Potentiometric Investigation of Fluoride Complexes of Zirconium(IV) and Hafnium(IV) in 1 <Emphasis Type="Italic">M</Emphasis> (H,Na)ClO<Subscript>4</Subscript> Medium Using a Fluoride Ion Selective Electrode

The stability constants of zirconium(IV) and hafnium(IV) fluoride complexes in 1 M (H,Na)ClO₄ medium were measured potentiometrically at 293 K for the first time using a fluoride ion selective electrode (F-ISE). This technique has been recommended by IUPAC as the best tool for studying fluoride complexes. A number of precautions were taken to ensure the stabilization of zirconium or hafnium in 1 M (H,Na)ClO₄ medium and to prevent the formation of polynuclear hydroxo complexes. The formation of only mononuclear complexes was indicated. The average log values of the overall stability constants of zirconium(IV)-fluoride complexes, ₁, ₂, ₃ and ₄ were computed by varying the concentration of metal ion and were found to be 8.49 ± 0.11, 15.76 ± 0.15, 21.57 ± 0.10, and 26.68 ± 0.16, respectively, whereas the corresponding values for hafnium(IV)-fluoride complexes were 8.22 ± 0.06, 15.48 ± 0.15, 21.76 ± 0.14, and 27.42 ± 0.15, respectively. The thermodynamic stability constant, ₁, calculated for these complexes follows the same trend as expected from the linear correlation based on the Brown Sylva Ellies (BSE) model for metal-fluoride complexes provided the effective charge on Zr is taken as +4.1 instead of the formal charge of +4. Without considering this adjustment of formal charge, an attempt has also been made to explain the trend in ₁ values of group(IV) metal-fluoride complexes based on electronegativity values. A good linear correlation was obtained that could explain the ability of these group(IV) ions to form different fluoride complexes with varying number of fluoride ions.     

Capillary zone electrophoretic (CZE) study of uranium(VI) complexation with humic acids

The interaction of UO₂ ²⁺ with various humic acids (HA's) has been studied by capillary zone electrophoresis (CZE). The experiments were done in 10 mM acetate buffer with pH 3.3 and 4.0, to avoid hydrolysis of uranium. It was found that in slightly acidic media and low HA concentration (<3 mM), two complexes with uranium(VI) are formed by fast kinetics and uranyl migrates as cationic species. Electrophoretic mobilities are decreasing with the increasing HA/uranium ratio and a low soluble neutral compound is also formed. In addition, it was found that at HA concentrations higher than 3 mM negatively charged species are formed. Similar results were obtained for HA's of different origin (soil, peat, coal derived, IHSS standards). Conditional stability constants of the complexes UO₂ ²⁺-HA for Fluka I HA, were estimated to be log ₁ = 4.18±0.06 and log ₂ = 7.28±0.18.     

Chromotropic acid,a fluorogenic chelating agent for aluminium(III)

Complexation of aluminium(III) with the fluorogenic ligand chromotropic acid (4,5-dihydroxynaphthalene-2,7-disulfonic acid) has been revisited with the aim of using enhancement of the fluorescence intensity as an analytical tool. Complexation at the optimum pH4 was shown to lead to a 1:1 complex with a stability constant log ₁₁₀=18.4±0.7. The fluorogenic effect was thoroughly investigated. Nearly selective excitation of the chelate rather than the ligand could be achieved at wavelengths longer than 360 nm. For analytical purposes the main interfering ion was Ga³⁺. The strongest competing ligand was shown to be citric acid. Competitive complexation by acetate or formate ions can also make their use in a buffer at the usual concentration, 0.2 mol L^–1, questionable, whereas a 10^–2 mol L^–1 formic acid buffer was shown to be a good alternative. The calibration plot showed that the dependence of response on Al(III) concentration was linear up to 500 g L^–1; the detection limit was 0.65 g L^–1 (3SD _blank, n=10, SD=±1.4% at 10 g L^–1 and ±0.8% at 100 g L^–1). The analytical procedure was successfully applied to several samples of tap water and the results were in good agreement with those from AAS determination.     

Interaction between Nickel(II) and Anions of Short-Chain Dialkyl Dithiphosphoric Acids in Aqueous Surfactant Solutions

Spectrophotometry was used to study the complexation of nickel(II) with anions L^– of diisopropyl and dibutyl dithiophosphoric acids in water and aqueous solutions of nonionic surfactant, Triton X-100 (T). Weak bis-complexes [NiL₂], whose formation are stimulated by the addition of nonionic surfactant, were found in water. Within the framework of simple model including equilibria of the formation of micelle-bound complexes {[NiL₂]T} and ligand associates {LT₂ ^-}, the values of logK = 3.87 ± 0.01 and 1.3 ± 0.3 for diisopropyl dithiophosphoric acid anions and logK = 5.47 ± 0.03 and 2.8 ± 0.2 for dibutyl dithiophosphoric acid anions, respectively, were calculated. The obtained results showed that the stability of associates of hydrophobic anions of dialkyl dithiophosphates and their nickel bis-complexes with nonionic micelles increases with the length of ligand chain.     

Fourier transform infrared study of mercury interaction with carboxyl groups in humic acids

An interaction between humic acid, an organic part of soil and mercury was studied by Fourier transform infrared spectroscopy (FTIR) and by ICP-AES analysis under given pH and concentration conditions. First the spectroscopic model was validated on the interaction of simple molecules representing the structural components of humic acid such as benzoic acid, catechol and salicylic acid with mercury. The interaction of carboxylic parts of humic acid with mercury is very interesting and easily characterised by infrared spectroscopy, an ideal mean for molecular study. Under the salt form (commercial humic acid Fluka TM: FHA), humic acid reacts with mercury in a different way from its acid form (FHA purified noted PFHA) and the Leonardite (LHA). Because of the straightforward exchange between Na⁺, Ca²⁺ and Hg²⁺, fixation of the latter is much more important with the salt form (FHA). However, this reaction is reduced under the acid form (PFHA, LHA) because the exchange with protons is difficult. The effect of this exchange was studied by FTIR showing the intensity decrease of ν_CO (COOH), the carboxylic functional group band of the acid, and the shifting of ν_as (COO⁻), the carboxylate functional group band under given pH and mercury conditions. For the FHA salt form, the characteristic band ν_CO (COOH) represented by a shoulder did not evolute, whereas the corresponding band to ν_as (COO⁻) strongly shifted (40 cm⁻¹) for a maximum Hg²⁺ concentration (1 g l⁻¹). On the other hand, for the acid form (PFHA, LHA), the intense band of ν_CO (COOH) disappeared proportionally to the increase of Hg²⁺concentration and the ν_as (COO⁻) band moved for about 20 cm⁻¹. The same results were reached with pH variations. Our results were confirmed by ICP-AES mercury analysis. This study shows that humic acids react differently according to their chemical and structural state.     

A study on the hydrolytic behavior of thorium (IV) and dioxo-uranium(VI) in the absence and presence of fluoride ions

The hydrolytic behavior of thorium(IV) and dioxo-uranium(VI) was studied in the absence as well as in the presence of small concentration of fluoride in the pH range of 2.0 to 4.0 and 2.0 to 5.5, respectively. Effects of metal ion concentration and time dependence of the hydrolytic process were also investigated. The log values of ₁₁ ^* and ₁₃ ^* computed from the best model of species distribution were found to be -3.51±0.03, and -10.75±0.14 for thorium(IV). Similarly, the best model for dioxo-uranium consisted of ₂₂ ^*, ₄₆ ^* and ₄₇ ^* species having values -6.15±0.05-18.43±0.09 and -23.36±0.07, respectively. Recently developed HYPERQUAD computer program was used for analyzing the various aspects of solution equilibrium species. Different models of chemically possible species distribution were invoked to identify the best model for which HYPERQUAD yields the best fitting of experimental data with least errors. The best model was also decided on the basis of chemical feasibility of the reaction. The species distribution of hydrolytic product remained unaffected in the presence of small quantity of fluoride ions (~1% of thorium and uranium concentration). Moreover, fluoride was found to be helpful in suppressing the early polymerization and colloid formation at the metal ion concentrations investigated. The small amounts of fluoride did not seem to affect the response of glass electrode significantly. The formations of fluoride containing ternary complexes were also not observed at 1% fluoride concentration.     

Influence of Excitation of the Lanthanide 4 fShell on Complexation with Organic Substrates in Solutions: 1. The Complexation of the β-Diketonate Tris(heptafluorodimethyloctanedionato)europium(III) with Sulfoxides in Benzene Solutions

Complexation of sulfoxides R¹R²S=O with the -diketonate Eu(fod)₃(fod is heptafluorodimethyloctanedionato) in the ground and excited states in benzene solutions was studied. Excitation of Eu(fod)₃was found to increase the formation constants and to reverse the sign of the enthalpy of complexation. The compensation effect was observed for the thermodynamic parameters: S ₀= (3.4 ± 0.4) × 10^–3H ₀+ (50.0 ± 4.7) in the ground state and S ^* ₀= (3.2 ± 0.4) × 10^–3H ^* ₀+ (62.0 ± 0.6) in the excited state of Eu(fod)₃. The enhancement of the stability of the complexes [Eu(fod)^* ₃· R¹R²S=O] is due to an increase in the entropy of complexation upon excitation of f–ftransitions in Eu(III).     

Complexation properties of phosphonocarboxylic acids in aqueous solutions

The concentration formation constants of phosphonoacetic acid (PAA) complexes with the Ca²⁺ and Mg²⁺ ions were determined in aqueous solution at 25°C by potentiometric and coulometric titrations at different ionic strengths and were extrapolated to I=0 in order to obtain thermodynamic values of the formation constants. Complexes were formed by the completely deprotonated K _f (ML) and monoprotonated K _f (MHL) forms of the PAA anion. The respective values for the complexes are: log K _f (CaL)=4.68±0.03, log K _f (CaHL)=2.61±0.08; log K _f (MgL)=5.58±0.09, log K _f (MgHL)=3.0±0.3. The enthalpy and entropy of complexation for the deprotonated Ca²⁺ and Mg²⁺ PAA species, determined from the temperature dependence of the log K _f (ML), are: H⁰(Ca) =0.6±0.2 kcal-mol^–1, S⁰(Ca)=21.4±0.6 cal-mol^–1-K^–1, H⁰(Mg)=3.0±0.7 kcal-mol^–1, and S⁰(Mg)=35±2 cal-mol^–1-K^–1. It is seen there-fore, that the complexes are entropy stabilized but enthalpy destabilized. Formation constants were also determined for Ca²⁺ and Mg²⁺ complexes with PAA analogs, phosphonoformic and 3-phosphonopropionic acids and the complexation of PAA was also studied at a single ionic strength, with Na⁺, Ag⁺, Tl⁺, Sr²⁺, Ba²⁺, Cd²⁺, Cu²⁺, and Pb²⁺ ions.     

Potentiometric Titrations of Rutile Suspensions to 250°C

A stirred hydrogen electrode concentration cell was used to conduct potentiometric titrations of rutile suspensions from 25 to 250°C in NaCl and tetramethylammonium chloride media (0.03 to 1.1m). Hydrothermal pretreatment of the rutile improved titration reproducibility, decreased titration hysteresis, and facilitated determination of the point of zero net proton charge (pHznpc). These pHznpc values are 5.4, 5.1, 4.7, 4.4, 4.3 (±0.2 pH units), and 4.2 (±0.3 pH units) at 25, 50, 100, 150, 200, and 250°C, respectively. The difference between these pHznpc values and pK_w(the neutral pH of water) is rather constant between 25 and 250°C (−1.45 ± 0.2). This constancy is useful for predictive purposes and, more fundamentally, may reflect similarities between the hydration behavior of surface hydroxyl groups and water. A three-layer, 1pKa surface complexation model with three adjustable parameters (two capacitance values and one counterion binding constant) adequately described all titration data. The most apparent trend in these data for pH values greater than the pHznpc was the increase in proton release (negative surface charge) with increasing temperature. This reflects more efficient screening by Na⁺relative to Cl⁻. Replacing Na⁺with the larger tetramethylammonium cation for some conditions resulted in decreased proton release due to the less efficient screening of negative surface charge by this larger cation.     

Thermodynamic Studies of the Complexation between Neodymium and Acetate at Elevated Temperatures

Complexation of neodymium (III) with acetate in 2.2 mol-kg^-1 NaClO₄ solution was studied at elevated temperatures (45 and 70°C) by potentiometry, calorimetry, and optical spectroscopy. The formation constants of the consecutive complexes, Nd(OOCCH₃),²⁺ Nd(OOCCH₃) ₂ ⁺ , and Nd(OOCCH₃)₃, and the molar enthalpies of complexation at these temperatures were determined. The stability of the three complexes increases with increased temperatures, because of increased positive entropy change at higher temperatures, which exceeds the increased values of the positive (endothermic) enthalpy. The molar heat capacity changes of complexation C_p,m(MLj) (J-K^-1-mol^-1) for Nd(OOCCH₃) _j ^(3-j)+ in the temperature range from 25 to 70°C were calculated to be: 102 ± 13 (j = 1); 122 ± 19 (j = 2); and 239 ± 27 (j = 3). The effect of temperature on the complexation is discussed in terms of the electrostatic model.     

Thermodynamics of Cm(III) in Concentrated Salt Solutions: Carbonate Complexation in NaCl Solution at 25°C

The carbonate complexation reactions of Cm(III) were studied by time-resolved laser fluorescence spectroscopy in 0–6 m NaCl at 25°C. The ionic strength dependence of the stepwise formation constants for the carbonato complexes Cm(CO₃) _n ^3–2n with n = 1, 2, 3, and 4 is described by modeling the activity coefficients of the Cm(III) species with Pitzer's ion-interaction approach. Based on the present results and literature data for Cm(III) and Am (III), the mean carbonate complexation constants at I = 0 are calculated to be: log ₁₀₁ ^o =8.1 ±0.3, log ₁₀₂ ^o =13.0 ± 0.6, log ₁₀₃ ^o =15.2 ± 0.4, and log ₁₀₄ ^o =13.0 ± 0.5. Combining these equilibrium constants at infinite dilution and the evaluated set of Pitzer parameters, a model is obtained, that reliably predicts the thermodynamics of bivalent actinide An(III) carbonate complexation in dilute to concentrated NaCl solution.     

The Influence of 4f Orbital Excitation in Eu(Fod)3 on Complexation with Sulfones in Benzene Solutions

Complexation of sulfones (S) with the -diketonate Eu(Fod)₃ (Fod–heptafluorodimethyloctanedione) in the ground and excited electronic states in benzene solutions was studied. The stability constants and thermodynamic parameters for the formation of complexes Eu(Fod)₃ · S in the ground state (K, H ₀, S ₀) and Eu(Fod)₃ ^* · S in the excited state (K*, H ₀ ^*, S ₀ ^*) were determined. The excitation of f–f transitions of Eu(III) was found to enhance the stability of Eu(Fod)₃ · S complexes, apparently due to an increase in the acceptor ability of the Eu(III) chelate. This fact confirms the involvement of the 4f orbital in the chemical bond formation. The compensation effect was observed for the thermodynamic parameters: S ₀ = (2.9 ± 0.3) × 10^–3H ₀ + (35.0 ± 4.0) in the ground and S ₀ ^* = (3.3 ± 0.3) × 10^–3H ₀ ^* + (49.0 ± 5.0) in the excited states of Eu(Fod)₃. It was shown that electronic excitation of the 4f orbital of Eu(Fod)₃ influences isotopic effects in complexation with sulfolanes.     

Spectra and structure of organophosphorus compounds: XLIX. Microwave,infrared, and Raman spectra,electric dipole moment,molecular structure,and ab initio calculations of dimethylphosphonothioic fluoride

The microwave spectra of (CH₃)₂PSF, (CH₃)(CD₃)PSF, (CD₃)₂PSF, and (CH₃)₂P³⁴SF have been investigated from 20.0 to 40.0 GHz. Botha-type R branch andc-type Q branch transitions have been measured in the ground states of each isotopic species. From a least-square adjustment to fit 12 rotational constants, the following structural parameters were obtained:r(P–F)=1.582 ± 0.003 Å;r(P=S)=1.902 ± 0.001 Å;r(P-C)=1.800 ± 0.001 Å;r(C-H)=1.088 ± 0.002 Å; HCP=109.28 ± 0.12°; SPF=114.50 ± 0.13°; and SPC=116.33 ± 0.06°. From Stark effect measurements, the dipole moment components have been determined to be ¦ _a ¦ =3.556 ± 0.005; ¦ _c ¦=2.026 ± 0.009; and ¦ _t ¦=4.093 ± 0.009 (D). The Raman spectra (3200 to 100 cm^–1) of each isotopic species have been measured for the solid, and liquid and qualitative depolarization values obtained. Additionally, the mid-infrared spectra (3200 to 500 cm^–1) of the solids have been recorded. Proposed assignments of the normal modes have been made on the basis of Raman depolarization values and group frequencies which are supported by normal coordinate analysis utilizing an ab initio force field. Optimized structural parameters have been obtained with both the 3-21G^* and 6-31G^* basis sets. These results are compared to the corresponding quantities for several similar molecules.For part XLVIII, seeJ. Raman Spectrosc.1922,23, 107.     

Thermochemical characterization of phenolic resins

Phenol-formaldehyde resins (I andII), synthesised at a monomer feed ratio of F/P = 1.0 and 1.5, were cured at 130C for 48 h without any catalyst (Ia, IIa), with 0.1% ferric acetyl acetonate (Ib, IIb) and with 0.1 %p-toluenesulphonic acid (Ic, IIc). Thermogravimetric studies indicate that the decomposition of the cured products takes place in two distinct stages: The first stage (T=340–480C; =0.045–0.16; E ₁ = 140±10-239±24 and 60±3–65±2 kJ mol^–1 for seriesI andII respectively) was attributed to the predominant cleavage of formal linkages. The main stage decomposition (T=460–640C; =0.114–0.393; E ₂=115±8–169±8 and 91±6–103±7 kJ mol^–1 for seriesI andII respectively) was attributed to reactions leading to graphitisation. E ₂ values were correlated to the extent of cure as measured by IR spectroscopy and pyrolysis-GC. The effect of catalysts on the extent of cure and on the activation energy was evaluated.The authors are grateful to Dr. S. Ganapathy, NCL, Pune for providing solid state NMR, Dr. K. Krishnan and Dr. A. G. Rajendran, VSSC, Trivandrum for thermoanalytical measurements.     

Complexation between Iron(III) and β-Hydroxyethylimino-N,N-Diethanoic and Dicarboxylic Acids in Aqueous Solutions

The reactions of iron(III) with -hydroxyethylimino-N,N-diethanoic acid (H₂Heida) and dicarboxy-lic acids (oxalic (H₂Ox), malonic (H₂Mal), and succinic (H₂Suc)) are studied using the spectrophotometric method. The equilibrium pattern in the binary and ternary systems is investigated. The complexation processes were shown to be complicated by hydrolysis and to depend strongly on the acidity of the medium. The following complexes were detected: FeHeida⁺, Fe(OH)Heida, Fe(OH)₂Heida^–, Fe(Heida)Ox^–, Fe(OH)(Heida)Ox^2–, Fe(OH)₂(Heida)Ox^3–, Fe(Heida)Mal^–, Fe(OH)(Heida)Mal^2–. Logarithms of the stability constants of these complexes calculated at = 0.1 (NaClO₄) and T = 20 ± 2°C are equal to 11.64 ± 0.05, 22.97 ± 0.05, 31.17 ± 0.05, 18.83 ± 0.03, 28.27 ± 0.02, 36.14 ± 0.02, 17.64 ± 0.03, and 26.39 ± 0.03, respectively.     

Divalent transition metal ion mixed ligand complexes with aliphatic dicarboxylic acids and imidazoles

Summary Mixed ligand metal complexes of Co^II, Ni^II and Cu^II with dicarboxylic aliphatic acids, H₂L (succinic, malic and tartaric) as primary ligands and with imidazoles, L (imidazole and 2-methylimidazole) as secondary ligands were prepared and characterized. MLL₂ and ML₄ molecular formulae were suggested for these complexes were Formation constants of the different complexes were determined pH-metrically at T = 25 ± 0.1°C and = 0.1 mol dm^–3 (NaClO₄). The stability of the mixed ligand complexes increased as the effective basicity of the dicarboxylic aliphatic acid anion increased, namely, tartarate < malate < succinate acid.     

Dissociation quotients of D-galacturonic acid in aqueous solution at 0.1 MPa to 1 molal ionic strength and 100°C

The molal dissociation quotients of D-galacturonic acid were measured potentiometrically in a newly-designed, hydrogen-electrode concentration cell from 5 to 100°C at four ionic strengths ranging from 0.1 to 1.0 mol-kg^–1 using sodium trifluoromethanesulfonate (NaF₃CSO₃) as the supporting electrolyte. These quotients were fitted in the all anionic (isocoulombic) form by an empirical equation incorporating three adjustable parameters. When combined with the known dissociation quotient for water in the same medium, this treatment yielded the following thermodynamic quantities for the acid dissociation equilibrium at 25°C and infinite dilution: log K_H=–3.490±0.011, H _H ⁰ =0.4±0.2 kJ-mol^–1, S _H ⁰ =–65±1 J-mol^–1-K^–1, and C _{p, H} ⁰ =–231±8 J-mol^–1-K^–1. Comparisons are made with the corresponding results of a limited number of previous studies carried out near ambient conditions.     

Dynamics of the formation of excited hydrogen,H(n=2,3,4), during a pulse discharge in methane

The dynamics of the formation and decay of excited hydrogen during a pulse discharge in methane at a pressure of 200 Pa and energy density of 0.05 J/cm³ has been studied. The population of hydrogen in the n=2 state was monitored by the laser absorption method. The time constant of the decay of the excited hydrogen was measured to be 95±15 ns. The concentration of free electrons reached a maximum value of 7×10¹⁴ cm^–3, and the time constant of their recombination was 220±50 ns. The formation of appreciable amounts of atomic hydrogen in the ground state during the discharge, H(n=1)>10¹⁶ cm^–3, was estimated on the basis of a kinetic model.Notation absorption coefficient - wave number, cm^–1 - c velocity of light - e electron charge - m _e mass of electron - A _mn Einstein coefficient - F _mn collisional deexcitation rate constant - S _m ionization rate constant - f ₂₄ oscillator strength - n _e electron concentration - n _H(n=2,3,4) excited-state hydrogen concentration - v _e electron velocity - q _mn excitation cross-section - (q _mnve) excitation rate constants - T _e electron temperature - E (t) electrical field strength - j current density - t _ei ^–1 electron-ion collision frequency - _2,m two-body recombination rate constant - _3,m three-body recombination rate constant     

Preparation and Thermochemistry of Complexes of Zinc Nitrate with Threonine

The solubility property of Zn(NO₃)₂–Thr–H₂O system (Thr—threonine) at 25°C in the entire concentration range has been investigated by the phase equilibrium semimicromethod. The corresponding phase diagram and refractive index diagram were constructed. From the phase equilibrium results, the incongruently soluble compounds of Zn(Thr)(NO₃)₂ · 2H₂O, Zn(Thr)₂(NO₃)₂ · H₂O, and Zn(Thr)₃(NO₃)₂ · H₂O were synthesized and characterized by IR, XRD, TG–DTG, chemical and elemental analyses. The constant-volume combustion energies of the compounds, _c E, determined by precision rotating bomb calorimeter at 298.15 K, were –6266.88 ± 3.72, –9263.28 ± 2.23, and –11 423.11 ± 6.81 J/g, respectively. The standard enthalpies of combustion for these compounds, _c H _m ^° (complex, s., 298.15 K), were calculated as –2147.40 ± 1.28, –4120.83 ± 0.99, and –6444.68 ± 3.85 kJ/mol and the standard enthalpies of formation, _f H _m ^° (complex, s., 298.15 K), are –1632.82 ± 1.43, –1885.55 ± 1.50, and –2770.25 ± 4.21 kJ/mol. The enthalpies of dissolution of the complexes in a medium of simulated human gastric juice (37°C, pH 1, in the solution of hydrochloric acid), _dis H _m ^° (complex, s., 310 K), which were also measured by a microcalorimeter to be 13.36 ± 0.06, 15.53 ± 0.06, and 17.04 ± 0.05 kJ/mol, respectively.     

Determination of Aqueous Nickel-Carbonate and Nickel-Oxalate Complexation Constants

An ion-exchange method was used to determine complexation constants for the Ni-oxalate and Ni-carbonate systems in a NaClO₄ background electrolyte. The Ni-oxalate data were interpreted in terms of a single Niox(aq) complex having log K ₁ values for Ni²⁺ + ox^2– Niox(aq) of 3.9 ± 0.1 (I.S. = 0.5 mol-L^–1 p[H] = 7.1) and 4.4 ± 0.1 (I.S. = 0.1 mol-L^–1 p[H] = 8.6) at 22 ± 1C. Specific ion-interaction theory (SIT) was used to obtain log K ₁ = 5.17 ± 0.05 (95% confidence level and = –0.23 ± 0.15) at I.S. = 0. The Ni-carbonate studies were carried out at p[H] values of 7.5, 8.5, and 9.6 in 0.5 mol-L^–1 NaClO₄/NaHCO₃ solutions. The NiCO₃(aq) species was the dominant complex in the [CO₃ ^2–] concentration ranges studied at all three p[H] values. A log K ₁ value for Ni²⁺ + CO₃ ^2– NiCO₃(aq) of 2.9 ± 0.3 was deduced at I.S. = 0.5 mol-L^–1. Extrapolating this value to zero ionic strength using the SIT approach yielded log K ₁ = 4.2 ± 0.3 (95% confidence level and = –0.26 ± 0.04). The data allowed upper bound values for the complexation constants for NiHCO₃ ⁺ and Ni(CO₃)₂ ^2– to be estimated, i.e., log K < 1.4 for Ni²⁺ + HCO₃ ^– NiHCO₃ ⁺, and log K ₂ < 2 for NiCO₃(aq) + CO₃ ^2– Ni(CO₃)₂ ^2–, respectively.