NaClO4 Medium Research Articles

Sulfate Protonation in Sodium Chloride and Sodium Perchlorate Media

The equilibrium (association) constant, KA, for the first protonation of the sulfate ion in aqueous solution: [H+(aq) + {text{SO}}_{4}^{2 - }(aq) ⇌ {text{HSO}}_{4}^{ - }(aq)] has been measured at 298.15 K in NaCl and NaClO4 media using glass-electrode potentiometry at five ionic strengths in the range 0.5 ≤ I/(mol·dm−3) ≤ 5.0. The present KA values vary smoothly with I and are in good agreement with reliable literature data but indicate that the previously reported value of KA at I = 5 mol·dm−3 in NaClO4 media is in error.

Spectroluminescence measurements of the stability constants of CanUO2(CO3)3(4-2n)- complexes in NaClO4 medium and the investigation of interaction effects.

The stability constants of ternary calcium uranyl tricarbonate complexes, CaUO2(CO3)32- and Ca2UO2(CO3)3(aq), were determined in NaClO4 medium at various ionic strengths using time-resolved laser-induced luminescence spectroscopy (TRLS) - also known as time-resolved laser-induced fluorescence spectroscopy (TRLFS). As in a previous study, the potential precipitation of schoepite (UO3·2H2O) and calcite (CaCO3) was avoided via titration of the triscarbonatouranyl complex with Ca2+ at varying pH values. The Ringböm coefficients relative to UO2(CO3)34- were individually evaluated under test sample conditions. Steadily enhanced luminescence intensity and increased decay-times were representative of complexation processes. The stoichiometry of calcium was quantified by slope analysis, and its measured intensity was corrected by using the corresponding Ringböm coefficient. The conditional formation constants, i.e. log10 Kn.1.3, were then assessed after rounding off the slope values to their nearest integers. Cumulative formation constants at infinite dilution log10 β°n.1.3, and specific ion interaction parameters ε were determined based on the experimental origin and slope values, respectively, over the range of 0.1-2.46 mol kgw-1 NaClO4 using the specific ion interaction theory (SIT) approach. The cumulative stability constants are log10 β°(CaUO2(CO3)32-) = 27.26 ± 0.04 and log10 β°(Ca2UO2(CO3)3(aq)) = 30.53 ± 0.06. The specific ion interaction coefficients are estimated to be ε(CaUO2(CO3)32-,Na+) = (0.02 ± 0.04) kgw mol-1 and ε(Ca2UO2(CO3)3(aq),NaClO4) = (0.18 ± 0.07) kgw mol-1. These latter values are different from the ones that were previously obtained in NaCl, and underlying causes are discussed from different aspects. This work provides valuable information to address the interaction effects between Ca-UO2-CO3 species and 1 : 1 type electrolytes. It is suggested that the affinity of the cation in a background electrolyte with CanUO2(CO3)3(4-2n)- (n = {1;2}) has to be taken into consideration at high ionic strengths, especially for globally non-charged species.

Extraction of lanthanides(III) from Perchlorate Solutions with Carbamoyl- and Phosphorylmethoxymethylphosphine Oxides and Tetrabutyldiglycolamide

ABSTRACT The solvent extraction of Ln(III) ions from perchlorate aqueous solutions into an organic phase containing neutral polyfunctional organophosphorus ligands R2P(O)CH2OCH2C(O)NBu2 R = Bu (I), R = Ph (II) and R2P(O)CH2OCH2P(O)R1 2 R = R1 = Bu (III); R = Bu, R1 = Ph (IV); R = R1 = Ph(V) has been studied. Their extraction behavior was compared with that of tetrabutyldiglycolamide (TBDGA), tetrabutylmethylenediphosphine dioxide (VI), P,P-dibutyl-P’P’-diphenylmethylenediphosphine dioxide (VII), tetraphenylmethylenediphosphine dioxide (VIII), dibutyl-N,N-dibutylcarbamoylmethylphosphine oxide (IX) and diphenyl-N,N-dibutylcarbamoylmethylphosphine oxide (X). The extraction equilibrium was investigated, and the equilibrium constants were calculated. It was found that the lanthanide(III) ions are extracted with the studied extractants from perchlorate solutions as LnL3(ClO4)3 complexes. In the NaClO4 media, TBDGA was found to possess a higher extraction efficiency towards Ln(III) ions than other neutral donor ligands studied. A successive replacement of the C(O)NBu2 groups in the diglycolamide extractant molecule by phosphoryl ones leads to a decrease in the extraction efficiency of Ln(III) ions. In the NaClO4 media, compounds II, IV and V with phenyl radicals at the P(O) group demonstrate a lower extraction efficiency towards Ln(III) ions than their butyl-substituted analogs. In contrast, phenyl-substituted diphosphine dioxides VIII, VII and carbamoylmethylphosphine oxide (X) extract Ln(III) ions more effectively than their butyl-substituted analogs VI and IX. The extraction of Ln(III) ions from HClO4 solutions is accompanied by HClO4 interaction with neutral donor extractants, which leads to a decrease of the free extractant concentration in the organic phase. By this reason, an increase in the HClO4 concentration higher than 0.1 M is accompanied by a decrease of the Ln(III) extraction with TBDGA. In the 3 M HClO4 system, diphosphine dioxide VIII outperforms TBDGA at the Ln(III) extraction.

Synthesis and characterization of phytate–uranium (VI) complexes

The phytate–uranium (VI) solid complexes were synthetized and characterized. A potentiometric study was firstly carried out in order to investigate acid–base properties of phytic acid and its interaction with uranium (VI) in 0.1 M NaClO4 medium at 25 °C. This study showed that the acid–base reactions for phytic acid occur from pH 2 to 12, and it is fully deprotonated at pH 13. A total of six protonation constants of phytic acid could be refined by fitting acid–base titration data using the SUPERQUAD program. The potentiometric study also revealed that the phytic acid reacts with uranium (VI) to form a precipitate, when an excess of uranyl ion is used. The phytate–uranium (VI) solid complexes were then prepared and characterized by fluorescence and X-ray photoelectron spectroscopies. The formation of phytate–uranium (VI) complexes with a preferential molar ratio of 1:2 was revealed in this study. The results of this study will thus contribute to development a viable and low-cost method that avoids the dispersion of anthropogenic actinides in the environment.

Influence of adipic acid on the speciation of Eu(III): Review of thermodynamic data in NaCl and NaClO4 media, and a new determination of Eu-adipate complexation constant in 0.5 mol·kgw−1 NaClO4 medium by time-resolved luminescence spectroscopy

The complexation of Eu(III) by adipic acid — AdipH2, hexane-1,6-dioic acid —, one of the major hydro-soluble degradation products (HDPs) from radio-oxidized polyesterurethane, is investigated by time-resolved luminescence spectroscopy (TRLS) in NaClO4 ionic medium (0.5 mol·kgw−1). The solubility and deprotonation constants of adipic acid, as well as specific interaction theory (SIT) coefficients of aqueous AdipH2(aq) and adipate ions (AdipH− and Adip2−) were estimated using available literature data in NaCl and NaClO4 ionic media. A 1:1 (EuAdip+) complex is evidenced in TRLS with log10β(0.5 m) = 2.09 ± 0.16. Using reaction constant at another NaClO4 ionic strength from literature, the logarithm of reaction constant log10β° = 3.88 ± 0.40 at 0 ionic strength and specific ion parameter ε(EuAdip+, ClO4-) = (0.10 ± 0.40) kgw·mol−1 is estimated. This implies that adipic acid can control the Eu(III) speciation from mildly acidic to mildly basic conditions, but not in highly basic solutions characteristic of cementitious interstitial waters.

Effect of carbonate on U(VI) sorption by nano-crystalline α-MnO2

Abstract U(VI) sorption on nano-crystalline α-MnO2 was studied in NaClO4 medium as a function of pH by batch sorption method in presence and absence of carbonate and subsequently employing surface complexation modeling (SCM) to predict species responsible for U(VI) sorption. The kinetic study of U(VI) sorption on nano-crystalline α-MnO2 was carried out to fix the time of equilibration. In presence of carbonate, U(VI) sorption on nano-crystalline α-MnO2 increases with pH of the suspension, leveling off in the pH range 5–8.5 thereafter decreasing at higher pH. However, in absence of carbonate, U(VI) sorption on nano-crystalline α-MnO2 remains close to 100% at pH>5. The difference in sorption behavior of uranium in the presence and absence of carbonate can be explained in terms of uranium speciation in the two systems. The dissolution of nano-crystalline α-MnO2 was studied in presence and absence of carbonate to ascertain its role in sorption. Surface complexation modeling was satisfactorily able to explain the sorption phenomena in all the systems. In addition, U(VI) sorption on nano-crystalline α-MnO2 was compared with literature data on U(VI) sorption by δ-MnO2.

Study of Complexation Equilibria Between the Iron(III) Ion and 2-Hydroxybenzamide in Aqueous Solution

The complexation of the Fe3+ ion with 2-hydroxybenzamide (salicylamide, HL) has been studied, at 25.0 °C, by potentiometric measurements with a glass electrode in 1 mol·dm−3 NaClO4 medium. The concentration of salicylamide (C L) has been varied between (1 × 10−3 and 10 × 10−3) mol·dm−3, while the concentration of metal cation (C M) ranged within (0.5 × 10−3 and 10 × 10−3) mol·dm−3. The ligand to metal concentration ratio has been varied between 1 and 10 and the hydrogen ion concentration was decreased stepwise until the incipient precipitation of a basic salt of the metal, which occurred at different values depending on the specific ligand to metal ratio. The experimental data were consistent with the formation of the complexes FeL2+, Fe(HL)L2+ and $$ {\text{FeL}}_{2}^{ + } $$ . Equilibrium formation constants have been given for the investigated ionic medium as well as for the infinite dilution reference state, evaluated according to the Specific Interaction Theory. The predominant species in neutral solution are $$ {\text{FeL}}_{2}^{ + } $$ and $$ {\text{Fe}}\left( {\text{OH}} \right)_{2}^{ + } $$ .

Stability, speciation and spectral properties of NpO2+ complexes with pyridine monocarboxylates in aqueous solution

Neptunyl ion as NpO2+ is the least reacting and most mobile radioactive species among all the actinides. The picolinic acid used for decontamination is co-disposed along with the radioactive waste. Thus, in long term storage of HLW, there is high possibility of interaction of actinides and long lived fission products with the picolinate and can cause migration. The complexation of NpO2+ with the three structural isomers of pyridine monocarboxylates provides an insight to explore the role of hetero atom (nitrogen) with respect to key binding moiety (carboxylate). In the present study, the log β values, speciation and spectral properties of NpO2+ complexes with pyridine monocarboxylates viz. picolinate, nicotinate and isonicotinate, have been studied at 298K in 0.1M NaClO4 medium using spectrophotometry. The complexation reactions involving protonated ligands are always accompanied by protonation/deprotonation process; thus, the protonation constants of all the three pyridine monocarboxylates under same conditions were also determined by potentiometry. The spectrophotometric data analysis for complexation of NpO2+ with pyridine monocarboxylates indicated the presence of ML and ML2 complexes with log β values of 2.96±0.04, 5.67±0.08 for picolinate, 1.34±0.09, 1.65±0.12 for nicotinate and 1.52±0.04, 2.39±0.06 for isonicotinate. The higher values of log β for picolinate were attributed to chelation while in other two isomers, the binding is through carboxylate group only. Density Functional Theory (DFT) calculations were carried out to get optimized geometries and electrostatic charges on various atoms of the complexes and free pyridine monocarboxylates to support the experimental data. The higher stability of NpO2+ nicotinate and isonicotinate complexes compared to simple carboxylates and the difference in log β between the two is due to the charge polarization from unbound nitrogen to the bound carboxylate oxygen atoms.

Solubility and acidic constants at 25 °C in NaClO4 aqueous solutions of 1-(2-hydroxyphenyl)ethanone

The acid–base equilibria of 1-(2-hydroxyphenyl)ethanone (HY) were studied, at 25 °C, by potentiometric measurements with a glass electrode in NaClO4 media for ionic strength ranging from 0.51 to 3.5 mol/kg. The solubility (S) of HY was determined in the same range of ionic strength in NaClO4 media. The total concentration (C L) of ligand was ranged from 1 × 10−3 mol/kg to 20 × 10−3 mol/kg; the experimental data were explained by assuming, for HY, the species HY2 − besides Y−. Elaboration of data, according to the specific interaction theory, had provided the constants valid in the infinite dilution reference state: HY ⇄ Y− + H+, log K°1-1 = − 10.11 ± 0.05 and 2 HY ⇄ HY2 − + H+, log K°2–1 = − 7.85 ± 0.05. The uncertainties were given as 3σ. The interaction coefficients (kg/mol) of ionic species Y− and HY2 − with the medium counter ion were e (Y−, Na+) = 0.50 ± 0.05 and e (HY2 −, Na+) = 0.59 ± 0.05, respectively.

The dependence on ionic strength of the protonation of water-insoluble flavonoids chrysin and daidzein in NaClO4 non-aqueous solutions

The acid–base equilibria of two water-insoluble flavonoids, chrysin and daidzein, were studied using combined spectrophtometric/potentiometric measurements at constant temperature (25.0±0.1) °C. The investigation was performed over a wide range of ionic strengths of 0.101–3.503molkg−1 NaClO4 medium in ethanol-water mixture (1:1 v/v). The Gran's method was used for glass electrode calibration to obtain pH readings based on the concentration scale (pcH). Data process for calculation of the protonation constants was done using the STAR computer program. The dependence on ionic strength of the constants was successfully modeled by means of the Brönsted–Guggenheim–Scatchard Specific Ion Interaction Theory (SIT). Other than the specific interaction parameters of the ionic species, the protonation constants at infinite dilution (zero ionic strength) were obtained. The thermodynamic functions ΔG0 (standard Gibbs free energy change) for the protonation processes were also calculated at different ionic strengths.

An improved thermodynamic model for the complexation of trivalent actinides and lanthanide with oxalic acid valid to high ionic strength

The dissociation constants of oxalic acid (Ox), and the stability constants of Am3+, Cm3+ and Eu3+ with Ox2− have been determined at 25°C, over a range of concentration varying from 0.1 to 6.60m NaClO4 using potentiometric titration and extraction techniques, respectively. The experimental data support the formation of complexes, M(Ox)n3−2n, where (M=Am3+, Cm3+ and Eu3+ and n=1 and 2). The dissociation constant and the stability constant values measured as a function of NaClO4 concentration were used to estimate the Pitzer parameters for the respective interactions of Am3+, Cm3+ and Eu3+ with Ox. Furthermore, the stability constants data of Am3+–Ox measured in NaClO4 and in NaCl solutions from the literature were simultaneously fitted in order to refine the existing actinide–oxalate complexation model that can be used universally in the safety assessment of radioactive waste disposal. The thermodynamic stability constant: log β0101=6.30±0.06 and log β0102=10.84±0.06 for Am3+ was obtained by simultaneously fitting data in NaCl and NaClO4 media. Additionally, log β0101=6.72±0.08 and log β0102=11.05±0.09 for the Cm3+ and log β0101=6.67±0.08 and log β0102=11.15±0.09 for the Eu3+ were calculated by extrapolation of data to zero ionic strength in NaClO4 medium only. For all stability constants, the Pitzer model gives an excellent representation of the data using interaction parameters β(0), β(1), and Cϕ determined in this work. The thermodynamic model developed in this work will be useful in accurately modeling the potential solubility of trivalent actinides and early lanthanides to ionic strength of 6.60m in low temperature environments in the presence of Ox. The work is also applicable to the accurate modeling transport of rare earth elements in various environments under the surface conditions.

Sorption of niobium on colloidal silica and the effect of humic acid

Sorption of niobium on silica colloids was studied using radiotracer technique in the pH range of 2–11 in NaClO4 medium. Silica was characterized using X-ray diffraction, light scattering and surface area measurements. The point of zero charge for silica colloids was about pH 2. The sorption of niobium was >99 % in the pH range of 3–9 and was slightly lower (~95 %) below pH 3 and above pH 9. The quantitative sorption in the pH range 3–9 could be explained by surface complexation model. The small reduction in sorption was attributed to the existence of small fraction of niobium as cationic species at pH ≤ 2 and anionic species above pH 9. Presence of humic acid was found to have negligible effects on the sorption characteristics.

A Potentiometric Study of the Association of Copper(II) and Sulfate Ions in Aqueous Solution at 25 °C

A study of the association between copper(II) and sulfate ions in aqueous solution has been made using copper ion-selective electrode potentiometry at constant ionic strengths (I) of 0.05, 0.1, 0.25, 0.5, 1.0, 3.0 and 5.0 mol·L−1 in NaClO4 media at 25 °C. Only one complex was detected, corresponding to the equilibrium: \( {\text{Cu}}^{ 2+ } ({\text{aq}}) + {\text{SO}}_{4}^{2 - } ({\text{aq}}) \rightleftarrows {\text{CuSO}}_{4}^{0} ({\text{aq}}). \) No higher order complexes were detected even at sulfate/copper(II) concentration ratios of up to 1,000. The present potentiometric values of log10K1(I) are shown to be consistently higher than those obtained by UV–Vis spectrophotometry because of the failure of the latter technique to detect all of the solvent-separated ion pairs present. Extrapolation of log10K1(I) to infinite dilution using an extended Guggenheim equation yielded a standard state value of log10\( K_{1} \{ {\text{CuSO}}_{4}^{0} ({\text{aq}})\} = 2.32 \pm 0.09 \), which is in excellent agreement with a recent IUPAC-recommended value.

Speciation of bioaccumulated uranium(VI) by Euglena mutabilis cells obtained by laser fluorescence spectroscopy

Abstract The ability of Euglena mutabilis cells – a unicellular protozoan with a flexible pellicle, which is typically found in acid mine drainage (AMD) environments – to bioaccumulate uranium under acid conditions was studied in batch sorption experiments at pH 3 and 4 using Na2SO4 and NaClO4 as background media. It was found that axenic cultures of Euglena mutabilis Schmitz were able to bioaccumulate in 5 days 94.9 to 99.2% of uranium from a 1 · 10 –5 mol/L uranium solution in perchlorate medium and 95.1 to 95.9% in sodium sulfate medium, respectively. The speciation of uranium in solution and uranium bioaccumulated by Euglena mutabilis cells, were studied by laser induced fluorescence spectroscopy (LIFS). The LIFS investigations showed that the uranium speciation in the NaClO4 systems was dominated by free uranyl(VI) species and that the UO2SO4 species was dominating in the Na2SO4 medium. Fluorescence spectra of the bioaccumulated uranium revealed that aqueous uranium binds to carboxylic and/or (organo)phosphate groups located on the euglenid pellicle or inside the Euglena mutabilis cells. Reduced uranium immobilization rates of 0.93 – 1.43 mg uranium per g Euglena mutabilis biomass were observed in similar experiments, using sterile filtrated AMD waters containing, 4.4 · 10 –5 mol/L uranium. These lower rates were attributed to competition with other cations for available sorption sites. Additional LIFS measurements, however, showed that the speciation of the bioaccumulated uranium by the Euglena mutabilis cells was found to be identical with the uranium speciation found in the bioaccumulation experiments carried out in Na2SO4 and NaClO4 media. The results indicate that Euglena mutabilis has the potential to immobilize aqueous uranium under acid condition and thus may be used in future as promising agent for immobilizing uranium in low pH waste water environments.

Improved thermodynamic model for interaction of EDTA with trivalent actinides and lanthanide to ionic strength of 6.60 m

The dissociation constants of ethylenediaminetetraacetic acid (H4EDTA), and the stability constants of Am3+, Cm3+and Eu3+ with EDTA4− have been determined at 25°C, over a range of concentration varying from 0.1 to 6.60m NaClO4 using potentiometric titration and an extraction technique, respectively. The formation of only 1:1 complex, M(EDTA)−, where (M=Am3+, Cm3+ and Eu3+), was observed under the experimental conditions. The observed ionic strength dependencies of the dissociation constants and the stability constants have been described successfully over the entire ionic strength range using the Pitzer model. The thermodynamic stability constant: logβ1010=20.55±0.18 for Am3+, logβ1010=20.43±0.20 for Cm3+ and logβ1010=20.65±0.19 for Eu3+ were calculated by extrapolation of data to zero ionic strength in an NaClO4 medium. In addition, logβ1010 of 20.05±0.40 for Am3+ was obtained by simultaneously modeling data both in NaCl and NaClO4 media. For all stability constants, the Pitzer model gives an excellent representation of the data using interaction parameters β(0), β(1), and Cϕ determined in this work. The improved model presented in this work would enable researchers to model accurately the potential mobility of actinides (III) and light rare earth elements to ionic strength of 6.60m in low temperature environments in the presence of EDTA.

Synthesis and Reactivity of L-Cystine with Di-Oxo-Bridged Cobalt(III) Complex

The kinetics of the oxidation of L-cystine by the di-oxo-bridged Co(III) complex have been studied at various temperatures, 298 K to 323 K, and pH, 3.30 to 4.45, using UV−vis spectroscopy in NaClO4 medium. The product of the reaction has been characterized by UV-vis spectra, IR, and 1H NMR spectroscopy. The activation parameters such as enthalpy, entropy, free energy, and activation energy have been evaluated. A new SNi mechanism has been derived for this reaction. The oxidation product was found to be cystine disulfoxide, R-SO-SO-R at T ≤ 318 K and disulfone, R- (SO2)2-R at T = 323 K. Supplemental materials are available for this article. Go to the publisher's online edition of Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry to view the supplemental file.

Characterization of Europium and Americium Dipicolinate Complexes

Dipicolinic acid (pyridine-2,6-dicarboxylic acid, DPA) is a well-known complexant that forms moderately strong complexes with a variety of metal ions, including trivalent lanthanides. A search of open literature reports and thermodynamic databases produced no information on equilibrium constants for dipicolinate complexes of trivalent actinides. Such information represents an essential starting point for assessing the potential for application of dipicolinate to the task of trivalent lanthanide/actinide separations, specifically as a potential substitute for diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) in the TALSPEAK process. The Am(III)/Eu(III)-dipicolinate system is the subject of the following report. Protonation constants of dipicolinic acid are pK1 = 2.15 (±0.02) and pK2 = 4.62 (±0.02) in µ = 1.0 M NaClO4 media, T = 25.0 (±0.1)°C. Stability constants of Eu3+ and Am3+ with DPA have been determined using solvent extraction distribution methods coupled with radiometric detection. The resolved americium stability constants are log β101 = 8.90 (±0.02), log β102 = 15.87 (±0.03), and log β103 = 21.88 (±0.04), while the corresponding europium constants are log β101 = 8.90 (±0.02), log β102 = 15.64 (±0.03), and log β103 = 21.65 (±0.04) under the same conditions. Stability constants were fit using weighted non-linear least squares analysis of multiple distribution experiments conducted as a function of pH. A preliminary assessment of the separation potential of dipicolinic acid in TALSPEAK-like systems is offered.

Interaction of Iron(III) with 2-Hydroxybenzoic Acid in Aqueous Solutions

The complex formation between the Fe3+ ion and 2-hydroxybenzoic acid (salicylic acid, H2L) has been studied at 298.15 K in 1.05 mol·kg–1 NaClO4 medium by both potentiometric titrations with glass electrodes and spectrophotometric measurements in the range of wavelengths between (410 and 600) nm. The concentrations of ligand (CL) and metal ion (CM) were varied between (0.105 × 10–3 and 21 × 10–3) mol·kg–1, and the ligand-to-metal ratio was varied between (1 and 1000) (1 ≤ CL/CM ≤ 1000). The hydrogen ion concentration was decreased stepwise to the incipient precipitation of a basic salt of metal. The experimental data have been explained with the formation of the complexes FeL+, FeL2–, FeL33–, Fe(HL)L, and Fe2(OH)2(HL)L+. Stability constants for their formation in 1.05 mol·kg–1 NaClO4 as well as in dilute solution are reported.

ChemInform Abstract: Solvent Extraction Investigation of Molybdenum(VI) Equilibria.

The extraction of molybdenum(VI) by tri-n-butyl phosphate (TBP) or a 50% TBP-cyclohexane mixture from 1.0 M NaClO4 medium has been investigated as a function of molybdenum concentration at various approximately constant pH values in the range 1.0–2.0. A satisfactory description of the extraction system could be obtained by computer treatment of the data when all the relevant protonation and condensation equilibria in both phases were taken into account. In the pH range investigated the existence of two large polyanions in the aqueous phase has been confirmed, i.e. [Mo36O112(OH2)16]8− and most probably [Mo18O56]4−. Formation constants have been calculated for these ions and also for some other species which agree well with values previously determined by other methods.

Solubility measurements of crystalline β-Ni(OH)2 in aqueous solution as a function of temperature and pH

Experimental results on the solubility of crystalline nickel hydroxide (theophrastite), according to the dissolution reaction, β-Ni(OH)2(cr) + 2H+ ⇄ Ni2+ + 2H2O(l), are reported as functions of temperature (0–200°C) and pH at pressures slightly exceeding saturation vapor pressure. The experiments were carried out in either various flow-through cells or a hydrogen-electrode concentration cell (HECC). The results were treated with a thermodynamic model incorporating only the unhydrolyzed aqueous nickel species (namely, Ni2+) whose solubility constants were fitted as a function of temperature. The thermodynamic quantities obtained at infinite dilution are: (25°C) = (11.67 ± 0.20), (25°C) = −(66.6 ± 1.1) kJ mol−1, (mean value 5–200°C) = −(82.1 ± 0.8) kJ mol−1, = (mean value 5–200°C) = −(52 ± 5) J K−1 mol−1, (mean value 5–200°C) = 0 J K−1 mol−1, where the uncertainties quoted are 2σ. These results are internally consistent, but differ from most of those reported in the literature. A significantly lower temperature for the thermodynamic stability of β-Ni(OH)2(cr) versus NiO(cr), namely, 77°C, is proposed based on the current measurements. Additional batch experiments at 50°C and 80°C within aqueous NaClO4 media established and quantified a particle-size dependence of the solubility constant of β-Ni(OH)2(cr) that extended to larger particle sizes than normally observed for metal oxide/hydroxides.