Deficiencies of the ferricinium-ferrocene redox couple for estimating transfer energies of single ions

The difference between the partial molal entropies of ferrocene and ferricinium has been determined in nine solvents from the temperature dependence of the formal potential of the ferricinium-ferrocene redox couple using a nonisothermal electrochemical cell arrangement in order to probe possible structural reasons for the limitations of the ferrocene assumption for estimating the transfer thermodynamics of single ions between different solvents. In contrast to the uniformly small positive values of predicted by the Born model, the experimental quantities varied widely from small or even negative values in hydrogen-bonded solvents (–5 to 3 e.u.) to substantially larger values (11–14 e.u.) in dipolar aprotic media. These variations appear to arise chiefly from additional solvent ordering in the vicinity of the ferricinium cation compared to the ferrocene molecule which is enhanced in the aprotic solvents. The variations in between water and a number of nonaqueous solvents provide a predominate contribution to the differences between the free energies of single ion transfer calculated using the ferrocene and alternative extrathermodynamic assumptions.     

Transfer activity coefficients of crown ethers and their complexes with univalent metal ions between pairs of polar organic solvents

From a comparison of transfer activity coefficients, [(LM⁺)]_PC,2 between propylene carbonate and solvent S₂ of alkali or silver ions complexed with dibenzo-substituted crown ethers (L=DB-18-cr-6, DB-21-cr-7, DB-24-cr-8, DB-30-cr-10) it can be concluded that in the complex LM⁺ both L and M⁺ are solvated, particularly in solvents of high donicity, e.g., N,N-dimethylformamide. From the abnormally low ionic mobility of DB-30-cr-10K⁺ in acetonitrile and the high value of the association constant of the ion pair DB-30-cr-10KBr it is concluded that the outer solvent shell is stripped upon formation of the ligand separated ion pair. A linear relation is found between [log (LM⁺)]_PC,2 and [log(M⁺)]_PC,2 only when L is 18-cr-6, B-18-cr-6, or DB-18-cr-6. Deviation from the linearity of complexes of the larger dibenzo crown ethers is attributed to the flexibility of L. It is shown that solution of 18-cr-6, DB-18-cr-6 and DB-30-cr-10 is enthalpy assisted to a greater extent in acetonitrile than in methanol, while the entropy of solution is more favorable in the latter.     

Stabilities in water and transfer activity coefficients from water to nonaqueous solvents of 15-crown-5 and 16-crown-5-metal ion complexes

Stability constants of 1 : 1 16-crown-5 (16C5)-metal ion complexes were determined in water at 25°C by conductometry and potentiometry with ion-selective electrodes. The selectivity sequences of 16C5 in water for univalent and bivalent metal ions are Ag⁺ > Na⁺ Tl⁺ > K⁺ and Sr²⁺ > Ba²⁺ Pb²⁺, respectively. The stability of a given 16C5-metal ion complex in water is much lower than might be expected on the basis of the solvation power (i.e. relative solubility of the metal ion) of water for the metal ion. The same tendency is observed for the cases of 15-crown-5 (15C5) -metal ion complexes. Transfer activity coefficients () of 15C5 and 16C5 for tetradecane (TD)/water, TD/methanol, TD/acetonitrile, and propylene carbonate/water systems were determined at 25°C. From these data, contributions of a methylene group and an ether oxygen atom to the log value of a crown ether were then obtained. The values from water to acetonitrile, propylene carbonate, and methanol of 15C5- and 16C5-univalent metal ion complexes were calculated, s, M⁺, and L being a solvent, a univalent metal ion, and a crown ether, respectively. The log value is greater than the corresponding log value. The log values are negative. This indicates that, although the M^- ions are more soluble in water than in the nonaqueous solvents, when the crown ether forms a complex with the M⁺ ion, the complex becomes more soluble in the nonaqueous solvents than in water, compared with the free crown ether. It was concluded from this finding that the unexpectedly low stability of the crown ether-M⁺ complex in water is attributed to strong hydrogen bonding between ether oxygen atoms of the free crown ether and water.     

Solvation of the potassium ion complexed with dibenzo-30-crown-10. Transfer activity coefficients between methanol and various solvents

The stability constants of the potassium complex with dibenzo-30-crown-10 have been determined from potentiometric or solubility measurements in the solvents: methanol, iso-propanol, n-butanol, propylene carbonate, acetonitrile and dimethylsulfoxide. The solubility of the ligand in these solvents has also been determined and the transfer activity coefficients of the potassium complex for transfer from methanol to solvent (S), _SM(KL⁺), have been computed. Although solid state studies indicate that dibenzo-30-crown-10 completely surrounds the potassium ion and shields it from water, the transfer activity coefficient of the potassium complex is found to be highly solvent dependent. Dibenzo-30-crown-10 is thus less effective for the removal of the solvation sphere of the potassium ion than previously estimated.     

Solvation of bivalent transition metal cations. I. Heats of transfer of the metal perchlorates between water,acetonitrile, and dimethylsulfoxide

Heats of solution of Mn(ClO₄)₂·6AN, Fe(ClO₄)₂·6AN, Co(ClO₄)₂·6AN, as well as Ni(ClO₄)₂·6AN, Cu(ClO₄)₂·4AN, and Zn(ClO₄)₂·6AN (AN=acetonitrile) have been determined in water, acetonitrile, and dimethylsulfoxide (DMSO). Coupled with the heats of solution of AN in the respective solvents they give the heats of transfer of the metal perchlorates. Heats of transfer of the cations as well as their heats of solvation are calculated using literature data for the heat of transfer of the perchlorate anion and the heats of hydration of the cations, respectively. The results are discussed in terms of the different contributions to the overall heat effects.     

Correlation of the gibbs free energy of transfer of fluoride ion from water to other solvents with solvent actidity and polarity parameters

Linear free energy relationships between the free energy of transfer of fluoride ion from water to other solvents or solvent mixtures and each of several parameters describing the acidity of the solvent were found. The correlation is about of the same quality for the A parameter of Swain or the acceptor number, AN, and almost as good for the parameter of Taft. The correlation was somewhat poorer with the E _T (30) parameter.     

Aqueous-phase ion solvation and the selectivity of anion exchange resins for monovalent ions

This research examines and quantifies the influence of ion solvation parameters on the affinity of monovalent anions for strong-base anion resins. A data set comprising resin selectivity coefficients and solvation parameters from the literature is statistically analyzed using correlation and multiple regression techniques. The affinity of monovalent anions for the resin phase correlated well to ionic radii. Solvation parameters such as the hydration number, and entropy, enthalpy and free energy of hydration are also strongly correlated to selectivity. Using the stepwise regression procedure on subsets of independent variables, the entropy of hydration, which characterizes the structure-influencing nature of ions in solution, is incorporated as the sole parameter in the predictive model for resin selectivity. The data are best correlated by the exponential form of the regression equation, and the physical meaning of the correlation is shown to be reasonable. A simple rule for categorizing ions as structure-makers and structure-breakers is proposed, and the results are consistent with conventional classifications.     

Stoichiometry, complex stability, molecular size and conformational variations of metal ion crown ether complexes subject to diffusion coefficients in nonaqueous solutions

Both the stoichiometry and complex stability constants of crown ether complexes with metal ions have been determined by examining gradual changes in their diffusional behavior in nonaqueous solution. Diffusion coefficients, D, were evaluated by pulsed field gradient (PFG) NMR titration experiments whilst complex stability constants were determined by nonlinear curve-fitting procedures, D versus c_sol., which also allow the treatment of multiple complexation equilibria (1:1 to 1:2 stoichiometries). Differences in the diffusion coefficients of the various free crown ethers with respect to their metal ion complexes indicate great sensitivity to both conformational changes and changes in molecular size upon complexation.     

Prediction of Henry''s constants by the UNIFAC-FV model for hydrocarbon gases and vapors in high-boiling hydrocarbon solvents

The design of absorption processes and gas liquid reactors requires the knowledge of gas solubility data. It is shown that a modified UNIFAC-FV model can be applied to predict Henry's constants for hydrocarbon gases and vapors in high-boiling hydrocarbon solvents. Very good agreement with experimental data has been achieved.     

Free energies of transfer of alkali metal halides in isodielectric acetonitrile and ethylene glycol mixtures at 25°C

Standard free energies of transfer (G _t ^o ) of alkali metal chlorides MCl (M=Li, Na, K, Rb, Cs) and of potassium bromide and iodide from ethylene glycol to its isodielectric mixtures containing 20, 40, and 60 wt. % acetonitrile have been determined from emf measurements at 25°C. The standard potentials of the M/M⁺, the Ag–AgBr, and the Ag–AgI electrodes have been calculated using auxiliary emf measurements. All electrolytes were found to be increasingly destabilized in mixed solvents of increasing acetonitrile content; the relative order of cation destabilization is Li⁺>Na⁺>K⁺>Rb⁺>Cs⁺ and that of anion destabilization is Cl^–>Br^–>I^–, both following (a) the order of increasing softness of ions and increasing interaction with the soft base, acetonitrile,and (b) the order of decreasing hydrogen-bonding propensity of ions and decreasing interactions with the hard base, ethylene glycol.     

Kinetics of anodic formation and cathodic reduction of MnO2 in the sulfate electrolyte solutions

The anodic formation of manganese dioxide is studied voltammetrically in a wide range of potential scan rate (V = 0.001–8 V/s). Using the diagnostic criteria of cronovoltammetric method, based on the original experimental data, the mechanism of electrooxidation of manganese ions in the acidic medium with subsequent reaction of disproportionation of the product of irreversible electrode reaction and hydrolysis yielding manganese dioxide is proposed. The kinetics of cathodic reduction of electrolytic manganese dioxide in the 0.5 M Na₂SO₄ solution is studied under the steady-state and non-steady-state potentiodynamic polarization conditions. From the experimental data, it is found that, in the acidic medium (pH 1–3), the mechanism of the electrode process changes depending on the cathodic potential scan rate: at the scan rate V < 0.5 V/s, MnOOH forms via one-electron transition leading, in its turn, to the partial deactivation of electrode surface with subsequent disproportionation of manganite. At the relatively high potential scan rates, manganite has no time to form, and the two-stage reduction via one-electron transitions at each stage is well pronounced. The parameters of the electrode processes are calculated.     

Osmotic and activity coefficients in the binary solutions of 1-butyl-3-methylimidazolium chloride and bromide in methanol or ethanol at T = 298.15 K from isopiestic measurements

Osmotic coefficients of the binary solutions of two room-temperature ionic liquids (1-butyl-3-methylimidazolium chloride and bromide) in methanol and ethanol have been measured at T = 298.15 K by the isopiestic method. The experimental osmotic coefficient data have been correlated using a forth-order polynomial in terms of (molality)^0.5, with both, ion interaction model of Pitzer and electrolyte non-random two liquid (e-NRTL) model of Chen. The values of vapor pressures of above-mentioned solutions have been calculated from the osmotic coefficients. The model parameters fitted to the experimental osmotic coefficients have been used for prediction of the mean ionic activity coefficients of those ionic liquids in methanol and ethanol.     

Importance of the Debye interaction in organic solutions: Henry's law constants for polar liquids in nonpolar solvents and vice versa

Henry's law constants have been determined for -butyrolactone (BL), ethyl acetate (EA), and 2-methyl-3-pentanol (MEP) in mixtures of iso-octane (ISO) and toluene (TOL), for BL, EA, TOL, and ISO in cinnamaldehyde (CIN) and for TOL and ISO in each other and in BL. From these data and published vapor pressures, the activity coefficients at infinite dilution and the standard molar Gibbs free energy of transfer, G ₂ ⁰ of the solutes from dilute solution in ISO to dilute solution in each solvent medium have been calculated. The different behavior patterns of BL and EA are attributed to differences in their abilities to exist in different conformations possessing different dipole moments. For polar solutes, G ₂ ⁰ decreases with increasing polarizability of the solvent and with increasing dipole moment of the solute, suggesting increased contributions from dipole-induced dipole (Debye) interactions. The sigmoidal plot of G ₂ ⁰ against the change in pair potential energy calculated from the classical expressions suggests that G ₂ ⁰ seriously underestimates the strength of the Debye interactions in comparison with the London interactions.     

Protonation constant of monoaza-12-crown-4 ether and stability constants with selected metal ions in aqueous solution in the presence of an excess of sodium ion: a potentiometric and differential pulse polarographic study at fixed ligand to metal ratio and varied pH

The ligand monoaza-12-crown-4 ether (A12C4) was studied in aqueous solution at 298 K and an ionic strength of 0.5 mol dm⁻³ in the presence of an excess of sodium ion (0.5 mol dm⁻³ NaNO₃). The protonation constant of A12C4, determined by glass electrode potentiometry (GEP) in the same background electrolyte, was found to be log K=9.36±0.03. Polarographic experimental and calculated complex formation curves (ECFC and CCFC) for labile metal–ligand systems, studied at a fixed total ligand (L_T) to total metal (M_T) concentration ratio and varied pH, were used for the modelling of the metal species formed and the refinement of their stability constants. The metal–ligand model and formation constants are optimised by solving mass-balance equations written for the assumed model and by fitting the CCFC to the ECFC. The CCFC can be generated for any metal–ligand model, including polynuclear metal species, for any L_T:M_T ratio, and for more than one ligand competing in the complex formation reaction. Three lead complexes with the ligand A12C4, viz. PbL²⁺, PbL(OH)⁺ and PbL(OH)₂, were found and their overall stability constants from differential pulse polarography (DPP), as log β, were estimated to be 3.75±0.03, 9.30±0.05 and 12.70±0.05, respectively. Two copper complexes CuL²⁺ and CuL(OH)₂ are reported and their stability constants (from DPP) were estimated to be 6.00±0.05 and 21.77±0.1, respectively. Two cadmium complexes CdL²⁺ and CdL(OH)⁺ are reported. The stability constant for CdL²⁺ was estimated from DPP and GEP as 2.80±0.05 and 2.68±0.03 (the latter value was obtained from a few potentiometric experimental points), respectively, and the stability constant for CdL(OH)⁺ from DPP was estimated to be 7.88±0.05. GEP could not be used for the stability constants determination of other metal complexes studied because of precipitation occurring prior the completion of a complex formation reaction.