Aqueous Sulphuric Acid Research Articles

On the effects of organic matter and sulphur-containing compounds on the CCN activation of combustion particles

Abstract. The European PartEmis project (Measurement and prediction of emissions of aerosols and gaseous precursors from gas turbine engines) was focussed on the characterisation and quantification of exhaust emissions from a gas turbine engine. The combustion aerosol characterisation included on-line measurements of mass and number concentration, size distribution, mixing state, thermal stability of internally mixed particles, hygroscopicity, cloud condensation nuclei (CCN) activation potential, and off-line analysis of chemical composition. Based on this extensive data set, the role of sulphuric acid coating and of the organic fraction of the combustion particles for the CCN activation was investigated. Modelling of CCN activation was conducted using microphysical and chemical properties obtained from the measurements as input data. Coating the combustion particles with water-soluble sulphuric acid, increases the potential CCN activation, or lowers the activation diameter, respectively. The adaptation of a Köhler model to the experimental data yielded coatings from 0.1 to 3 vol-% of water-soluble matter, which corresponds to an increase in the fraction of CCN-activated combustion particles from ≤10−4 to ≌10−2 at a water vapour saturation ratio Sw=1.006. Additional particle coating by coagulation of combustion particles and aqueous sulphuric acid particles formed by nucleation further reduces the CCN activation diameter. In contrast, particles containing a large fraction of non-volatile organic compounds grow significantly less at high relative humidity than particles with a lower content of non-volatile OC. The resulting reduction in the potential CCN activation with an increasing fraction of non-volatile OC becomes visible as a trend in the experimental data. While a coating of water-soluble sulphuric acid increases the potential CCN activation, or lowers the activation diameter, respectively, the non-volatile organic compounds, mainly found at lower combustion temperatures, can partially compensate this sulphuric acid-related enhancement of CCN activation of carbonaceous combustion aerosol particles.

Speciation of the Fe(II)–Fe(III)–H 2SO 4–H 2O system at 25 and 50 °C

This work presents theoretical and experimental results on the speciation of the Fe(II)–Fe(III)–H 2SO 4–H 2O system in concentrated solutions (up to 2.2 m H 2SO 4 and 1.3 m Fe). The aim was to study the chemical equilibria of iron at 25 and 50 °C in synthetic aqueous sulphuric acid solutions that contain dissolved ferric and ferrous ion species. Raman spectroscopy, volumetric titration and conductivity measurements have been carried out in order to study the presence of specific ions and to characterize the ionic equilibrium. A thermochemical equilibrium model incorporating an extended Debye–Hückel relationship was used to calculate the activities of ionic species in solution. Model calculations were compared with experimental results. Model simulations indicate that anions, cations and neutral complexes coexisted in the studied system, where the dominant species were HSO 4 −, H +, Fe 2+ and FeH(SO 4) 2 0. This indicated that these solutions showed a high buffer capacity due to the existence of bisulphate ions (HSO 4 −), which presented the highest concentration. A decrease in the concentration of H + and Fe 3+ took place with increasing temperature due to the formation of complex species. Standard equilibrium constants for the formation of FeH(SO 4) 2 0 were obtained in this work: log K f 0 = 8.1 ± 0.3 at 25 °C and 10.0 ± 0.3 at 50 °C.

Oxygen reduction on platinum in aqueous sulphuric acid in the presence of ammonium

The oxygen reduction reaction (ORR) was studied using a rotating disk electrode. The kinetic current density (cd) at 0.7 V RHE was reduced by more than a factor of 10 on pre-reduced Pt and a factor of 2.5 on pre-oxidised Pt when comparing ORR rate for pure sulphuric acid to that in 0.18 M H 2SO 4 + 0.1 M NH 3 solution which was the highest ammonium concentration studied. The Tafel slope at low cd on pre-reduced Pt is about − 90 mV/decade in the presence of ammonium compared to about − 74 mV/decade in pure acidic solution. The Tafel slopes at high cd is not affected by ammonium. The Tafel slope of ORR on pre-oxidised Pt surface is not significantly affected by ammonium. The exchange cd is lower in the presence of ammonium both for pre-reduced and pre-oxidised Pt. The direct origin of the loss of ORR activity has not been firmly resolved. Adsorption and oxidation of ammonium including intermediate species as well as nitrogen–oxygen species formed at high potentials may be involved.

Kinetics and mechanism of stripping of Mn(II)–D2EHPA complex by sulphuric acid solution

The kinetics of stripping of Mn(II)–D2EHPA chelate (MnA 2.HA.H 2A 2) existing in kerosene phase by aqueous sulphuric acid solutions at constant sulphate ion concentration of 0.50 M have been investigated by the constant interfacial area stirred (Lewis) and non-stirred (Hahn) cells. Both pseudo-rate constant ( q) or (rate / area) and flux ( F) methods of the rate data treatment have been applied. The empirical rate equations have been derived. Results have been compared among themselves and other published works on Mn(II)–D2EHPA chelate stripping kinetics. Rate constants obtained from ( q) or (rate / area) and ( F) methods differ in magnitude and units and an explanation of this has been given. An analysis of rate equations obtained from the Lewis cell experimentation ( F b = 10 − 4.1 [MnA 2.HA.H 2A 2] (o)(1 + 0.005[H +] (i) − 1 ) − 1 (1 + 7.94[H 2A 2] (o) 0.5) − 1 ) suggests that the process is chemically controlled at lower aqueous acidity and higher free D2EHPA concentration regions; whereas, it is diffusion-controlled at higher aqueous acidity and lower free D2EHPA concentration regions. In the investigated D2EHPA concentration and aqueous acidity regions, majority of data fall in the intermediate controlled region. The suggested mechanisms are supported by the values of activation energy ( E a). In the kinetic regime, the reaction step: MnA + → Mn 2+ + A − occurring in the bulk aqueous phase is the slowest step which proceeds through S N2 mechanism as indicated by the comparable higher negative value of Δ S ± in the stated condition. In Hahn cell technique, the empirical rate equation is: F b = 10 − 5.9 [MnA 2.HA.H 2A 2] (o)(1 + 0.0025[H +] (i) − 1 ) − 1 (1 + 2.8[H 2A 2] (o) 0.5) − 1 . Analysis of this equation together with the E a value of 12–42 kJ/mol indicate that the stripping process is completely diffusion controlled in the low pH and extractant concentration region and in other parametric condition, that is intermediate controlled. For both cells, the ratio of ( k f) to ( k b) equals almost to K ex of 10 − 2.44 obtained from the distribution study.

Kinetics and Mechanism of Acetoxymercuration and Acid-Catalysed Hydration of α-Alkylstyrenes

Nine α-alkylstyrenes carrying the following substituents have been synthesised: methyl, ethyl, propyl, butyl, pentyl, isobutyl, isopropyl, sec-butyl and tert-butyl. Kinetics has been measured for the reaction of these compounds with mercuric acetate in anhydrous acetic acid at 25 °C. The rate constants are very sensitive to inductive effects (ρI = –49.5 in AISE theory) and steric effects (ψ = –1.59 according to Charton). These results support the presumed existence of an intermediate with an asymmetrically bound acetylmercuric cation to carbon atoms of the vinyl group of styrene. The kinetics of an acid-catalysed hydration of the above-mentioned α-alkylstyrenes were also measured in aqueous sulphuric acid at 25 °C. The derivatives exhibited kinetically a consecutive isomerisation reaction giving the more stable substituted styrenes. The values of the slope m‡m* of the dependence upon the excess acidity function (X) were evaluated, and exceptionally low values were found for the butyl and pentyl substituents (1.02 and 0.73, respectively), while on the other hand tert-butyl showed an exceptionally high value (3.28). On the basis of the facts, a mechanism has been suggested for the acid-catalysed hydration, involving the reaction of a relatively stable and sterically hindered carbocation with water as the rate-limiting step of the reaction.

Catalyzed Oxidation of Tetrahydrofurfuryl Alcohol by Cerium(Ⅳ) in Aqueous Sulphuric Acid Medium

The kinetics of trace Ir(Ⅲ) catalyzed oxidation of tetrahydrofurfuryl alcohol(THFA) by Ce(Ⅳ) in aqueous sulphuric acid medium was studied by titrimetric technique of redox in the temperature range of 298~313 K. The order was found to be unity with respect to both Ce(Ⅳ) and Ir(Ⅲ), and for tetrahydrofurfuryl alcohol the apparent reaction order was a positive fraction. It was found that the pseudo first order ([THFA]»[Ce(Ⅳ)]) rate constant, k(subscript obs) increased with the increase of [H(superscript +)] but decreased with the increase of [HSO(superscript - subscript 4)]. Under the protection of nitrogen, however, the reaction system could induce polymerization of acrylonitrile, indicating the generation of free radicals. A reasonable mechanism involving a preequilibrium was proposed. From the relation between k and [HSO(superscript - subscript 4)], Ce(SO4)2 was found to be the kinetically active species. The equilibrium constants and rate constants of the rate-determining step along with the activation parameters were evaluated.

Iridium(III) catalyzed oxidation of cyclic ketones by cerium(IV)

IrCl3which is considered to be a sluggish catalyst in alkaline media, was found to surpass the catalytic efficiency of even osmium and ruthenium in acidic media in the oxidation of cyclopentanone and 2-methylcyclohexanone by cerium(IV) sulphate in aqueous sulphuric acid medium. It was observed that the order of the reaction shows direct proportionality with respect to low concentrations of the oxidant and alcohols, but tends to become independent of concentration at higher concentrations. On increasing the concentrations of externally added Cl-, H+ and CeIIIions, the rate of the reaction decreases sharply initially but the decrease in rate becomes less prominent as their concentration is increased. The rate of reaction is directly proportional with respect to IrCl3concentrations. Kinetic data suggest that the production of CeIII ion occurs before the rate-determining step. Parameters such as the energy of activation, free energy of activation and entropy data collected at five different temperatures suggest that cyclopentanone forms the activated complex more easily.

Selective oxidations by nitrosating agents in liquid and solid acid catalysts

The oxidative properties of N2O3 and NO+ ion towards methyl or methylene groups of enolizable ketones are tested at 25°C in homogeneous and heterogeneous acid systems. The results in concentrated aqueous sulphuric acid show that reasonable good yields (∼95%) of benzoylformic acid from acetophenone are obtained at selected acidity ranges. A kinetic study for the conversion reagents–products and a thermodynamic treatment of the rates was also attempted by using the acidity dependence of rates, the equilibria of the reacting species and the departure from the ideality of the catalytic acid system. On the replacement of aqueous acid solutions with solid acids, benzoyl cyanide is instead obtained, with high yields and selectivity over catalysts with suitable acidic properties. Additional studies of different ketones and of certain alcohols are also reported.

Selective oxidations by nitrosating agents in liquid and solid acid catalysts

The oxidative properties of N 2 O 3 and NO + ion towards methyl or methylene groups of enolizable ketones are tested at 25 °C in homogeneous and heterogeneous acid systems. The results in concentrated aqueous sulphuric acid show that reasonable good yields (∼95%) of benzoylformic acid from acetophenone are obtained at selected acidity ranges. A kinetic study for the conversion reagents–products and a thermodynamic treatment of the rates was also attempted by using the acidity dependence of rates, the equilibria of the reacting species and the departure from the ideality of the catalytic acid system. On the replacement of aqueous acid solutions with solid acids, benzoyl cyanide is instead obtained, with high yields and selectivity over catalysts with suitable acidic properties. Additional studies of different ketones and of certain alcohols are also reported.

Antimony Solubility and Speciation in Aqueous Sulphuric Acid Solutions at 298 K

AbstractThis work is an experimental and theoretical study of antimony speciation and the solubility of Sb2O5, Sb2O3 and Sb2(SO4)3 at 298 K in aqueous sulphuric acid solutions (0 to 6 kmol/m3). A thermodynamic model was developed to quantify the solubility products and aqueous metal speciation. Main dissolved species were H3SbO3(aq) and H3SbO4(aq). The solubility of antimony increases with increasing solution acidity and with the presence of hydrogen peroxide in the solution. The following standard equilibrium constants at 298 K were obtained: Log K0(Sb2O5) = 11.7±0.5, Log K0(Sb2O3) = 32±1, and Log K0(Sb2(SO4)3) = 55±2.

Kinetics and Mechanism of Iridium (III) Catalyzed Oxidation of Isobutyl Alcohol by Cerium(IV) in Aqueous Sulphuric Acid Medium

在酸性介质中用氧化还原滴定法研究了铈(IV)离子在痕量铱(III)离子催化作用下,于25～40℃区间氧化异丁醇(BA)的反应动力学.结果表明反应对铈(IV)离子为一级,对异丁醇的表观反应级数为正分数.准一级速率常数kobs随[H+]及催化剂[Ir(Ⅲ)]增加而增大,随[]增加而减小.在氮气保护下,反应不能引发丙烯酰胺聚合,说明在反应中没有自由基产生.提出了催化剂、底物和氧化剂间生成双核加合物的反应机理,通过kobs与的依赖关系,找到本反应体系的动力学活性物种是Ce(SO4)2+,并计算出平衡常数、速控步骤的速率常数及相应的活化参数.

Anodically generated manganese(III) sulphate for the oxidation of dipeptides in aqueous sulphuric acid medium: A kinetic study

The kinetic of oxidation of dipeptides (DP) namely valyl-glycine (Val-Gly), alanyl-glycine (Ala-Gly) and glycyl-glycine (Gly-Gly), by Mn(III) have been studied in the presence of sulphate ions in acid medium at 26°C. The reaction was followed spectrophotometrically at λmax = 500 nm. A first-order dependence of the rate on both [Mn(III)]o and [DP]o was observed. The rate is independent of the concentration of reduction product, Mn(II) and hydrogen ions. The effects of varying the dielectric constant of the medium and addition of anions such as sulphate, chloride and perchlorate were studied. The activation parameters have been evaluated using Arrhenius and Eyring plots. The oxidation products were isolated and characterized. A mechanism involving the reaction of DP with Mn(III) in the rate-limiting step is suggested. An apparent correlation was noted between the rate of oxidation and the hydrophobicity of these dimers, where increased hyphobicity results in increased rate of oxidation

Electrochemical study of molybdenum oxide film electrodes

Several different electrochemical techniques were used to study the redox reaction in an aqueous sulphuric acid solution at molybdenum oxide film electrode. It was determined by chronopotentiometry, along with cyclic voltammetry and impedance spectroscopy, that in the redox process, the H + ion combines with an electron from the film to form an intermediate before entering the film, and that the transport of the H atom within the layer of oxide occurs purely by diffusion. The diffusion coefficient of the H atom was deduced from these data ( D=10 −12 cm 2 s −1).

Surface enhanced infrared spectroscopy—Au(1 1 1-20 nm)/sulphuric acid—new aspects and challenges

The structure and orientation of water molecules on Au(1 1 1-20 nm) film electrodes in contact with aqueous sulphuric acid solution was studied by surface enhanced infrared reflection–absorption spectroscopy (SEIRAS) employing an attenuated total reflection (ATR) configuration (ATR-SEIRAS) with a vertical spectroelectrochemical cell. The spectrum of interfacial water is strongly dependent on electrode potential, ionic strength and pH. Coadsorption of hydronium ions with weakly hydrogen-bonded water molecules was found at E< E pzc. At E> E pzc strong hydrogen-bonding among water molecules and to coadsorbed sulphate species exists. Based on pH-dependent electrochemical, spectroscopic and in situ STM investigations, a new model is suggested to represent the ordered ( 3 × 7 ) adlayer at maximum sulphate coverage θ=0.2: a Zundel or hydrated hydronium ion H 5O 2 +, in which one proton is shared between two water molecules, bridges adjacent sulphate species via hydrogen-bonds along the main diagonal of the ( 3 × 7 ) unit cell. This alternating arrangement gives rise to a long-range ordered, 2D network of sulphate and water species interconnected by hydrogen-bonds, and capable to form hydrogen-bonds with second-layer water species. The suggested model is consistent with all experimental observations as well as predictions from quantum-chemical and molecular dynamic simulations.

Charge dynamics in conducting polyaniline-metal oxalate composites

Polyaniline (Pani) and its metal oxalate composites (∼10 wt.%) of trivalent metal ions of Cr, Fe, Mn, Co and Al were synthesized by chemical oxidative polymerization technique with potassium perdisulphate oxidant in aqueous sulphuric acid medium. These materials were characterized by UV-VIS and EPR spectral techniques. Their d.c. electrical conductivities at room temperature and also as a function of temperature (307-453 K) were measured by four- probe technique. Presence of radical cation/polaron transition was indicated by UV- VIS absorption peak and EPR signals. Further, a close correlation existed between the conductivities and EPR parameters such as line width and peak ratio, which demonstrated that both mobile and fixed spins are involved in these composites. The dependence of conductivity on temperature, when analysed graphically by VRH, GB and TC mechanisms, pointed out that VRH is the predominant charge transport mechanism in these materials.

Metal oxalate complexes as novel inorganic dopants: Studies on their effect on conducting polyaniline

Doped polyaniline materials with metal oxalate complexes of Cr, Fe, Mn, Co and Al were synthesized byin situ chemical oxidative polymerization of aniline using potassium perdisulphate as oxidant in aqueous sulphuric acid medium. These polymer materials were characterized by chemical analyses, spectral studies (UV-visible and IR), X-ray diffraction and thermal techniques and also by conductivity measurements by four-probe technique. The presence of complex anion in polyaniline material was confirmed by chemical and spectral analyses. The yield and conductivity of metal oxalate doped polyanilines were found to be high when compared to the simple sulphate ion doped polyaniline prepared under similar condition. UV-visible and IR spectral features not only confirmed the polyaniline doping by complex anions but also substantiated their facilitating effect on conductivity. The X-ray diffraction patterns indicated some crystalline nature in metal oxalate doped polyaniline and amorphous in polyaniline sulphate salt. The conductivity of the polymer samples strongly depended on the degree of crystallinity induced by complex counter anions as dopant. All the polymer materials, as evident from TGA curves, were observed to undergo three-step degradation of water loss, de-doping and decomposition of polymer. Further, the thermal stability of polyaniline was found to improve on doping with metal oxalate complex.

Determination of oxygen solubility in aqueous sulphuric acid media

Oxygen is an essential element in numerous hydrometallurgical leaching processes where it acts as a primary or a secondary oxidant in the dissolution of sulphic concentrates. A common acidic leaching medium is sulphuric acid. It was found crucial to define experimentally the solubility and the rate of mass transfer of oxygen in various sulphuric acid environments prior to direct leaching experiments of sulphuric zinc concentrates, and a laboratory scale equipment was developed for the analysis of oxygen concentrations in aqueous acid media. This paper discusses the solubility and dissolution of oxygen as a function of temperature and concentration of sulphuric acid. Also, the effect of temperature (25–55 °C), gas flow rate (0.75–2.25 l/min) and acid concentration (up to 2 M) on the mass transfer coefficient k L a between gas and liquid is evaluated.

The effect of Cu(II) coordination on the structure and electric properties of polyaniline–poly(vinyl alcohol) blend

Polyaniline (Pani) and its blends with poly(vinyl alcohol) (PVA) and PVA+Cu(II) complex were synthesised by in situ chemical oxidative polymerisation technique with potassium perdisulphate (PDS) oxidant in aqueous sulphuric acid medium. The polymer samples were characterised by conductivity, XRD, TGA and spectral (IR and UV–visible) methods. Both conductivity and thermal stability of Pani decreased largely with PVA. XRD and spectral studies revealed that PVA-modified structural and chemical characteristics of Pani through intermolecular hydrogen bonding and thereby effected these changes. In PVA+Cu(II) complex, however, the hydroxyl groups of PVA were tied and as a result the complex as a whole interacted with Pani. Consequently, it redeemed the characteristics of Pani substantially with the additional advantage of imparting higher thermal stability to Pani.

Ruthenium(III) Mediated Cerium(IV) Oxidation of Thallium(I) in Aqueous Sulphuric Acid--A Kinetic and Mechanistic Study

The cerium(IV) oxidation of thallium(I) has been studied in an aqueous sulphuric acid medium. A minute amount (10 6 mol dm 3 ) of ruthenium(III) is sufficient to catalyse the reaction. The reaction is first order in [cerium(IV)] in the presence of ruthenium(III) catalyst. The order in [thallium(I)] varies from zero to unity, whereas that in [ruthenium(III)] is unity. Increase in sulphuric acid concentration accelerates the reaction rate. The added products cerium(III) and thallium(III) do not significantly effect the reaction rate. The active catalyst, substrate and oxidant species are [Ru(H 2 O) 6 ] 3+ , Tl(SO 4 ) and H 3 Ce( SO 4 ) 4 , respectively. Possible mechanisms are proposed and the reaction constants involved have been determined.

Rigorous treatment of time-dependent trace gas uptake by droplets including bulk diffusion and surface accommodation

Absorption of a trace gas by a liquid droplet is simulated by solving the coupled time-dependent diffusion equations for gas and liquid phase. A comprehensive numerical model and a new approximate analytical solution are compared with previous simplified expressions which describe the dependence of gas uptake on solubility, diffusion and mass accommodation. The model and the novel formula are suited for the evaluation of laboratory droplet experiments. Specifically, we re-evaluate measured uptake data of HCl on aqueous sulphuric acid droplets. Our treatment corroborates the earlier finding that the mass accommodation coefficient γ 0 of HCl on aqueous H 2SO 4 is about 0.15, but this value might be influenced by the presence of forced convection and the Stefan flows as well as the inaccuracy in the Henry's law coefficient of HCl.