Effect of methanol on the dissolution of neodymium oxide in hydrochloric acid solutions

The effect of methanol on the solubility of Nd₂O₃ in HCl medium in the temperature range of 25–55°C were studied. HCl-CH₃OH was observed to be a better solvent than HCl-CH₃OH-H₂O and hydrochloric acid for the dissolution of Nd₂O₃ mainly due to prevention of neodymium hydroxide and dissolution increased with increase of HCl concentration even at high HCl concentrations. Presence of methanol in the medium was determined to cause corresponding decreases in conductivity, E_SCE, pH values with corresponding increase in E_R values. Neodymium compounds formed in HCl-CH₃OH medium at 25°C were isolated and identified using X-ray diffraction, thermal and chemical analysis methods and their thermal behaviour were investigated between 25–1100°C. NdCl₃ × x ₁CH₃OH and NdCl₃ × x ₂CH₃OH × 4.14H₂O (x ₁ + x ₂ = 2.8) were formed in non-aqueous medium and these compounds were converted into NdCl₃, NdCl_2.37 and NdCl₃ × 0.47CH₃OH with complete and partial loss of water and methanol around 240°C respectively. Methanol was completely removed around 390°C with formation of NdCl_2.37 (77.5%) and NdOCl (22.5%) and NdOCl was detected as the major stable phase above 700°C.     

Effect of hydroacoustic treatment on chitosan dissolution in aqueous acetic acid solutions

Effect of a hydroacoustic treatment in a rotary-pulsatory apparatus on the dissolution rate and the achievable limiting solubility of chitosan in aqueous acetic acid solutions of various concentrations was studied. The rheological properties of solutions produced by the conventional method and with mechanical activation were compared     

Electrochemical dissolution of magnetite in acid solutions

The present paper deals with the electrochemical behavior of magnetite microcrystals in an acid medium. A voltammetric method employing a carbon-paste electroactive electrode (CPEE) with an organic binder was used. It was found that the cathodic voltammograms, which were recorded at different scan rates, formed a set bounded in the space of i–E parameters by a generalizing voltammetric curve corresponding to the effective potential scan rate _eff. In other words, all curves are situated under one enveloping curve, just as the smaller dolls sit in the largest doll of a Russian doll. Reverse currents (a cathodic current in the anodic direction of the potential scan) were observed on the cyclic voltammogram. Forward and reverse currents obey the same laws and have one and the same generalizing curve, which could be taken as the magnetite characteristic.     

Mechanism of anodic dissolution of copper in aqueous acidified solutions of different anions

The potentials of copper anodes were measured as a function of current density in aqueous acidified solutions of different anions. Tafel lines were obtained, whose slopes depend on the nature of the anion. This denoted the participation of the anion in the anodic dissolution of the metal. Hence, mechanisms were suggested based on the formation of intermediate cuprous compounds, which are further oxidized electrochemically, or undergo chemical disproportionation to cupric salts.The results in phosphate solutions indicated that the second electrochemical oxidation is the rate determining step. In chloride, nitrate and sulphate solutions, both the second electrochemical step and the disproportionation reaction govern the overall reaction rate.Measurements in acidified copper salt solutions showed that Cu²⁺ ions affect the reaction mechanism. Thus, in chloride and sulphate solutions, the disproportionation reaction becomes predominating, whereas in nitrate solutions the intermediate is mainly oxidized electrochemically. An interpretation is provided, based on the adsorption of Cu²⁺ ions.     

Effect of amalgamation on the passivation of transition metals in their anodic dissolution in tetrahydroadipic acid solutions

Effect of anode amalgamation on the degree of passivation of the electrodes and on the current efficiency in electrosynthesis of coordination compounds of copper(II), nickel(II), zinc(II), and cadmium(II) with galactaric acid was studied. An improvement of anodic synthesis characteristics was observed when amalgamated zinc, copper, and cadmium were used. Complex compounds of 1 : 1 composition were obtained and the structure of the coordination environment of the metal ion was determined by IR spectroscopy.     

A thermometric study of the dissolution of copper in acid solutions

The dissolution of Cu in solutions of HNO₃ of different concentrations has been studied by the thermometric method. Starting from the initial temperature, T_i, the temperature—time curves exhibit an induction period followed by a rapid rise in temperature to a maximum value, T_m, attained t min after the start of the reaction. T_m increases and t decreases with increase of the acid concentration, M. ΔT (i.e.T_m ? T_i) and the reaction number (R.N. = (T_m ? T_i)/t) vary with M according to: ΔT = k(M ? M₀) and R.N. = A₁Mⁿ, where k, M₀, A₁ and n are constants.The effect of varying concentrations of HCl, H₂SO₄ and H₃PO₄ on the R.N. of Cu in 3.5 M HNO₃ was examined. Small amounts of these acids lower the R.N. (inhibition) due to the displacement of an active species on the surface of the metal by the anion of the acid. Larger additions of the acids accelerate dissolution. The concentration at which the added acid changes from corrosion-inhibitor to accelerator varies as HCl < H₂SO₄ < H₃PO₄. This sequence is considered to parallel the strength of adsorption of the respective anions. The results of experiments with salt additions confirm this view; all salts act only as dissolution-retardants. Calculations pertaining to the effect of the various ions on the R.N. support the conclusion that the dissolution of Cu in HNO₃ is autocatalytic in nature, and depends on the [H⁺]/[NO₃^?] ratio.Cu does not dissolve in air-free, cold HCl. Attack takes place, however, in the presence of KNO₃. Under these conditions attack is of the pitting- rather than the general type. The temperature rises suddenly after an incubation period, which decreases in length with increase of the amount of the added salt.Proof of the involvement of HNO₂ in the autocatalytic cycle of Cu dissolution in HNO₃ is obtained from the results of urea additions to the solution.     

Anodic dissolution of cobalt suicides in sulfuric acid solutions

Anodic dissolution of suicides Co₂Si, CoSi, and CoSi₂ in 0.5 M H₂SO₄ solutions with and without NaF is studied by polarization methods coupled with an atomic-absorption analysis. In the absence of NaF, anodic stability of cobalt silicides is higher than that of cobalt itself by 3–6 orders of magnitude and increases in the series Co₂Si < CoSi < CoSi₂. Cobalt selectively dissolves from the lattice of silicides, while silicon simultaneously oxidizes to SiO₂. The process is controlled by the diffusion of elemental and oxidized cobalt in the suicide and the SiO₂ surface layer, respectively. The anodic dissolution rate of silicides markedly increases in the presence of NaF, while their anodic stability becomes less different.     

Effect of cadmium and lead adatoms on the reduction kinetics of peroxodisulfate anions at platinized platinum in acid solutions

The reduction of peroxodisulfate anions on a rotating Pt/Pt disk electrode in solutions of sulfuric and perchloric acids with or without cadmium and lead salts is studied. At E _r > 0.2V (RHE) the reduction rate in HClO₄ exceeds that in H₂SO₄, but at E _r < 0.2V, the process in HClO₄ is strongly inhibited upon approaching E _r = 0. Lead adatoms catalyze the process, while cadmium adatoms inhibit it in H₂SO₄ and accelerate it at E _r < 0.2V in HClO₄. The results are interpreted by taking account of the specific adsorption of sulfuric-acid anions and their co-adsorption with cadmium cations, as well as the adsorption of peroxodisulfate anions on the Pt surface modified by lead and cadmium ions in perchlorate solutions.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 137–141.Original Russian Text Copyright © 2005 by Nikiforova, Petrii.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Silver dissolution in acid thiocarbamide solutions containing sulfide ions

Regularities of silver dissolution in acid thiocarbamide electrolytes are studied. The kinetics of the process is shown to be severely affected by the admixture of hydrogen sulfide molecules that form upon inserting sodium sulfide or accumulate in electrolyte with the passage of time elapsed since its preparation. Catalytic effect increases with increasing length of time of the electrode’s contact with solution prior to the beginning of experiment or following an increase in the concentration of sulfide ions. Experiments with the surface renewed in the course of potential scans show that the catalytic effect is connected with the adsorption of sulfide ions on an interface. At large values of the surface coverage with sulfide ions, the dissolution rate increases so much that the dissolution process starts to be limited largely by the process of supply of thiocarbamide molecules toward the electrode surface.     

The mechanism of pulse anodic activation of iron dissolution in acid solutions

Basing on the model of spatial separation of the dissolution and passivation of hydrophilic metals, formation of stationary surface morphology during pulse anodic activation of iron dissolution in sulfuric acid solutions is analyzed.     

Specific adsorption of anions preceding the dissolution of CuO in acidic solution

The specific adsorption of sulfate and chloride ions on powdered CuO was studied by radiotracer techniques in acidified of 0.5 mol dm^–3 NaClO₄ supporting electrolyte. It was found that the pH and concentration dependence of the adsorption of labelled sulfates could be determined over a wide pH range (2.0≤pH≤7.0), despite the gradual dissolution of the copper oxide. No specific adsorption was found at pH≥7.0. It was postulated that the specific adsorption was preceded by the protonation of the CuO surface. No reliable relationships were found for labelled chlorides, owing to the relatively high desorption rate of the surface complexes formed in the sorption process. The apparent adsorbability of sulfate ions was significantly higher than that of the chloride ions, as shown by the results of competitive adsorption of the two species. This observation was in agreement with the postulated lower desorption rate of the surface complex formed with sulfates than that of species formed with chlorides. Electronic Publication     

Chemical dissolution of iron in aqueous solutions

By anodic oxidation of hydrogen on the Pt ring of a metal-disk-Pt-ring rotating electrode in aqueous acid (pH 0.5–3) sulfate solutions, it is shown that during the polarization of the metal disk hydrogen coevolves by two parallel reactions. One is the well known electrochemical charge transfer process, while the other one is a direct chemical reaction of the metal, i.e. iron, with water molecules from the electrolyte. This process was proposed a long time ago by Kolotyrkin and co-workers, but their experimental results were subjected to serious criticism which is taken care of in the present paper. The chemical reaction is potential- and pH-independent and, depending on the actual conditions during the corrosion processes (actual potential, pH, etc.), can be of greater or smaller importance in the overall process. The consequences of the existence of this direct chemical reaction of metal with water molecules (i.e. H₂O-induced corrosion or chemical corrosion) on pitting and stress corrosion cracking of metals is discussed. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth. This article was submitted by the authors in English.     

Thermodynamics of ferrocene dissolution in water-methanol solutions

The polythermal solubility of ferrocene in water-methanol solutions in a wide range of their compositions was studied. Ferrocene solubility grows as temperature and methanol content increase. Thermodynamic functions of ferrocene transfer from water into water-methanol solutions were calculated. The results obtained were compared with the data on neon solubility in the same solvent.     

Effect of boric acid on tetrafluoroborate ion hydrolysis in solutions of tetrafluoroboric acid

The presence of boric acid in tetrafluoroboric acid solutions enhances the BF₄^- ion hydrolysis. An increase of the boric acid content in a 32% HBF₄ solution from 0.1% to 3.8% is accompanied by a simultaneous increase of the degree of hydrolysis from 4.8% to 30.5%.     

Effect of specific adsorption of anions on the kinetics of the current relaxation after pulsed anodic activation of dissolution of hydrophilic metals

Based on the model of spatial separation of the dissolution and passivation of hydrophilic metals, the effect of strong specific adsorption of anions on the shape of curves for the relaxation of anodic current of metals in acid solutions after switching an anodic activation pulse off is considered by using iron as an example.     

Relationship between dissolution rate of uranium dioxide pellets in nitric acid solutions and their porosity

Dissolution of uranium dioxide pellets /10 mmø×11 mm/ in nitric acid was studied. A dependence of the initial dissolution rate on acid concentration and temperature was observed. The effective surface area of the pellet was measured during the dissolution process, and compared with its apparent surface area. The experimental dissolution curve was in good agreement with the curve obtained by simulations assuming that the dissolution rate depends on the effective surface area.