Effect of a catalyst on the kinetics of reduction of celestite (SrSO4) by active charcoal

Reduction of celestite (SrSO₄) powder with particles of active charcoal has been studied extensively in the absence and presence of catalysts. The optimum temperature at the charging zone has been optimized to get a maximum water-soluble strontium sulfide value. The strontium value has been analyzed using a chemical method, which was verified by the instrumental method using an inductively coupled plasma-optical emission spectrophotometer (ICP-OES). The conversion-time data have been analyzed by using a modified volume-reaction (MVR) model, and the effect of the catalyst on kinetic parameters has been elucidated. It was found that potassium carbonate, potassium dichromate, sodium carbonate, and sodium dichromate catalysts were found to enhance the reaction rate quite satisfactorily in the reduction of the celestite (SrSO₄).     

Water and solute activities in the aqueous H2SO4 -(NH4)2SO4 solutions

The activities of water in H₂SO₄-(NH₄)₂SO₄-H₂O solutions, the concentrations of which ranged from 0.1 to 4.0 mol kg_-1 H₂SO₄ and 0.1 to 5.0 mol kg_-1 (NH₄)₂SO₄, were determined by an isopiestic method at 298 K. The experimentally determined water activities showed considerable deviation from the Zdanovskii rule. The activities of water were not in agreement with those calculated from the Robinson-Bower equation, even though the formation of sulfato-complexes of NH₄(I) and the dissociation of sulfuric acid were taken into consideration. The behavior of H₂SO₄-(NH4)₂SO₄ solu-tions is quite different from other systems. Water activities of aqueous solutions of H₂SO₄-Li₂SO₄ and H₂SO₄-Na₂SO₄ can be calculated from the Robinson-Bower equation with satisfactory accuracy. The interesting behavior of (NH₄)₂SO₄ can be attributed to the water structure-breaking property of NH₄ ⁺ ions. An additivity equation for estimating the water activity of aqueous H₂SO₄ solutions con-taining (NH₄)₂SO₄ was proposed by considering the contributing effects of solutes on the structure of water. The proposed equation satisfactorily described the solution system under investigation. The mean activity coefficients of H₂SO₄ and (NH₄)₂SO₄ in the solution system, H₂SO₄-(NH₄)₂SO₄-H₂O, at 298 K, were calculated by the McKay-Perring method using the water activities experimentally determined. It is believed that the calculated values for concentrated solutions of more than one sol-ute are sufficiently accurate since the mean activity coefficients of H₂SO₄ agree with the values measured by the emf method.     

Transformation of α-Fe2O3 to Fe3O4 Realized by Mechanochemical Reaction of α-Fe2O3 and SrCO3

A transformation from ??-Fe₂O₃ with rhombohedral structure to Fe₃O₄ with spinel face-centered cubic (fcc) structure was realized by ball milling the mixture of SrCO₃ and ??-Fe₂O₃ powders with molar ratio of 1:3 under ambient air in the current work, but it was not observed by ball milling the ??-Fe₂O₃ powders. The transformation is due to a mechanochemical reaction between SrCO₃ and ??-Fe₂O₃ in the milling process, and the ??-Fe₂O₃ catalyzes the decomposition of SrCO₃, while the decomposition promotes the transformation of ??-Fe₂O₃ to Fe₃O₄. X-ray diffraction, X-ray photoelectron spectrometry, and magnetic measurements indicate that the transformation finishes completely upon milling of 120?hours, and the mixture milled for 120?hours consists of Fe₃O₄ and a Sr element. However, Sr does not incorporate into the Fe₃O₄ but exists in grain boundary of the Fe₃O₄ in a form of a Sr-O bond. It is found that the product obtained by ball milling the mixture is different from that prepared by sintering the mixture at atmospheric pressure, which product is SrFeO_2.97 with cubic structure. The mechanism of the transformation and origin of difference in products obtained in the ball milling and sintering processes are also discussed in the current work.     

Coupled process of reaction and solvent extraction: I. The reaction between CO2 and SrCl2 coupled with solvent extraction of HCl

In order to ensure the continual reaction between CO₂ and SrCl₂ to produce SrCO₃, it is necessary to neutralize produced HCl or to remove it to another phase. Coupling the reaction with solvent extraction is a way to lower the acidity of the aqueous solution and to allow the reaction to proceed continually. Based on this coupled process, a new flow sheet for producing strontium carbonate from celestite is proposed. The factors influencing equilibrium of the coupled process of strontium carbonate production with solvent extraction have been studied.     

Chemical conversions of osmium (VI) sulfite complexes in ammonia–sulfate solutions

It has been found that osmium (VI) sulfite complexes having composition [OsO₂(SO₃)₂(H₂O)₂]²⁻ in ammonia and sulfuric acid solutions enter into reaction of intrasphere substitution resulting in the formation of new complexes of osmium (VI). The rate of their formation depends on the concentration of constituents in the system and the temperature of solutions. Water-soluble ammonia–sulfite complexes of osmium (VI) (final form is [OsO₂(SO₃)₂(NH₃)₂]²⁻) are formed in ammonia-sulfate solutions. These complexes are converted into sulfate derivatives—water-soluble ([OsO₂(SO₃)(SO₄)(NH₃)₂]²⁻) and insoluble ([OsO₂(SO₄)₂(NH₃)₂]²⁻) in solutions containing (NH₄)₂SO₄ and H₂SO₄.     

Preparation of Nd(III) carbonate by precipitation stripping of Nd(III)-loaded VA10

The preparation of fine particles of Nd(III) carbonate from kerosene solution, from which Nd(III) was extracted with versatic acid 10 (VA10) by a precipitation stripping technique using an aqueous NH₃-(NH₄)₂CO₃ solution as stripping medium, was studied. In preliminary experiments, we were unable to recover simple Nd(III) carbonate from Nd(III)-loaded VA10 by CO₂ gas bubbling, when water, (NH₄)₂CO₃, NH₄HCO₃, NaHCO₃, or NA₂CO₃ solution saturated with CO₂ was used as the stripping solution. To obtain simple Nd(III) carbonate, it is necessary to use more than the stoichiometric amount of NH₃ compared to VA10 and about 10 times as much (NH₄)₂CO₃ as Nd(III). The solution mixture of NH₃-(NH₄)₂)CO₃ acts as a pH buffer, an adductor for VA10, and a CO ₃ ²⁻ ion source. Although it was concluded that the precipitates are Nd₂(CO₃)₃·xH₂O (x⊧4), their X-ray pattern does not coincide with that quoted by JCPDS. By heating these precipitates, cubic Nd₂O₃ was obtained at 823 K, while, at 973 K, hexagonal Nd₂O₃ was formed. Since the stripping solution consisting of NH₃-(NH₄)₂CO₃ was highly alkaline, VA10 was also stripped in the aqueous phase. To use a closed-circuit system for the precipitation stripping of Nd(III) carbonate from Nd(III)-loaded VA10, it is important to regenerate VA10 in the organic phase. For this purpose, evaporation of NH₃ by air bubbling was studied. By bubbling air into a stripping solution warmed at 333 K, almost all the VA10 can be transferred to the organic phase.     

Leaching of chrysocolla with Ammonia-Ammonium carbonate solutions

Chrysocolla was leached in solutions of ammonium hydroxide and ammonium carbonate as a function of the variables: temperature (25 to 55 ‡C), ammonia-ammonium ratio (0.0:1.0 to 1.0:0.0), total ammonia concentration (0.25 to 6.0 M), and particle size (100 to 400 mesh). A model of the leaching behavior was deduced based on: (1) the activation energy of 60.75 kJ/mole (14.51 kcal/mole) for 3 M total NH₃ which was dependent on both total ammonia concentration and temperature; (2) first-order dependence of rate on [(NH₄)₂CO₃]; (3) dependence of initial reaction rate on reciprocal of particle diameter; and (4) morphological evidence from SEM and ED AX measurements of diffusion and leaching occurring primarily in surface microcracks and not in the submicroscopic pores. In addition to the importance of diffusion through microcracks in rate control chemical reaction at active surface sites to produce the species, CuNH ₃ ^{2 +}, is also important. Only a fraction of the Cu atoms react that are exposed to lixiviant. Higher ammonia-ammonium ion concentrations, higher temperatures, or much longer times are required for more refractory Cu atoms to dissolve. Formerly Graduate Student, Department of Metallurgy and Metallurgical Engineering, University of Utah     

Gradient solid electrolytes for thermodynamic measurements: System Na2CO3-Na2SO4

The thermodynamic properties of Na₂CO₃-Na₂SO₄ solid solution with hexagonal structure have been measured in the temperature range of 873 to 1073 K, using a composite-gradient solid electrolyte. The cell used can be represented as Pt, CO₂′ + O₂∥ Na₂CO₃Na₂ (CO₃)_x(SO₄)_1-x) ∥ CO₂″ + O₂″, Pt The composite-gradient solid electrolyte consisted of pure Na²CO³ at one extremity and the solid solution under study at the other, with variation in composition across the electrolyte. A CO₂ + O₂ + Ar gas mixture was used to fix the chemical potential of sodium at each electrode. The Nernstian response of the cell to changes in partial pressures of CO₂ and O₂ at the electrodes has been demonstrated. The activity of Na₂CO₃ in the solid solution was measured by two techniques. In the first method, the electromotive force (emf) of the cell was measured with the same CO₂ + O₂ + Ar mixture at both electrodes. The resultant emf is directly related to the activity of Na₂CO₃ at the solid solution electrode. By the second approach, the activity was calculated from the difference in compositions of CO₂ + O₂ + Ar mixtures at the two electrodes required to produce a null emf. Both methods gave identical results. The second method is more suitable for gradient solid electrolytes that exhibit significant electronic conduction. The activity of Na₂CO₃ exhibits positive deviation from Raoult’s law. The excess Gibbs’ energy of mixing of the solid solution can be represented using a subregular solution model such as the following: ΔG^E = X(1 - X)[6500(±200)X + 3320(±80)(l - X)] J mol^-1 whereX is the mole fraction of Na₂CO₃. By combining this information with the phase diagram, mixing properties of the liquid phase are obtained.     

Separation of cobalt from nickel in ammonia-ammonium carbonate solutions using pressurized ion exchange

High resolution pressurized ion exchange has been used successfully to study and separate the various cobalt and nickel complexes present in commercial ammonia-ammonium carbonate solutions produced by the Caron process. Using chromatographic elution from Dowex 50W-X8 (15–25 micron) resin with ammonium carbonate solutions, three cobalt species, identified as the purple carbonato tetrammine complex, [Co(NH₃)₄CO₃]⁺, the red carbonato pentammine complex, [Co(NH₃)₅CO₃]⁺, and the yellow hexammine complex [Co(NH₃)₆]³⁺, were separated from a single nickel species. Nickel sorption was found to be a strong function of pH, whereas sorption of the cobalt complexes was essentially independent of pH over a rather wide range, extending from ～pH 7.8 to 10. Distribution ratios for all species increased significantly with decreasing ammonium carbonate concentration. With ammonium carbonate solution at pH 9.5, the complexes were eluted in the following order: [Co(NH₃)₄CO₃]⁺, [Co(NH₃)₅CO₃]⁺, [Ni(NH₃)_6-x(H₂O)_x]²⁺, and [Co(NH₃)₆]³⁺. From 4 M (NH₄)₂CO₃, distribution ratios were 5.0, 7.5, 18, and 75 for the respective complexes identified in the order above. This study points out some of the difficulties and opportunities in developing a viable ion exchange process for the recovery and separation of these metal ions.     

The ion-association-interaction approach as applied to aqueous H2SO4-Al2(SO4)3-MgSO4 solutions at 250 °C

A hybrid ion-association-interaction approach is implemented to describe the chemistry and thermodynamics of aqueous H₂SO₄-Al₂(SO₄)₃-MgSO₄ solutions at 250 °C. These solutions are relevant to the sulfuric acid pressure leaching of nickeliferous laterites. Strong complexes in solution are handled via the ion-association approach. Nonidealities, including weak ion pair formations, are treated through the Pitzer ion-interaction theory. The existing complexes in solution and the Pitzer ion-interaction parameters were identified through processing solubility data in the binary (H₂SO₄-Al₂(SO₄)₃ and H₂SO₄-MgSO₄) as well as the ternary (H₂SO₄-Al₂(SO₄)₃-MgSO₄) electrolyte solutions at or near 250 °C. The existing aqueous aluminum-bearing species identified were Al³⁺, Al(SO₄)⁺, and Al₂(SO₄) ₃ ⁰ , with Al₂(SO₄) ₃ ⁰ as the dominant species at moderate to high H₂SO₄ concentrations. The existing aqueous magnesium-bearing species found were Mg²⁺ and MgSO ₄ ⁰ , with Mg²⁺ being dominant except at low concentrations of H₂SO₄. The dominant species identified for Al and Mg explain why a higher H₂SO₄ concentration in solution is required during the processing of high-magnesium laterites.     

Effect of distribution of B4C on the mechanical behaviour of Al-6061/B4C composite

In this work, the effect of mixing parameters on the distribution of B₄C in 6061-Al alloy and its correlation with mechanical behaviour was studied. 6061-Al alloy powder was mixed with 10 mass-% B₄C powder in a ball mill and powder rotator mixer by varying mixing time from 1 to 5?h. Mixing was performed in both wet and dry conditions in a ball mill while only dry condition was used in the powder rotator mixer. The green compacts were sintered at 630°C. The quadrat method was used to quantify the distribution of B₄C particles in the microstructures of sintered Al/B₄C composite. The results showed that the distribution was improved with mixing time but the density, hardness and compression strength of Al/B₄C composites were reduced with time during ball milling. On the other hand, the distribution of reinforcement, density, hardness and compressive strength of Al/B₄C composites was improved with mixing time in the powder rotator mixer.     

Effects of iron and temperature on solubility of light rare earth sulfates in multicomponent system of Fe2(SO4)3-H3PO4-H2SO4 synthetic solution

Numerous light rare earth elements (LREE) minerals containing Fe and P were processed by sulfuric acid roasting method, and the leaching solution mainly comprises LREE sulfate, Fe₂(SO₄)₃, H₃PO₄, and H₂SO₄, however, the solubility data of LREE sulfates in this system is few. This work studies the solubility of LREE sulfates in independent LREE sulfate system RE₂(SO₄)₃-Fe₂(SO₄)₃-H₃PO₄-H₂SO₄ (RE = La, Ce, Pr or Nd) and mixed LREE sulfates system (La,Ce,Pr,Nd)₂(SO₄)₃-Fe₂(SO₄)₃-H₃PO₄-H₂SO₄ at different temperature (25–65 °C) and concentrations of Fe₂(SO₄)₃ (Fe₂O₃, 0–50.13 g/L), H₂SO₄ (0.5 mol/L), and H₃PO₄ (P₂O₅, 20.34 g/L) based on the industrial operating condition at low liquid and solid ratio 2:1. The solubility of each LREE sulfate in the independent system (La₂O₃, 12.25–20.88 g/L; CeO₂, 41.93–62.35 g/L; Pr₆O₁₁, 37.34–56.69 g/L; Nd₂O₃, 26.60–37.63 g/L) is much higher than that of the mixed system (La₂O₃, 6.95–11.03 g/L; CeO₂, 10.63–21.51 g/L; Pr₆O₁₁, 11.56–20.36 g/L; Nd₂O₃, 12.36–19.79 g/L) under the same other conditions. The results also indicate that, in the two systems, both Fe and the temperature have negative effects on the solubility of LREE sulfates. That may occur due to the complication reactions between the complexes of RESO₄⁺ and Fe(SO₄)₂^–. However, the influence degree of temperature and iron concentration on the LREE sulfates solubility varies in the two systems and among different LREE species. This research is of theoretical significance for optimizing the conditions of the sulfuric acid process for recovering the LREE from the mixed LREE bearing minerals as well as the single LREE containing secondary rare earth scraps.     

Sm-MnOx catalysts for low-temperature selective catalytic reduction of NOx with NH3: Effect of precipitation agent

A series of Sm-Mn mixed oxide catalysts were prepared via precipitation using various precipitants,namely Na₂CO₃(NH₄)₂CO₃,and NH₃·H₂O,and evaluated for the selective catalytic reduction(SCR) of NO_x with NH₃ at low temperatures.Various characterisation techniques were used to determine the physicochemical properties of the catalysts,and it is found that their catalytic performance is greatly influen...     

Oxidation-reduction equilibrium of Cu2+/Cu+ in binary alkaline sulfate melts

Oxidation-reduction equilibria for a Cu²⁺/Cu⁺ couple in binary alkaline sulfate melts, Li₂SO₄ + R₂SO₄ (R = Na, K, Rb, Cs), were determined at temperatures of 973, 1023, and 1073 K by equilibrating these melts with gas mixtures of Ar + O₂ + SO₂. RedOx equilibria are well interpreted by the average ionic radii of alkaline metals: r(average) = X(Li₂SO₄) r(Li) + X(R₂SO₄) r(R), wherer andX denote, respectively, mole fraction and the ionic radii of alkaline metal. The oxygen anion activities would increase with an increase in R₂SO₄ mole fractions of binary sulfates Li₂SO₄ + R₂SO₄.     

Dissolution kinetics of malachite in ammonia/ammonium carbonate leaching

The leaching of oxide copper ore containing malachite, which is the unique copper mineral in the ore, by aqueous ammonia solution has been studied. The effect of leaching time, ammonium hydroxide, and ammonium carbonate concentration, pH, [NH₃]/[NH₄⁺] ratio, stirring speed, solid/liquid ratio, particle size, and temperature were investigated. The main important parameters in ammonia leaching of malachite ore are determined as leaching time, ammonia/ammonium concentration ratio, pH, solid/liquid ratio, leaching temperature, and particle size. Optimum leaching conditions from malachite ore by ammonia/ammonium carbonate solution are found as ammonia/ammonium carbonate concentrations: 5 M NH₄OH+0.3 M (NH₄)₂CO₃; solid/liquid ratio: 1:10 g/mL; leaching times: 120 min; stirring speed: 300 rpm; leaching temperature: 25 °C; particle size finer than 450 μm. More than 98% of copper was effectively recovered. During the leaching, copper dissolves as in the form of Cu(NH₃)₄⁺² complex ion, whereas gangue minerals do not react with ammonia. It was determined that interface transfer and diffusion across the product layer control the leaching process. The activation energy for dissolution was found to be 15 kJ mol⁻¹.     

A novel synthesis method and up-conversion properties of hexagonal-phase NaYF4:Er nano-crystals

A novel synthesis method for hexagonal(β)-phase NaYF4:Er nano-crystals(NCs)which showed up-conversion(UC)from infrared to visible spectral region was developed.The NaYF4:Er NCs were synthesized in oleic acid(OA)and 1-octadecene(ODE)with Y2(CO3)3· xH2O,Er2(CO3)3· xH2O,Na2CO3 and NH4F as precursors.This proposed method was simple and less toxic compared with generally used method so far.The XRD results showed that the molar ratio of OA/ODE and the temperature were key factors for phase control of NaYF4:Er NCs.The UC emission spectra were obtained with the emission wavelength at about 980 nm(4I11/2→4I15/2),800 nm(4I9/2→4I15/2),660 nm(4F9/2→4I15/2)and 540 nm(4S3/2→4I15/2)from Er3+ ions,by excitation wavelength of 1550 nm.The slope values,n,in the pump-power dependence,showed that the emission at 980 and 800 nm were generated by 2-step UC and at 660 nm and 540 nm were 3-step UC.The optical process for the UC excitation was discussed.     

Activities of water and solutes in the solution system H2SO4−Cr2(SO4)3−H2O

The activities of water in the solution systems of Cr₂(SO₄)₃−H₂O and H₂SO₄−Cr₂(SO₄)₃−H₂O were determined at 298 K by an isopiestic method. Properties of the green and violet isomers of Cr₂(SO₄)₃ were considered separately. The activities of water in aqueous solutions containing violet Cr₂(SO₄)₃ alone are considerably smaller than those of the green isomer. The water activities of aqueous H₂SO₄ solutions containing the violet isomer of Cr₂(SO₄)₃ satisfactorily obey the Zdanovskii rule, while those of the green isomer show an upward deviation from the Zdanovskii linearity. The activities of water in these solution systems were also calculated, using the Robinson-Bower empirical equation, and showed satisfactory agreement with the calculated values, whether the formation of the sulfato-complex, CrSO ₄ ⁺ , and the dissociation of H₂SO₄ were taken into consideration or not. The mean activity coefficients of H₂SO₄ and Cr₂(SO₄)₃, for the solution systems studied in this work, were calculated by applying the McKay-Perring method and its modified form, using the measured water activity data of these solution systems. The concentration dependencies of the mean activity coefficients of H₂SO₄ and Cr₂(SO₄)₃ in the solutions containing the green isomer are quite different from those of the violet isomer. Particularly, it was found that the activity values of water and solutes in the solution system of H₂SO₄-violet Cr₂(SO₄)₃−H₂O are similar to those of the solution system of H₂SO₄−Fe₂(SO₄)₃−H₂O. formerly Graduate Student with Kyoto University     

Simulation of in situ uraninite leaching. part I: A partial equilibrium model of the NH4HCO3-(NH4)2CO3-H2O2 leaching system

In situ leaching of uraninite and calcite by H₂O2-NH₄HCO₂-(NH₄)₂CO₃ solutions has been simulated using a partial equilibrium model which incorporates a one-parameter mixing cell model of solution flow. Rate laws for UO₂ dissolution and for CaCO₂ dissolution/precipitation were taken from the literature, as were equilibrium constants for solution phase reactions. Parameters of the model include the UO₂ and CaCO₃ ore grades, the concentrations of the H₂O₂, NH₄HCO₃, and (NH₄)₂CO₃ components, porosity, exit solution flow rate, ore and mineral densities, and mineral rate constants and surface areas. Mineral conversions, component and species concentrations, and porosity are among the time-dependent quantities calculated using the model. For the conditions simulated, calcite dissolved somewhat faster than uraninite. The results emphasize the importance of the coupling between the mineral reactions and solution flow. Changes in the concentrations of the CO ₃ ^2- and HCO ₃ ^- species are particularly complicated and not predictable from the calcite kinetics alone or from a purely equilibrium model; although the simulations did not reveal any conditions under which the solution would become saturated with CaCO₃, the pH continued to change throughout the calcite dissolution and is buffered only after calcite has been consumed.     

Catalytic ozonation of NH4+-N in wastewater over composite metal oxide catalyst

Large amounts of water containing-ammonium nitrogen(NH₄⁺-N)have attracted increasing attention.Catalytic ozonation technology,involving the generation of hydroxyl radical(OH)with strong oxidation ability,was originally utilized to degrade organic-containing wastewater.In this paper,Ce/MnOx composite metal oxide catalysts prepared with different preparation conditions were used to degrade wastewater containing inorganic pollutant(NH₄⁺-N).The as-prepared catalyst features were characterized using X-ray diffraction(XRD),Brunauer-Emmett-Teller method(BET),scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS),Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS)and H₂-temperature programmed reduction(H₂-TPR)techniques.The results show that the catalyst,prepared by conditions with precipitant Na₂CO₃ and Ce/Mn molar ratio 1:2 calcined at 400℃for 3 h in pH 11.0,displays the optimal performance,with the removal rate of NH₄⁺-N and selectivity to gaseous nitrogen,88.14 wt%and 53.67 wt%,respectively.The effects of several operating factors including solution pH,initial NH₄⁺-N concentrations and scavengers were evaluated.In addition,XRD patterns of catalyst with the best performance and the comparative study on decontamination of NH₄⁺-N by various processes(O₃,catalyst and catalyst/O₃)show that the primary metal oxides are CeO₂ and MnO₂ in Ce/MnOx composite metal oxide catalysts,which have a synergistic effect on the catalytic ozonation of NH₄⁺-N,and the new phase MnO₂ plays a great role.After 5 consecutive use cycles,the degradation efficiency is declined slightly,and can still achieve better than 70 wt%over 1 h reaction.Additionally,the application of catalytic ozonation for actual wastewater on the removal rate of NH₄⁺-N was investigated.Possible mechanism and degradation pathway of NH₄⁺-N were also proposed.In a word,the application of CeO₂-MnO₂ composite metal oxide catalysts in catalytic ozonation can be regarded as an effective,feasible and promising method for the treatment of NH₄⁺-N.     

Water and solute activities of H2SO4-Fe2(SO4)3-H2O and HCl-FeCl3-H2O solution systems: Part II. Activities of solutes

The mean activity coefficients of solutes such as H₂SO₄ and Fe₂(SO₄)₃ in the solution system H₂SO₄-Fe₂(SO₄)₃-H₂O, and HCl and FeCl₃ in the solution system of HCl-FeCl₃-H₂O at 298 K were calculated by the McKay-Perring method using the water activities of these solution systems. The mean activity coefficients calculated by the McKay-Perring method were sufficiently accurate for use on concentrated mixed solutions. A Gibbs-Duhem equation, which is applicable for calculating mean activity coefficients of one component using the activity data of the other components, was also used to calculate the mean activity coefficient of Fe₂(SO₄)₃ in aqueous solutions of H₂SO₄-Fe₂(SO₄)₃ by using both the activity of water measured by an isopiestic method and that of H₂SO₄ determined by the McKay-Perring method. The Gibbs-Duhem method was more tedious for calculation than the McKay-Perring method, and it required thermodynamic data other than water activities.