakage is quantified from the value of the proton transport number. The results are analyzed assuming that the rate determining step of proton leakage is the interfacial transfer reaction of protons from the aqueous anodic solution to the membrane. The proton leakage is enhanced by the polarizing power of the cation. The transfer of protons into the membrane seems to be catalyzed by the presence of a layer of adsorbed cations on the surface of the membrane. The presence of salt decreases the proton leakage but it is always greater with H₂SO₄ solutions compared to HCl solutions.     

Effect of cathodic polarization of a steel membrane and the acidity of alcohol solutions of hydrochloric acid on the balance between rates of the reaction of evolution of hydrogen and its solid-phase diffusion

The effect of cathodic polarization of the entrance side of a steel membrane and the concentration of hydrochloric acid (0.99–0.01 M) on the balance between rates of the reaction of evolution of hydrogen and its diffusion through steel (ρ) is studied in hydrochloric-acid solutions of ethanol and ethylene glycol with a constant ionic strength equal to unity. It is demonstrated that ρ lowers down with a shift of the potential of the polarization side of the membrane in the negative direction and increases with diminishing concentration of hydrochloric acid. The results are interpreted with allowance made for the surface coverage by two forms of adsorbed hydrogen, namely, a raised form and a subsurface form.     

Exaltation effects in the metal electrodeposition from acid electrolytes

The exaltation of mass transfer of discharging cations, caused by the concurrent hydrogen evolution from acidified solutions, results in a sharp increase in the metal deposition rate. In this case the limiting process rate depends linearly on the hydrogen evolution current density; it depends but weakly on the solution agitation and temperature. Under the electrolysis of solutions containing weak acids as a supporting electrolyte, the higher the acid formation constant, the more pronounced is the dependence of the electrodeposition limiting rate on the hydrogen current density. When microelectrodes are used, the varying of the background acid nature may affect the hydrogen evolution rate markedly, while the metal electrodeposition rate mainly depends on the cell voltage. When metals are electrodeposited from complex anions, the parallel hydrogen evolution hinders the mass transfer; this process depends on the anion stability constant and charge, all other conditions being the same. The found peculiarities can be used in the inversion voltammetry for the lowering of the metal detection limit, improving of the analysis selectivity, and time saving.     

A 3D MIL-101@rGO composite as catalyst for efficient conversion of straw cellulose into valuable organic acid

Efficient conversion of straw cellulose to chemicals or fuels is an attracting topic today for the utilization of biomass to substitute for fossil resources. The development of catalysts is of vital importance.In this work, a composite catalyst metal-organic frameworks(MOFs) immobilized on three-dimensional reduced graphene oxide(3D-r GO) were synthesized by in situ growth of the MIL-101(Cr) within the 3Dr GO matrix. The supporting of 3D-r GO guaranteed the dispersion and acid site density of MI...