Sorption properties of indium oxohydroxides

The influence of the pH of precipitation and of the ionic medium on the sorption properties (the surface charge and the kinetic characteristics of sorption) of indium oxohydroxides obtained by alkaline hydrolysis of indium nitrate was investigated. It was found by the method of zero charge point, by the indicator reactions of heterogeneous hydrolysis, by drop titration, and by thermography that In(OH)₃·0.5 H₂O·(0.2–0.3) NaOH with a very small degree of crystallinity is formed at pH>6. The basic properties of the oxohydroxide increase with increasing pH of precipitation up to 9–10 and then remain constant. The rates of the indicator reactions depend slightly on the pH of precipitation. The sorption properties of indium oxohydroxides differ markedly from those of ferrogels obtained under similar conditions. The results obtained can be explained by the formation of quasistable microstructures during the precipitation of indium oxohydroxide.     

Sorption properties of zirconogels

The effect of the pH of precipitation and the nature of the initial zirconium salt and the ionic medium on sorption properties (surface charge, kinetic parameters of sorption) of zirconogels formed due to alkaline hydrolysis of zirconium(iv) sulfate and zirconyl nitrate was studied. Zero point of charge methods, indicator reactions of heterogeneous hydrolysis, drop titration, and thermography were used in the study. The properties of the zirconogels are very close to those of Ferro- and titanogels obtained under similar conditions. The composition of zirconogels, the activation energies of the indicator reactions of heterogeneous hydrolysis, and the dependences of the rate constants of these reactions on the nature and concentration of the ionic medium were determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2653–2657, November, 1996,     

Sorption of [Fe(CN)6]3− and [Fe(CN)6]4− ions on the surface of Fe(III), Cr(III), and Zr(IV) oxyhydroxide hydrogels from aqueous solutions

The sorption of [Fe(CN)₆]^3? and [Fe(CN)₆]^4? anions on the surface of Fe(III), Cr(III), and Zr(IV) oxyhydroxide hydrogels at various pH values of hydrogel precipitation from solutions without a support electrolyte and from NaCl and Na₂SO₄ solutions with an ionic strength of 0.5 was studied. It was found that isotherms of sorption of [Fe(CN)₆]^3? and [Fe(CN)₆]^4? anions from solutions without a support electrolyte and from NaCl solutions and those of sorption of [Fe(CN)₆]^4? from Na₂SO₄ solutions are described by the Langmuir equation. It was established that the sulfate background suppresses the sorption of [Fe(CN)₆]^3? on Fe(III) and Zr(IV) oxyhydroxides. Both anions are sorbed only when the surface of the oxyhydroxides is charged positively; the Langmuir equation parameters A _max and K tend to decrease to the point of zero charge as the pH value of oxyhydroxide precipitation increases. An electrostatic mechanism of the sorption of [Fe(CN)₆]^3? and [Fe(CN)₆]^4? anions was suggested.     

Sorption properties of hydrogels of transition and p-metal oxide hydroxides

The experimental data on the sorption properties of hydrogels of Fe^III, Ti^IV, Zr^IV, Sn^IV, In^III, Cr^III, Sm^III, Co^II, and Zn^II oxide hydroxides are summarized. The data were obtained by the point of zero-charge method and by measurin the rates of the indicator reaction,viz., heterogeneous hydrolysis of the chloro complexes of platinum group metals. The compositions of the oxide hydroxides, the pH of the point of zero charge in various ionic media, and the rate constants of the indicator reactions were determined. The values obtained were analyzed in terms of their correlation with the charge, the radius, and the electronic struture of the central ion and with the composition and the preparation conditions of the oxide hydroxide. A data base that allows one to choose an appropriate oxide hydroxide sorbent for extraction of particular complex-forming metals from complexing media is presented. The data obtained confirm the model of the mechanism of heterogeneous hydrolysis and extend its predicting ability. The model was tested experimentally in relation to tin(IV) oxide hydroxide. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 228–238, February, 1999.     

Estimation of oxyhydroxide specific surface area using the amounts of OH groups adsorbed

A procedure for pH-metric determination of the limiting adsorption of OH groups (A _OH) by Fe^III, Zr^IV, Cr^III, and In^III oxyhydroxide hydrogels from 0.1 and 1.0 M solutions of NaCl is described. Data on the molecular area occupied by a single OH group on the hydrogel surface (S _OH) and the S _spec values, which were calculated from A _OH and S _OH, are presented. The S _spec value does not depend on the pH of hydrogel precipitation; the true S _spec value can be determined only from sorption of the OH groups at the actual point of zero charge of the hydrogel. The A _OH values for hydrogels were found to change only slightly during aging of hydrogels in electrolyte solutions.     

氢氧化铝镁钠米颗粒的零电荷点及电荷密度研究

探讨了电位滴定(PT)法测定的零电荷点的物理意义, 认为是零净电荷点(ZPNC)。并对零净电荷点pH(pHZPNC), 零可变电荷点pH(pHZPVC), 永久电荷密度(σP), 可变电荷密度(σV)和净电荷密度(σT)之间的关系进行了理论分析。用PT法测定了氢氧化铝镁纳米颗粒的pHZPNC和σP, 探讨了电解质浓度和pH对各电化学性质的影响规律。另外, 还考察了CO3^2^-对PT法测定结果的影响。     

Comparison of the sorption properties of crystalline and amorphous samarium(III) oxohydroxides

The influence of the pH of precipitation (pH₁) and the ionic medium on the sorption properties of as-precipitated samarium(III) oxohydroxides,i.e., the pH of zero charge point and the rate of heterogeneous hydrolysis of the IrCl₆ ²⁻, RhCl₆ ³⁻, and PtCl₄ ²⁻ complexes, was studied. The composition of precipitates was studied by the drop titration of solutions of samarium nitrate and thermography. It was found that as-predipitated samarium oxohydroxides are amorphous and the substitution of NO₃ ⁻ ions by OH⁻ is not complete even at pH₁ 11. Heterogeneous hydrolysis occurs on the surface of samarium oxohydroxide, and its rate increases with increasing pH₁. The as-precipitated samarium oxohydroxides have much higher sorption activities than crystalline Sm₂O₃, and their activities are similar to those of ferrogels. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 583–588, April, 1998.     

Electrometric indicators in the amperometric titration of cations and acids

Amperometric titration is carried out at zero potential in the presence of hydroquinone and p-aminophenol as electrometric indicators. Solutions of sodium carbonate, sodium arsenite, sodium tetraborate and potassium cyanide have been used as reagents in neutralisation, precipitation and complex formation reactions for the determination of cations separately, the simultaneous determination of several cations in one sample and of acid and cations present simultaneously. After the equivalence point has been reached, hydroxyl ions appear in the solution as a result of salt hydrolysis and a considerable increase in the current voltage occurs. The indicator in alkaline solution induces the depolarisation of the electrode.     

Determination of Point of Zero Charge of Tunisian Kaolinites by Potentiometric and Mass Titration Methods

This paper deals with determining points of zero charge of natural and Na⁺‐saturated mineral kaolinites using two methods: (1) acid‐base potentiometric titration was employed to obtain the adsorption of H⁺ and OH^? on amphoteric surfaces in solutions of varying ionic strengths in order to determinate graphically the point of zero net proton charge (PZNPC) defined equally as point of zero salt effect (PZSE); (2) mass titration curve at different electrolyte concentrations in order to estimate PZNPCs by interpolation and to compare with those determined by potentiometric titrations. The two methods involved points of zero charge approximately similar for the two kaolinites between 6.5‐7.8, comparable to those reported previously and were in the range expected for these clay minerals. The comparison of potentiometric surface titration curves obtained at 25 °C and those published in the literature reveals significant discrepancies both in the shape and in the pH of PZNPCs values.     

Changes in the adsorption activity of metal oxyhydroxide hydrogels during their ageing in electrolyte solutions

Adsorption properties of Ti(IV), Zr(IV), In(III), Cr(III) and Fe(III) oxyhydroxides aged in sodium chloride and sulphate solutions have been studied. The main research method was determination the rates of heterogeneous hydrolysis reactions for RhCl₆³⁻ and IrCl₆²⁻ complexes. The rate constants of these reactions were a measure of adsorption activity of the oxyhydroxides on which surface they occur. For comparison the corresponding data for freshly formed oxyhydroxides are presented. It was found that adsorption activity (the values of rate constants K_obs) of ferro- and indiogels decrease slowly with increasing of duration and temperature of ageing. The sulphate electrolyte promotes the decrease of sorption activity more than chloride electrolyte. In contrast, the samples of titano- and chromogels aged during 2 h at 80°C have much greater adsorption activity than freshly formed ones, and this activation holds for a very long time. The zirconogel keeps in these conditions stable adsorption activity for a very long time. Apparently, after precipitation the chromo-, titano- and zirconogels some metastable non-crystalline or micro-crystalline structures stable to further ageing form rapidly. The oxyhydroxides behaviour is obviously connected with the electronic structure of outer electronic shell of their central ions. The chromium(III) oxyhydroxide with a unsymmetrical, partially filled 3d-sublevel is the most effective sorbent and has the most stable sorption properties.     

An experimental comparison between the aqueous pH scale and the electron donicity scale

We have made an experimental comparison of the aqueous pH scale with the Lewis base donicity scale. This requires an indicator that can function in both kinds of system. We find that the zeta-potential zero charge point for certain oxide materials provides a suitable indicator. The materials are: SiO₂, Al₂O₃, TiO₂, and MgO. For each material, we measure the zeta potential in aqueous media as a function of pH. In each case, the zeta potential varies systematically with pH, and changes sign at some value of pH, that we refer to as the zero charge point. We then do similar measurements in a series of organic liquids having different donicities. In this case, the zeta potential varies systematically with the donicity and may change sign. In such cases, we determine the donicity that corresponds to the zero charge point. For a given oxide material, we can then relate a zero charge point on the pH scale to one on the donicity scale. This gives us a consistent experimental alignment of the pH and donicity scales, relative to one another. We compare our results with thermodynamic data, relating to the pH scale, and the hydrogen electrode, and find good agreement.     

Changes in the acid-base properties of hydrogels of metal oxide hydroxides induced by aging in solutions of electrolytes

Changes in the acid-base properties of titanium(IV), zirconium(IV), iron(III), chromium(III), and indium(III) hydrogels of oxide hydroxides induced by aging in sodium chloride and sulfate solutions were studied by the point of zero charge method. On aging in a solution of the sulfate electrolyte the hydrogels loose their basic properties much more rapidly than in the chloride solution. The most changes were observed in the region of high pH of the point of zero charge of the hydrogels. The changes in the acid-base properties can be explained by a decrease in the content of surface OH groups with are displaced by the supporting electrolyte ions, and due to the oxolation of the hydrogels. Published inIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1329–1332, August, 2000.     

Study on electric property at interface of positive sol of magnesium aluminum hydroxide

The isoelectric point (IEP) and zero point of charge (ZPC) of magnesium aluminumhydroxide were studied by electrophoresis method,potentiometric titration method,and elemental analysis.Results showed that the charge of magnesium aluminum hydroxide was composed of variable charge and permanent positive charge.Because of the permanent positive charge,the IEP and ZPC obtained were higher than calculated.The IEP decreased and ZPC increased as X increased.     

Copper(II) ion hydrolysis in aqueous solution

A copper(II) ion-selective-electrode potentiometric method was used to determine the first and second hydrolysis constants of Cu²⁺. Special techniques prevented copper(II) hydroxide precipitation, and copper(II) carbonate and cipper(II) organic complexation during the titration of the experimental solution over the pH range 6.8–8.4. The large change in the total copper concentration during the titration due to adsorption of copper onto the vessel walls was accounted for by measuring the total copper concentration at each pH by atomic absorption spectrophotometry. The two hydrolysis constants were determined at 25°C in 0.7 and 0.05m NaClO₄ media. The measured stability constants are independent of the copper concentration and yield similar zero ionic strength values. Also, the stepwise equilibrium constants decrease as the ligand number increases.     

Investigation of pH-dependent gel structure in amorphous carbonate-containing aluminum hydroxide by viscometry, fiber optic doppler anemometry, and freeze fracture/freeze etch scanning electron microscopy

Particle interactions in amorphous carbonate-containing aluminum hydroxide suspensions were studied by rotational viscometry, fiber optic Doppler anemometry, and freeze fracture/freeze etch scanning electron microscopy. The particle interactions were pH dependent; however, they were not symmetrical in relation to the point of zero charge. Particle networks were present further from the point of zero charge when the pH was above the point of zero charge. It is hypothesized that negatively charged edge groups, which exist when the pH is above the point of zero charge, interact with the hydroxyls on the faces of the sheet-like particles more strongly than do uncharged or positive edge groups, thus producing the observed asymmetrical behavior of pH-dependent properties.     

Properties of aluminated silica sols

The properties of colloidal dispersions of “Positive Sol 130M” (E. I. du Pont de Nemours and Co.), which contains silica particles coated with alumina, have been studied in detail. Drifts of pH, leaching of aluminum species, and changes of electrokinetic mobility occurred during aging at room temperature for periods of up to a month, and were attributed to surface reactions. Despite its name, the sol has a point of zero charge (PZC) at pH 9, and consequently the particles carry a positive charge only below this pH. The alumina of the coating dissolves in acidic environments; the extent of leaching of aluminum species is dependent upon pH and length of aging at room temperature. The effect of different anions on this process was examined by using various inorganic acids; it was shown that phosphate and sulfate ions exhibit the strongest effects. The removal of alumina changed the surface characteristics as well as the stability of the sol 130M. The mobilities of the particles were found: to be directly related to the composition of the particle surface, regardless of the time of aging or the pH.     

Studies on Zero Point of Charge and Permanent Charge Density of Mg-Fe Hydrotalcite-like Compounds

The theoretical analysis on the zero point of charge (ZPC) and charge density of colloidal particle possessing permanent charges indicates that ZPC determined directly by means of potentiometric titration (PT) should be zero point of net charge (ZPNC) and the permanent charge density (σ_P) can be obtained from the adsorption amount of H⁺ and OH^-(Γ_OH--Γ_H+) at ZPNC. ZPNC does not change with the electrolyte concentration while the zero point of variable charge (ZPVC) changes with the electrolyte concentration. When σ_P is zero, ZPNC equals to ZPVC, and only under this condition is ZPC measured directly by PT equal to ZPVC. The relationship between ZPNC, ZNVC, σ_P, the variable charge density (σ_V)5 or the total net surface charge density (σ_T) with pH or electrolyte concentration is derived.     

Surface charge and adsorption of Na+ and Cl− ions at the zirconium dioxide/water interface

The surface charge and adsorption densities of Na⁺ and Cl^– ions at the zirconium dioxide/electrolyte interface have been determined as a function of pH for 0.1, 0.01 and 0.001 M solutions of NaCl. Using potentiometric titration of the surface hydroxy groups, it was found that the point of zero charge occurred at pH 4.3±0.15. The results are discussed in terms of site binding model of the electric double layer. The ionization and complexation constants have also been determined.     

The point of zero charge of manganese dioxides

Chemically prepared δ-MnO₂ (sample A) was characterised by chemical and size analyses and by t.g.a. and d.t.a. Sample B was the well-characterised International Common Sample ICS 5, which is a chemical λ-MnO₂. Potentiometric acid/base titration of sample A gave a point of zero charge, p.z.c., at pH 3.3±0.5, which is close to the isoelectric point obtained from sedimentation-coagulation results. Fast acid/base titration of sample B gave a p.z.c. at pH 5.9±0.3, which is close to the isoelectric point estimated from electrophoretic measurements. Temperature in the range 298–346 K (25°–73°C) had a minor effect on the titration results.     

Stoichiometry-Controlled Synthesis of Nanoparticulate Mixed-Metal Oxyhydroxide Oxygen Evolving Catalysts by Electrochemistry in Aqueous Nanodroplets

Mixed-metal oxyhydroxides—especially those of Ni and Fe—are one of the most active classes of materials known for catalyzing the oxygen evolution reaction (OER). Here, nanoparticulate mixed metal oxyhydroxides (of Ni, Fe, and Co) were prepared on an electrode surface by electrochemical reaction of a precursor solution encapsulated in aqueous nanodroplets (AnDs), with each of the droplets containing 10 s of attoliters of fluid. Electrode reactions and synthesis can be monitored in situ by electrochemistry as single AnD stochastically lands and interacts with the working electrode. Resultant metal oxyhydroxide nanoparticles can be size and composition controlled precisely by modulating the precursor solution stored in the AnD. Nanoparticulate metal oxyhydroxides were implemented as catalysts for the OER and exhibited superior catalysis compared to their thin-film counterparts, demonstrating a hundred-thousand-fold enhancement in atom efficiency at comparable turnover rates.