Electrochemical impedance of passive iron in sulfuric acid water-organic solutions

The passive state of iron in sulfuric acid solutions in dimethylformamide and dimethyl sulfoxide containing different amounts of water is studied by the impedance method in a wide range of ac frequencies and also by the methods of polarization curves and open-circuit potential transients. In the studied systems, the metal passivation potential is shown to shift in the positive direction with a decrease in the water content in the solvent. The open-circuit potential of the destruction of the passive layer (Flade potential) is independent both of the nature of the solvents and the water amount in them. The impedance characteristics of passive iron can adequately be modeled by an equivalent circuit consisting of two parallelR-C circuits, i.e. similarly to a circuit used earlier for modeling the impedance of a passive Fe electrode in neutral aqueous solutions. The physical meaning of the equivalent-circuit elements calculated from the impedance measurements is analyzed. The effects the electrode potential, the organic-solvent type, the water content in the latter, etc. have on the elements are discussed. The obtained results suggest that the potential difference applied to the interface between passive iron and studied solutions is located mainly in the Helmholtz layer. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

Current oscillations during copper electrodissolution under solution sparging in acidic NaCl solutions

Potentiostatic current oscillations were observed during electrodissolution of a helical coiled copper wire electrode in acidic sodium chloride when nitrogen was sparged through the solution. The oscillations were studied as a function of solution pH, chloride ion concentration and sparging rate over the potential range of 0.2 to 0.7 V versus Ag/AgCl (3 M NaCl). The surface morphology of the copper was observed during the oscillatory process using in situ, real-time optical microscopy, and the formation of a surface film was correlated with the oscillations.     

Protonation acidity constants for benzotoluidides in sulfuric acid solutions

The protonation of o-, m-and p-benzotoluidide in sulfuric acid solutions is studied by UV spectroscopy in the 190–350 nm region. Principal component analysis is applied to estimate the contributions of the effect of protonation and the medium effect. For the substances studied in this work, the first principal component (PC) captures about 98 % of the variance and the second PC ∼100 % of the cumulative percentage variance in the 210–350 nm region. The same spectral region is used for calculation of the ionization ratio from the coefficients of the first PC and mole fractions of the base and its conjugate acid. Using these data and Hammett's equation (pK _{BH +} = H _X + log I), the pK _{BH +} values for the protonation reaction are obtained. The dissociation constants as well as the solvent parameters m* (∼0.43) and ϕ (∼0.60) are calculated using the Excess Acidity Method (-pK _{BH +}= 2.28–2.30) and the Bunnett-Olsen Method (-pK _{BH +}= 2.24–2.28). The probable sites of protonation are discussed.     

Reaction of sphene with sulfuric acid solutions

The kinetics of titanium(IV) extraction from sphene with boiling 4–6 M sulfuric acid was studied. The rate constants of the sphene dissolution were determined. The influence of the particle size of the starting mineral on the rate of its dissolution was established. The feasibility of performing sphene dissolution by a liquid-phase procedure yielding stable titanium-containing sulfate solutions suitable for the synthesis of multifunctional composite materials was examined.     

Protonation equilibria of rauwolfia alkaloids in sulfuric acid solutions

The pK_a values for the indole ring protonation equilibria of the Rauwolfia alkaloids, yohimbine, ajmalicine, reserpine and reserpiline have been measured in strongly sulfuric acid solutions. The alkaloids obey the H_I acidity function stablished by Hinman and Lang for indole ring protonation, but they are considerably weaker bases than alkylindoles. Evidences indicating that these compounds behave as carbon bases are reported.     

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.     

Vanadium(V) complexes in sulfuric acid solutions

The ionic state of vanadium(V) is studied spectrophotometrically over a wide range of sulfuric acid concentrations from 1.0 to 16.8 mol/l. Existence regions for monomeric and dimeric vanadium(V) complexes are determined. The equilibrium constant of vanadium(V) dimerization in 12 M H₂SO₄ is determined.     

Electrochemical amination of benzene in aqueous-acetic solutions of sulfuric acid

Indirect cathodic amination of benzene with hydroxylamine in the presence of Ti(IV)/Ti(III) mediator system in aqueous media containing 4–11 mol/L H₂SO₄ and 13–5.5 mol/L AcOH has been studied. Aniline, diphenyl, and isomeric phenylenediamines are the electrolysis products at 25–60°C. The increase in temperature favors the formation of the monoamino compound. Aniline yield with respect to hydroxylamine at complete conversion of the latter has reached 78.7%, mass fraction of aniline being 97.1%.     

Electronic spectroscopy of solutions of heterocyclic compounds in sulfuric acid

The behavior of furan derivatives in strongly acidic media was investigated by means of electronic spectroscopy. It is shown that at concentrations of 10^–5 to 10^–6 mole/liter a number of the investigated compounds are stable in sulfuric acid of any concentration because of a decrease in the probability of polymerization processes.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1594–1596, December, 1972.     

State of rhodium(III) in sulfuric acid solutions

The formation of rhodium(III) sulfate complexes under moderately rigorous temperature conditions was studied by ¹⁰³Rh and ¹⁷O NMR spectroscopy. The complexes [Rh₂(μ-SO₄)₂(H₂O)₈]²⁺, [Rh₂(^*μ-SO₄)(H₂O)₈]⁴⁺, and [Rh₃(μ-SO₄)₃(μ-OH)(H₂O)₁₀]²⁺ were found to be the most stable species in aged solutions.     

Ab initio simulations of sulfuric acid solutions

Ab initio molecular dynamics simulations are presented for a solution consisting of a single H₂SO₄ molecule in a periodic box with 63 water molecules. Three trajectories lasting 35–55 ps were calculated at temperatures in the 320–326 K range, for two functionals, and double- and triple-zeta quality basis sets. All three trajectories displayed prompt ionization of the first proton. The ion displayed varying extents of ionization in the different trajectories. Proton hopping was observed between the oxygen atoms. Moreover, hopping of the solvating water molecules also occurred between the different oxygen atoms of the anions, with bidentate water, hydrogen bonding to two different oxygen atoms, serving as an intermediate.     

Chemistry of rhenium sulfide deposition from sulfuric acid solutions

Rhenium sulfi de deposition from sulfuric acid solutions of sodium thiosulfate was examined.