Studies on the recovery of Pu(IV) from hydrochloric acid-oxalic acid solutions using an anion exchange method

Sorption of Pu(IV) from hydrochloric acid-oxalic acid solutions has been investigated using different anion exchangers, viz., Dowex-1X4, Amberlite XE-270 (MP) and Amberlyst A-26 (MP) for the recovery of plutonium from plutonium oxalate solutions. Distribution ratios of Pu(IV) for its sorption on these anion exchangers have been determined. The sorption of Pu(IV) from hydrochloric acid solutions decreases drastically in the presence of oxalic acid. However, addition of aluminium chloride enhances the sorption of plutonium in the presence of oxalic acid, indicating the feasibility of recovery of plutonium. Pu(IV) breakthrough capacities have been determined with a 10 ml resin bed of each of these anion exchangers at a flow rate of 60 ml per hour using a solution of Pu(IV) with the composition: 6M HCl+0.05M HNO₃+0.1M H₂C₂O₄+0.5M AlCl₃+100 mg.l^–1 Pu(IV). The 10% Pu(IV) breakthrough capacities for Dowex-1X4, Amberlite XE-270 (MP) and Amberlyst A-26 (MP) are 15.0, 8.9 and 6.2 g of Pu(IV) l^–1 of resin respectively.     

Synthesis and applications of poly(acryl<Emphasis Type="Italic">p</Emphasis>-aminobenzenesulfonamideamidine-<Emphasis Type="Italic">p</Emphasis>-aminobenzenesulfonylamide) chelating fiber for pre-concentrating and separating trace Bi(III), Hg(III), Au(III) and Pd(IV) from solution samples

Poly(acrylp-aminobenzenesulfonamideamidine-p-aminobenzenesulfonylamide) chelating fiber containing "S", "N", and "O" elements was synthesized from polyacrylonitrile fiber and p-aminobenzene sulfonamide and used to enrich and separate trace Bi(III), Hg(III), Au(III), and Pd(IV) ions from wastewater and ore sample solution. The enrichment acidity, flow rate, elution conditions, reuse, interference ions, saturated adsorption capacity, constant of adsorption rate, analytical accuracy, and actual samples on chelating fiber were investigated by means of inductively coupled plasma optical emission spectrometry (ICP-OES) with satisfactory results. Solutions of 100 ng mL^–1 of Bi(III), Hg(III), Au(III), and Pd(IV) ions can be enriched quantitatively by this chelating fiber at a rate of 1.0 mL min^–1 at pH 4 and desorbed quantitatively with 20 mL of 0.25 M HCl and 2% CS(NH₂)₂ solution at 50 °C (with recovery 97%). When the chelating fiber was reused for 20 times, the recoveries of the analyzed ions enriched by the fiber were still over 95% (except for Hg(III)). One thousand-fold excesses of Mn²⁺, Ca²⁺, Zn²⁺, Mg²⁺, Fe³⁺, Cu²⁺, Ni²⁺, Al³⁺, and Ba²⁺ ions and thousands-fold excesses of Na⁺ and K⁺ cause little interference in the pre-concentration and determination of the analyzed ions. The saturated adsorption capacity of Bi(III), Hg(III), Au(III), and Pd(IV) was 4.850×10^–4, 3.235×10^–4, 2.807×10^–4, and 3.386×10^–4 mol g^–1, respectively. The constants of adsorption rate were 0.409 min^–1 for Bi, 0.122 min^–1 for Hg, 0.039 min^–1 for Au, and 0.080 min^–1 for Pd. The relative standard deviations (RSDs) for the enrichment and determination of 10 ng mL^–1 Bi(III), Hg(III), Au(III), and Pd(IV) were lower than 2.3%. The results obtained for these ions in actual samples by this method were basically in agreement with the given values with average errors of less than 1.0%. FT-IR spectra shows that the existence of –SO₂–Ar, –H₂N–Ar, O=C–NH–, HN=C–NH–, and –HN–SO₂ functional groups are verified in the chelating fiber. From the FT-IR spectroscopy, we can see that Hg(III), Au(III), and Pd(IV) are mainly combined with nitrogen and sulfur (or oxygen), and Bi(III) is mainly combined with nitrogen (or oxygen) of the groups to form a chelating complex.     

Recovery of plutonium from phosphate containing aqueous analytical waste solutions using macroporous anion exchange resin

Distribution ratios of Pu(IV) between 7.5M HNO₃+0.75M H₃PO₄+0.3M H₂SO₄ media and a macroporous anion-exchange resin Amberlyst A-26 (MP) increased from 40 to 250 when 1M aluminium nitrate was added to the aqueous medium. When 1M ferric nitrate was used in place of aluminium nitrate the distribution ratio further increased to 850. The 10% Pu(IV) breakthrough capacities with a 5 ml bed resin column, using synthetic feed solutions containing 1M aluminium nitrate, were 1.4 g l^–1, 3.2 g l^–1 at flow rates of 30 ml per hour and 10 ml per hour, respectively. The corresponding 10% Pu(IV) breakthrough capacities in the presence of 1M ferric nitrate were 8.5 g l^–1 and 12.8 g l^–1. More than 97% of plutonium could be recovered from actual analytical phosphate waste solutions.     

Studies on the sorption of Pu(IV) on alumina microspheres from nitric acid-oxalic acid solutions

Sorption of Pu(IV) on alumina microspheres prepared by the sol-gel procedure has been investigated for the recovery of plutonium from nitric acid-oxalic acid solutions. Distribution ratio for Pu(IV) between alumina microspheres and nitric acid-oxalic acid have been determined. The influence of the mode of preparation and heat treatment of these microspheres, on the sorption of Pu(IV) have been investigated. Pu(IV) breakthrough capacities have been determined using a 5 ml bed of alumina with solutions of Pu(IV) in 1M HNO₃+0.05M H₂C₂O₄ and 0.5M HNO₃+0.05M H₂C₂O₄. The elution behavior of Pu(IV) loaded on these microspheres were studied using nitric acid solutions containing different oxidising and reducing agents. Investigations were also carried out to fix the activity in the alumina matrix by heat treatment.     

Static and Kinetic Studies on the Adsorption Behavior of Sulfadiazene

To investigate the nature of interactive forces between sulfadiazene molecules and alumina surface the experiments were performed for the adsorption of sulfadiazene (SD) from its aqueous sulution onto the alumina surfaces at 25 ± 0.2°C and the influence of factors such as increasing concentration of SD (4.0–20.0 × 10^–3 mol cm^–3), the time required for adsorption equilibrium, pH (2.0–12.0) and temperature (5–45°C) of the adsorption medium, the presence of ions like Cl^–, SO^2– ₄ and PO^3– ₄ (0.01–0.30 M) and organic solvents (5% v/v) were observed on the course of adsorption of SD. Various adsorption and kinetic parameters such as adsorption coefficient, the rate constants for adsorption and desorption were also evaluated. The results of the above cited studies facilitated to formulate the mechanisms of interaction between SD and alumina surfaces. From application view point the present work may be a potential tool for an effective chromatographic separation of sulfa drugs from industrial effluents.     

Voltammetric evaluation of the electrode material on the oxidation of cyanide catalyzed by copper ions

The cyanide oxidation on vitreous carbon (VC), stainless steel 304 (SS 304) and titanium (Ti) was investigated through a voltammetric study of cyanide solutions also containing copper ions. Results showed that cyanide oxidation occurs by means of a catalytic mechanism involving adsorbed species as CN^–, Cu(CN)₄^3– or Cu(CN)₄^2– depending on the electrode material. It was observed that on VC, the adsorption of Cu(CN)₄^3– controlled the oxidation rate. Instead, for SS 304 and Ti, the adsorption of CN^– controlled the global process. However, in all cases, the adsorption of Cu(CN)₄^3– on the electrode surface was required for the catalytic oxidation of CN^–. Voltammetric experiments for solutions containing cyanide oxidation products, such as cyanogen (CN)₂ and cyanate (CNO^–), confirmed that the adsorbed species mentioned above controlled the catalytic oxidation of CN^– depending on the electrode material. A voltammetric identification of the oxidation products showed that cyanogen, (CN)₂ tended to adosorb on VC, while the formation of cyanate, CNO^– predominated on SS 304.     

Redox determination of uranium in presence of plutonium in low phosphoric acid medium and the recovery of plutonium

Quantitative determination of uranium in (U, Pu)O₂ fuels is usually done by the DAVIES-GRAY method. High concentrations of phosphoric acid in the analytical waste generated by this method make the revocery of plutonium rather complex. Studies on the recovery of plutonium from nitric acid medium containing different concentrations of H₃PO₄ by conventional anion-exchange procedure reveal that more than 90% of the plutonium can be easily recovered when the phosphoric acid concentration is less than 0.5 M in the solution. A method was developed for the determination of uranium in the presence of plutonium, which involves the reduction of U(VI) to U(IV) by Fe(II) in a medium of 3.5M H₃PO₄ +4.5M H₂SO₄ instead of 10–11M H₃PO₄ so as to have the H₃PO₄ concentration 0.6M in the waste. A number of determinations of uranium in UO₂(NO₃)₂ working standard solutions and (U, Pu) synthetic solutions with uranium at the 3–7 mg level were carried out by this method. The precision obtained was better than ±0.2% and the accuracy was also within the precision limits. The resulting analytical waste generated was directly subjected to anion exchange separation for the recovery of plutonium which was found to be more than 90%.     

Determination of uranium and plutonium in plutonium based fuels by sequential amperometric titration

A method is described for the sequential determination of uranium and plutonium in plutonium bearing fuel materials. Uranium and plutonium are reduced to U(IV) and Pu(III) with titanous chloride and then titrated with dichromate to two end points which are detected amperometrically using two polarized platinum electrodes. Uranium-plutonium solutions of known concentrations containing plutonium in the proportions of 4, 30, 50, and 70% were analyzed with precisions better than 0.3%, maintaining the amounts of plutonium per aliquot in the range of 2–10 mg. No significant bias could be detected. Several samples of (U, Pu)O₂ and (U, Pu)C were analyzed by this procedure. The effects of iron, fluoride, oxalic acid and mellitic acid on the method were also studied.     

Adsorption of Cu(II) and Co(II) complexes on a silica gel surface chemically modified with 2-mercaptoimidazole

The isotherms of adsorption of MeX₂ (Me = Cu²⁺, Co²⁺; X = Cl^–, Br^–, ClO ₄ ^– ) by silica gel chemically modified with 2-mercaptoimidazole (SiMI) were studied in acetone and ethanol solutions, at 25 °C. Covalently attached 2-mercaptoimidazole molecule to silica gel surface adsorbs MeX₂ from solvent by forming a surface complex. The metal is bonded to the surface through the nitrogen atom of attached 2-mercaptoimidazole. At low loading, the electronic and ESR spectral parameters indicated that the Cu²⁺ complexes are in a distorted-tetragonal symmetry field. The d-d electronic transition spectra showed that for Cu(ClO₄)₂ complex, the peak of absorption did not change for any degree of metal loading and for Cl^– and Br^– complexes, the peak maxima shifted to higher energy with lower metal loading. The CoX₂(X = Cl^–, Br^–, ClO ₄ ^– ) analogues possess a distorted-tetrahedral field.     

ESR and TPD Study of the Interaction of Nitromethane and Ammonia with HZSM-5 and CuZSM-5 Zeolites

The properties of complexes formed on HZSM-5 and CuZSM-5 zeolites in the course of ammonia and nitromethane adsorption are studied. Ammonia adsorbs on CuZSM-5 and forms two species, which decompose at different temperatures T _dec. One is due to the formation of the Cu²⁺(NH₃)₄ complex (T _dec = 450 K), and the other is assigned to ammonia adsorbed on copper(II) compounds, Cu²⁺O^– and Cu²⁺–O^2––Cu²⁺, or CuO clusters (T _dec = 650–750 K). Ammonia adsorption on Cu⁺ and Cu⁰ is negligible compared with that on the Brönsted acid sites and copper(II). Nitromethane adsorbed on HZSM-5 and CuZSM-5 at 400–500 K transforms into a series of products including ammonia. Ammonia also forms complexes with the Brönsted acid sites and copper(II) similar to those formed in the course of adsorption from the gas phase, but the Cu²⁺(NH₃)₄ complexes on CuZSM-5 are not observed. Possible structures of ammonia and nitromethane complexes on Brönsted acid sites and the Cu²⁺ cations in zeolite channels are discussed. The role of these complexes in selective NO _x reduction by hydrocarbons over the zeolites is considered in connection with their thermal stability.     

Adsorption characteristics of traces of radium on membrane filters,glass and polyethylene

Adsorption of radium was studied on glass and polyethylene from aqueous solutions containing 8–40 pg·dm^{–3 224}Ra and on membrane filters, glass and polyethylene bottles from waste and river waters containing 2–170 pg·dm^{–3 226}Ra. The adsorption from aqueous solutions was determined as a function of pH and composition of the solutions and interpreted as due to ion exchange of Ra²⁺ ions for counter ions in the electric double layer on glass and polyethylene or due to chemisorption of RaSO₄ (RaCO₃) ion pairs on glass. Borosilicate glass adsorbed radium substantially more than polyethylene. The adsorption of dissolved forms of radium from the waste and river waters during storage and membrane filtration of the waters was negligible, but a significant loss of particulate forms of radium was sometimes observed during the storage. It has been recommended to separate dissolved and particulate forms of radium soon after the sampling and to prefer polyethylene to glass as container material for storage of dissolved forms of radium.     

Anionic exchange studies on uranium(VI) in aliphatic dicarboxylic acids

Summary Uranium (VI) forms anionic complexes with aliphatic dicarboxylic acids namely oxalic, succinic and adipic acids. These complexes are stable in solutions of p_H up to 5 and retained quantitatively on Lewatit MN (Cl^–). The break-through capacities of the ion exchange column used are given for the different uranyl complexes. Li, Cu²⁺, Ni, Co, Cr³⁺, Fe³⁺, Al, Mn²⁺, Th, Zr, Ce⁴⁺ andPO₄ ^3– which may interfere with uranium are tested under the prescribed conditions in order not to affect uranium determination.Thorium and zirconium form anionic oxalate complexes in 1% oxalic acid-ammonium hydroxide solution of p_H range 2.5–5 and are retained quantitatively on Lewatit MN (Cl^–). 0.4 N hydrochloric acid solution is used for eluting uranium or zirconium while thorium is eluted with 4–5 N hydrochloric acid solution.

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Zusammenfassung Uran(VI) bildet mit aliphatischen Dicarbonsäuren (Oxal-, Bernstein, Adipinsäure) anionische Komplexe, die in Lösung bis zu p_H 5 beständig sind und von Lewatit MN (Cl^–) quantitativ zurückgehalten werden. Die Durchbruchskapazitäten der verwendeten Austauschersäule für die einzelnen Urankomplexe werden angegeben. Störungen durch Li, Cu²⁺, Ni, Co, Cr³⁺, Fe³⁺, Al, Mn²⁺, Th, Zr, Ce⁴⁺ und PO₄ ^3– werden untersucht. Thorium and Zirkonium bilden anionische Oxalatkomplexe in 1%iger Oxalsäurelösung, die mit Ammoniak auf p_H 2,5 bzw. 5 eingestellt ist; diese Komplexe werden von Lewatit MN (Cl^–) quantitativ zurückgehalten. Uran oder Zirkonium werden mit 0,4 n Salzsäure, Thorium mit 4–5 n Salzsäure eluiert.

     

Bismuthiol I as an analytical reagent

Summary Bismuth and palladium have been determined volumetrically after precipitation as bismuthiol I complexes. From 0.1 N hydrochloric acid solutions they are separated from Fe²⁺, Al, Cr, Ce³⁺, Zr, Ti, Zn, Th, UO₂ ²⁺, Be, Mg, Mn, Co, Ni, alkalis, alkaline and rare earths.A mixture of tartrate or citrate and EDTA has been found to be useful for the separation of palladium at a p_H 3.5–8.5 from As, Zn, Bi, Sn⁴⁺, Sb, Fe³⁺, Tl⁺, Cu²⁺, Cd, Pb, Ru³⁺, Os⁴⁺, PO₄ ^3–, Ce⁴⁺, Ir⁴⁺, Rh³⁺, VO₃ ^–, CrO₄ ^2–, AsO₄ ^3–, WO₄ ^2–, MoO₄ ^2– and from all the other ions referred to above. Potassium iodide at p_H 6.0–8.0 and thiosulphate at 6–7 keep Ag, Pb, Hg²⁺ and Au³⁺, Ir⁴⁺, Os⁴⁺ respectively in solution and thus allow a selective precipitation of palladium.Hg²⁺, Pb, Cu²⁺, Ag, Tl⁺, Cd and Pd when present along with bismuth are first removed by the reagent and from the filtrate bismuth is estimated. Sn²⁺, Sb³⁺, Fe³⁺, F^–, VO₃ ^–, PO₄ ^3–, AsO₄ ^3– and CrO₄ ^2– interfere in bismuth determination while only Sn²⁺, Pt⁴⁺ and CN^– interfere in palladium estimation.     

Potentiometric Study of Reactions of Titanium Complexes in Phosphate–Perchlorate Acid Solutions

The potentiometric method is used to measure the equilibrium potential in the Ti(IV)/Ti(III) system and determine that monophosphate Ti(IV) complexes and Ti³⁺hydrated complexes dominate in phosphate–perchlorate acid solutions, 4M(H, Na)ClO₄, at of 5 × 10^–2to 4 × 10^–1M. Equations that describe the total electrode reaction are proposed. Decreasing the concentration of free hydrogen ions from 3 to 0.12 M results in the deprotonation of TiO(H₂PO₄)⁺complexes and the formation of TiO(HPO₄) complexes. Equilibrium constants for reactions of the formation of Ti(IV) monophosphate complexes and the protonation of TiO(HPO₄) complex are calculated.     

Formation Mechanism of O2– Radical Anions in the Adsorption of NO + O2 and NO2 + O2 Mixtures on ZrO2 According to EPR and TPD Data

The effects of various factors on the formation of O₂ ^– radical anions in the adsorption of an NO + O₂ or NO₂ + O₂ mixture on ZrO₂ were studied. It was found that the thermal stability of the O₂ ^– species depends on the composition of the adsorbed gas. It was suggested that nitrogen oxide complexes on ZrO₂ centers are responsible for the formation of O₂ ^–. These centers are formed upon the treatment of the oxide in a vacuum; however, they are different from both coordinatively unsaturated Zr⁴⁺ cations (NO adsorption centers at 77 K) and Zr⁴⁺–O^––O^––Zr⁴⁺ centers, at which O₂ ^– are formed because of the adsorption of H₂ + O₂. Based on the experimental data, the mechanism of O₂ ^– formation in the adsorption of an NO + O₂ mixture is discussed.     

Spectrophotometric determination of thorium(IV) using 3-nitroso-4-hydroxycoumarin

Summary 3-Nitroso-4-hydroxycoumarin is suggested as a new reagent for the spectrophotometric determination of 125 g to 0.50 mg Th(IV) in 3: 1 dioxan: water medium as 1: 1 complex having orange red colour with absorption maximum at 419 nm, at pH 4.5–6.0. For the estimation of9.6 ppm Th(IV) 100-folds acetate, citrate, tartrate; 50 ppm UO₂ ²⁺, 75 ppm Ce³⁺, La³⁺, Gd³⁺; 4.5 ppm Ce⁴⁺; 25 ppm Tm³⁺, Zr⁴⁺; and 100 ppm Ti⁴⁺, V⁵⁺, MoO₄ ^2– and WO₄ ^2– do not interfere.

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Zusammenfassung 3-Nitroso-4-hydroxycoumarin wird als neues Reagens für die spektrophotometrische Bestimmung von 125 g bis 0,50 mg Th(IV) in Dioxan: Wasser = 3: 1 als 1: 1-Komplex mit orange-roter Farbe mit einem Absorptionsmaximum bei 419 nm bei pH 4,5–6,0 empfohlen. Bei einem Einsatz von 9,6 ppm Th(IV) stört die hundertfache Menge Acetat, Citrat, Tartrat nicht. Auch 50 ppm UO₂ ²⁺, 75 ppm Ce³⁺, La³⁺, Gd³⁺, 4,5 ppm Ce⁴⁺, 25 ppm Tm³⁺, Zr⁴⁺, 100 ppm Ti⁴⁺, V⁵⁺, MoO₄ ^2– bzw. WO₄ ^2– stören nicht.

     

Bismuthiol II as an analytical reagent

Summary The estimation of bismuth by the reagent Bismuthiol II is studied critically. The effect of acidity, reagent concentration and interfering ions are given in detail. The maximum acidity that may be tolerated for the complete precipitation of bismuth is 0.3 N in nitric acid, 0.5 N in hydrochloric acid and 1N in sulphuric acid. Higher acidity than 0.1 N decomposes the reagent present in excess. In 0.1 N nitric acid bismuth has been separated from a number of ions like Al³⁺, Cr³⁺, Th⁴⁺, rare earths, Zr⁴⁺, Ti⁴⁺, UO₂ ²⁺, Be²⁺, Mn²⁺, Co²⁺, Ni²⁺, Mg, alkalis and alkaline earths, SO₄ ^2–, Cl^–, C₂O₄ ^2–- and from Fe²⁺ and Ce³⁺ in 0.1 N hydrochloric acid. In presence of a citrate or a tartrate it can be separated from As³⁺, Ce⁴⁺, MoO₄ ^2–- and WO₄ ^2–-at p_H 1.5 to 2.5. When Hg²⁺, Pb²⁺, Pd²⁺, Cd²⁺, Cu²⁺, Ag⁺ and Tl⁺ are present they are first precipitated by the reagent at p_H 6 to 8 in presence of a citrate or a tratrate and the bismuth is estimated gravimetrically in the acidified filtrate. Ions as F^– and PO₄ ^3– that form insoluble compounds with bismuth, Sb³⁺ and Sn²⁺ that form less soluble compounds with the reagent and Fe³⁺, VO₃ ^–, CrO₄ ^2–, AsO₄ ^3– that act as oxidising agents, interfere.     

Raman Spectroscopic Study of Aluminum Silicate Complexes at 20°C in Basic Solutions

Raman spectroscopic measurements were performed at ambient temperature onaqueous silica-bearing solutions (0.005 < m _Si < 0.02; 0 < pH < 14). The spectraare consistent with the formation of monomeric Si(OH)^o ₄, SiO(OH)^– ₃ andSiO₂(OH)^2– ₂ species at acid to neutral, basic, and strongly basic pH, respectively.Raman spectra of aqueous Al-bearing solutions at basic pH confirm thepredominance of the Al(OH)^– ₄ species in a wide concentration range (0.01 < m _Al < 0.1).Raman spectra of basic solutions (12.4 < pH < 14.3), containing both Al andSi, exhibit a strong decrease in intensities of SiO(OH)^– ₃, SiO₂(OH)^2– ₂, andAl(OH)^– ₄ bands in comparison with Al-free Si-bearing and Si-free Al-bearingsolutions of the same metal concentration and pH, suggesting the formation ofsoluble Al—Si complexes. The amounts of complexed Al and Si derived fromthe measurements of the Al and Si band intensities in strongly basic solutions(pH 14) are consistent with the formation, between Al(OH)^– ₄ andSiO₂(OH)^2– ₂, of the single Al—Si dimer SiAlO₃(OH)^3– ₄ according to the reactionSiO₂(OH)^2– ₂ + Al(OH)^– ₄ SiAlO₃(OH)^3– ₄ + H₂OAt lower pH ( 12.5) the changes in band intensities are consistent with theformation of several, likely more polymerized, Al—Si complexes.     

Sorption and separation of some elements of nuclear importance using Chelex-100

Chelex-100, in the anionic form has been studied for its ability to perform selective separation and concentration of some metal ions of nuclear importance from mineral acid solutions. The sorption behavior of Zr(IV)–Nb(V), Mo(VI), Tc(VII), Te(IV) and U(VI) from solutions of hydrochloric and sulphuric acids on Chelex-100 has been studied under static and dynamic conditions. Mo(VI) and Tc(VII) have been concentrated on the resin from hydrochloric or sulphuric acid solutions at low acidities probably, as the anions MoO ₄ ^2– and TcO ₄ ^– , respectively. Te(IV) has been isolated from hydrochloric acid solutions of normalities 6 in the form of the anionic chloro complex TeCl ₆ ^2– . Optimum conditions for elution and separation of Mo(VI), Tc(VII), Te(IV) and U(VI) were recommended.     

Ion association studies in aqueous NiSO4 solutions by positron lifetime spectroscopy

Ion association has been studied by positron lifetime spectroscopy in aqueous solutions containing the Ni²⁺ and SO ₄ ^2– ions at 294 K with the double aim of assessing the reliability of the method for quantitative determination of complex formation constants and of probing the validity of various expressions to calculate single-ion activity coefficients at high ionic strength. The existence of two complexes, identified as NiSO₄ and Ni₂SO ₄ ²⁺ , is shown by the data analysis. Considering the formation constant of the former, K_I=(196±10)M^–1, determined in previous works leads to discarding several of the expressions commonly used for activity corrections. Two possible values are retained for K_I, (193±20)M^–1 and (179±20)M^–1, while K_II related to Ni₂SO ₄ ²⁺ is better defined, as (2.57±0.14)M^–1.