Heats of solution of 1:1 electrolytes in 1,2- and 1,1-dichloroethane and derived enthalpies and entropies of transfer of electrolytes from water to these solvents

Heats of solution of nine electrolytes in 1,2-dichloroethane and of three electrolytes in 1,1-dichloroethane have been determined calorimetrically at various electrolyte concentrations and extrapolated to zero concentration to yield H _s ^o values for these electrolytes. It is shown that values of H _t ^o for transfer from water to the dichloroethanes of 11 electrolytes are often negative, so that these electrolytes can be more stable enthalpically in the less polar solvents. Combinations of the H _t ^o values with previously determined G _t ^o values yield values of S _t ^o for transfer of 11 electrolytes from water to the dichloroethanes. These S _t ^o values are mostly very negative; they can be correlated very well by the method of Abraham, and in this way S _t ^o values for transfer of numerous other anions and cations have been predicted. The Ph₄As⁺/Ph₄B^– convention yields single-ion entropies of transfer from water to the dichloroethanes in reasonable agreement with values calculated by the correspondence-plot method.     

Thermodynamics of transfer of non-ionic solutes between immiscible liquid phases. I. Transfer of normal alcohols fromn-octane to water at 25°C

A recently developed calorimetric method has been employed to estimate the thermodynamic functions for transfer of 1-propanol, 1-butanol, 1-pentanol, 1-hexanol and 1-heptanol from n-octane to water at 25°C. A linear correlation for G _t ^o as a function of the number of carbon atoms of the alchohol molecule has been found but for H _t ^o and S _t ^o the dependence gave well defined minima.     

Solubilities in aqueous mixtures oft-butanol andtrans-dichlorotetra(3-alkylpyridine) cobalt(III) hexachlororhenate(IV): Gibbs energies of transfer of the complex cations

The solubilities of the hexachlororhenate(IV) salts of the complex cations trans-[Co(3Mepy)₄Cl₂]⁺ and trans-[Co(3Etpy)₄Cl₂]⁺ have been determined in water+t-butyl alcohol mixtures. By reference to the solubilities of Cs₂ReCl₆ and the Gibbs energies of transfer of Cs⁺ from water into water+t-butyl alcohol mixtures, G _t ^o (Cs⁺), G _t ^o [Co(3Mepy)₄Cl ₂ ⁺ ] and G _t ^o [Co(3Etpy)₄Cl ₂ ⁺ ] are calculated. These latter values, when introduced into the equation for a free energy cycle applied to the process of the initial state going to the transition state for the solvolyses of these two cations, produces values for G _t ^o [Co(3Mepy)₄Cl^2+*] and G _t ^o [Co(3Etpy)₄Cl^2+*] for the Co³⁺ cations in the transition state. These values are compared with (G _t ^o (i) for i=[Co(Rpy)₄Cl₂]⁺, [Co(Rpy)₄Cl]^2+*, [Coen₂XCl]⁺ and [Coen₂X]^2+* to investigate the influence of the hydrophobicity of the surface of the complex on its stability in the mixtures. G _t ^o (i) (solvent sorting) are compared with G _t ^o (i) (TATB).     

Kinetics of the solvolysis oftrans-dichlorotetra-(4-t-butylpyridine)-cobalt(III) ions in water +t-butyl alcohol mixtures

Rates of solvolysis of the complex cation [Co(4tBupy)₄Cl₂]⁺ have been determined in mixtures of water with the hydrophobic solvent, t-butyl alcohol. The solvent composition at which the extremum is found in the variation of the enthalpy H^* and the entropy S^* of activation correlates well with the extremum in the variation of the relative partial molar volume of t-butyl alcohol in the mixture and the straight line found for the variation of H^* with S^* is coincident with the same plot for water + 2-propanol mixtures. A free energy cycle is applied to the process initial state (C ⁿ⁺) going to the transition state [M⁽ⁿ⁺¹⁾⁺...Cl^–] in water and in the mixture using free energies of transfer of the individual ionic species, G _t ^o (i), from water into the mixture. Values for G _t ^o (i) are derived from the solvent sorting method and from the TATB/TPTB method: using data from either method, changes in solvent structure on going from water into the mixture are found to stabilize the cation in the transition state, M⁽ⁿ⁺¹⁾⁺, more than in the initial state, C ⁿ⁺. This is compared with the application of the free energy cycle to the solvolysis of complexes [Co(Rpy)₄Cl₂]⁺ and [Coen₂LCl]⁺ in mixtures of water with methanol, 2-propanol or t-butyl alcohol: the above conclusion regarding the relative stabilization of the cations holds for all these complexes in their solvolyses in water+alcohol mixtures using values of G _t ^o (Cl^–) from either source.     

Solubility of dichlorotetraalkylpyridinecobalt III hexachlororhenate(IV) in water + methanol mixtures: Free energies of transfer of the complex cation from water into the mixtures

The solubility of salts [Co(3Rpy)₄Cl₂]₂]ReCl₆] has been determined in water + methanol mixtures. By comparing these with the solubilities of the salt Cs₂ReCl₆ and using calculated activity coefficients for the ions in the water+methanol mixtures, values for {G _t ^o (Co(3Rpy)₄Cl ₂ ⁺ )–G _t ^o (Cs⁺)} can be determined where G _t ^o is the standard Gibbs free energy of transfer from water to an aqueous mixture. G _t ^o (Cs⁺) from the solvent sorting scale and from the TPTB scale are then used to calculate G _t ^o (Co(3Rpy)₄Cl ₂ ⁺ ). These two sets of values for G _t ^o (Co(3Rpy)₄Cl ₂ ⁺ ) on the differing scales are then inserted into a free energy cycle applied to the bond extension Co(3Rpy)₄Cl ₂ ⁺ (initial state)Co(3Rpy)₄Cl²⁺+Cl^– (transition state) for the solvolysis in water and in water + methanol mixtures to produce values for G _t ^o (Co(3Rpy)₄Cl²⁺) using both scales. Data for the solubilites of [Copy₄Cl₂]₂[ReCl₆] and [Co(4Rpy)₄Cl₂]₂[ReCl₆] have been re-calculated to compare free energies of transfer for these complex cations with those specified above.     

Free energy of transfer of cobalt(III) complexes from water into water—t-butyl alcohol mixtures

The solubility of trans-(Coen₂Cl₂)₂ReCl₆ has been determined in water and in water +t-butyl alcohol mixtures. By comparzng these values with the solubility of Cs₂ReCl₆ in similar mixtures, values for the difference in free energy of transfer, G _t ^o (i) between water and water + t-butyl alcohol can be calculated for i =[Coen₂Cl₂]⁺ and Cs⁺. The introduction of G _t ^o (Cs⁺) then produces values for G _t ^o (Coen₂Cl₂ ⁺). The difference in G _t ^o (i) for i=[Coon₂Cl]²⁺ in the transition state and i=[Coen₂Cl₂]⁺ in the initiad' tate for the solvolysis of the trans-[Coen₂Cl₂]⁺ ion in water + t-butyl alcohol can be derived from the application of a free energy cycle: using G _t ^o (Coen₂Cl₂ ⁺) determined from the solubility measurements allows the calculation of values for G _t ^o (Coen₂Cl²⁺). G _t ^o (i) in water + t-butyl alcohol for bis (1,2-diamino) cobalt (III) ions are compared with G _t ^o (i) for tetrapyridinecobalt (III) ions.     

Ions in Aqueous Acetic Acid Mixtures: Solvent Reorganization around Protons and Gibbs Energies of Transfer from Water

The photo-absorbing, basic sensor, 4-nitroaniline, has been used to determine theequilibrium constant for solvent reorganization around the proton in mixtures ofvarying composition of water with acetic acid. In all the mixtures used, theself-ionization of the acetic acid was suppressed. In contrast to mixtures of waterwith the related ethanol or acetone, this equilibrium is shifted more toward thewater-solvated species as the mole fraction x ₂ of the cosolvent increases. TheGibbs energy of transfer of protons from water into the mixture G ^o _t (H⁺) can bederived with the aid of this equilibrium constant for the solvent reorganization.Using G ^o _t (H⁺), G ^o _t (i) for i denoting anions and other cations can be evaluated.In comparison the G ^o _t (i) for cations have lower negative values than when eitherethanol or acetone is added to water. Correspondingly, for halide anions, thepositive G ^o _t (i) with added acetic acid are rather less than is found with eitherethanol or acetone added. The influence on the ion-solvent interaction of bothelectron withdrawing hydroxy and carbonyl groups in acetic acid may beresponsible for this. Although G ^o _t (i) for C10^– ₄ and Re0^– ₄ are also positive, both picrateions and OH^– give negative values with acetic acid added to water. With picrateions, the hydrophobic effect of the carbon ring produces stabilization in themixture relative to water. With OH^–, complete conversion to acetate anionsoccurs. As is found with other cosolvents, the contribution of the charge onacetate anion to G ^o _t (CH₃COO^–) is found to increase as x ₂ rises. The aciddissociation constant K _a for acetic acid is found to decrease slowly as x ₂ rises to0.5, followed by a rapid decrease for x ₂ greater than 0.7 where dimerization ofacetic acid occurs.     

Solubility of inert gases and liquid hydrocarbons in water

The thermodynamic statistical model based on the distribution of molecular populations among energy levels has been employed for the analysis of the solubility of hydrocarbons and other inert gases or liquids in water at different temperatures. The statistical distribution is described by a convoluted partition function Z_G·_s. The product of a grand canonical partition function Z_G represents the distribution of the species in the reaction while the canonical partition function Z_G represents the properties of the solvent. The first derivative of the logarithm of the partition function with respect to 1/T is the apparent enthalpy which is the result of the contributions of the separate partition functions, {H_aap}_T=H^o+n_wC_p,wT, where {H_app}_T refers to Z_G, n_wC_p,wT=–H_w to _s, and H^o is the change in enthalpy of hydrocarbon-water reaction. The plot {H_app}_T vs/ T results in a straight line with slope n_w at constant C_p,w. The apparent enthalpy is obtained from the coefficients of the polynomial fitting of the solubility data, as a function of 1/T. Alternatively, the apparent enthalpy can be determined calorimetrically. The enthalpy thus obtained is a linear function of the Kelvin temperature. The values of n_w range from 1.6, 1.9, 5.6 to 5.8 for helium, hydorgen, butane and hexane, respectively. For fluorocompounds the range of n_w is 10.1 to 11.1 indicating that n_w is a function of the number of water molecules expelled from the cage of solvent to form a cavity to host the solute molecule. The analysis of several sets of calorimetric or solubility data with the present molecular thermodynamic model yields values of H^o and n_w consistent with the size of the dissolved molecules.List of Symbols p pressure - H _ij average enthalpy - H _i level enthalpy (=H) - H _i enthalpy difference - H _ij intersublevel energy difference - i index of level - j index of sublevel - Z_G grand canonical partition function - _S canonical partition function - Z_G- convoluted partition function - –G ^o/RT standard Gibbs energy normalized toRT - –H ^o/RT standard enthalpy normalized toRT - S ^o/R standard entropy normalized toR - C _p molar heat capacity - T absolute temperature - H _G- enthalpy of the convoluted ensemble - H _G enthalpy of the solute - H enthalpy of the solvent - H _app apparent enthalpy - H _w enthalpy of water - C_yH_z hydrocarbon - W water - K _s solubility equilibrium constant - x ₂ molar fraction of solute - C_yH_zW_(x-nw) hydrocarbon molecule trapped in a cavity - K _H Henry constant - P _s solubility product - [W] concentration of water - reference temperature - a, b, c, d coefficients of the fitting polynomial - {H _app}_T apparent enthalpy at temperatureT - {H }_T standard enthalpy at temperatureT - {H _w}_T water contribution to enthalpy at temperatureT - C _p,w isobaric molar heat capacity of water - L Ostwald coefficient - C _p isobaric heat capacity difference - Bunsen coefficient - C _p,app apparent isobaric heat capacity difference - n _C number of carbon atoms in the chain - h _w interaction enthalpy of one water molecule - H ₀ intercept for the extrapolated enthalpy     

Medium effects on the reactions of coordination complexes: Base hydrolysisof(αβS)(p-hydroxybenzoato)(tetraethylenepentamine) cobalt(III) in iso-propanol–water,tert-butanol–water and dimethyl sulfoxide–water media

The base hydrolysis of (S)(p-hydroxybenzoato)-(tetraethylenepentamine)cobalt(III) has been investigated in aqueous–organic solvent media using i-PrOH, t-BuOH and dimethyl sulfoxide (DMSO) as cosolvents at 20.0 T (°C) 40.0 (I=0.02 mol dm ^-3) with 80% (v/v) of cosolvents. Only the base-catalysed path (k_obs=k_OH[OH^-]) is observed. The relative second order rate constant k _OH ^os /k _OH ^ow at I=0 increases nonlinearly with increasing mol fraction (x_O.S.) of the cosolvents, the rate acceleration in alcoholic cosolvents being greater than in DMSO. The destabilization of ^-OH in mixed solvent media alone does not explain the observed rate acceleration. The solvent composition dependence, log k _OH ^os = log k _OH ^ow + a_ix _os ⁱ [i=1,2,k _OH ⁰ denotes k_OH at I=0 in mixed solvent(s) and water (w)] indicates specific solute–solvent interactions. The values of the relative transfer free-energy data [_TG(t.s.) - _TG^o (i.s.)](sw)(25 °C)(G), where t.s. and i.s. denote the transition state and initial state of the substrates respectively, are positive for all substrates at all compositions, indicating a greater destabilizing effect of the mixed solvent on the transition state than on the initial state. The G values also correlate with G^E(G = ax_O.S. + cG^E) for all solvents, supporting the fact that solvent structural effects mediate the rates and energetics of the reaction. However, the solvent effects on the solvation components of H and S are mutually compensating, thus indicating that there is no change in the mechanism.     

Model calculations of changes of thermodynamic variables for the transfer of nonpolar solutes from water to water-d2

A recently introduced modified hydration shell hydrogen bond model for rationalizing the thermodynamic consequences of hydrophobic hydration is adapted for use with heavy water. The required adjustment of parameters employs the assumption that breaking hydrogen bonds in water-d₂ involves a greater enthalpy change and a larger entropy increase than bond breaking in ordinary water. It also makes some use of information derived from studies of gas solubilities in the two solvents, although a review of the data leads to serious questions about the reliability of results obtained in this way. The model permits calculations of hydrogen bonding contributions to the changes, G _t ^o , H _t ^o , S _t ^o , and C _p,t ^o , for transfer of nonpolar solutes from water to water-d₂ and implies that such data should show regular trends. Although some of the numerical results depend strongly on the values chosen for the parameters, the pattern defined by these trends is nearly independent of parameters. Predicted values of C _p,t ^o are large and positive for all nonpolar solutes, while S _t ^o is expected to be negative near 0°C, becoming progressively less negative on warming and eventually positive. Both of these quantities should be proportional to the molecular surface area of the solute. Analogous predictions regarding G _t ^o and H _t ^o can also be made, but only if it is permissible to neglect possible contributions to these quantities from van der Waals interactions.     

Absorption spectra of 2-styryl-4-phenyl-thiazole ethiodides in organic solvents of varying polarities and determination of the formation constant of the charge transfer complex formed

The electronic absorption spectra of some 2-styryl-4-phenyl-thiazole ethiodides are studied in organic solvents of different polarities. The shorter wavelength band appearing in the visible region is assigned to an intramolecular charge transfer (CT)-transition originating from the phenyl moiety to the positively charged hetero ring, while the longer wavelength one is due to an intermolecular CT-transition from the iodide ion to the 2-styryl-4-phenyl-thiazolinium cation. These assignments are based on the nature of the aldehydic residue and effects of solvent, concentration, and temperature on both the position and absorptivity of the CT complex-band. It is concluded that the CT complex formed will be highly solvated inDMF, DMSO, ethanol and methanol relative to in CHCl₃, dioxane and acetone. The formation constant of the CT complex in solutions of different polarities is determined at different temperatures. Furthermore, the thermodynamic parameters H ^o, G ^o and S ^o for complex formation are calculated and discussed.

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Absorptionsspektren von 2-Styryl-4-phenyl-thiazol-ethiodiden in verschiedenen Lösungsmitteln und Bestimmung der Bildungskonstanten der Charge-Transfer-Komplexe

Zusammenfassung Die Elektronenanregungsspektren einiger substituierter 2-Styryl-4-phenyl-thiazol-ethiodide wurden in einigen Lösungsmitteln unterschiedlicher Polarität untersucht. Die Absorption bei kürzerer Wellenlänge wird einem intramolekularen Charge-Transfer (CT)-Übergang zugeordnet, die langwellige Bande einem intermolekularen CT-Übergang (Jodid—organ. Kation). Die Diskussion erfolgt basierend auf Substitutions-, Lösungsmittel-, Konzentrations-, und Temperatur-Effekten. Die Komplexbildungskonstanten und die thermodynamischen Parameter H ^o, G ^o und S ^o werden angegeben.

     

Thermodynamics of Protonated Amine–Hexacyanoferrate(II) Complex Formation in Aqueous Solution

Thermodynamic parameters, G°, H° and TS° are reported for the formation of proton amine–hexacyanoferrate(II) complexes, in aqueous solution, at 25°C. H° were determined by the temperature dependence of formation constants and/ or by direct calorimetry in aqueous solution, at T = 25°C. Enthalpy changes for the reaction H_iAⁱ⁺ + H_j Fe(CN) ₆ ^j-4 = AFe (CN)₆H _i+j ^i+j-4 (where A = methylamine, ethylenediamine, and tetraethylenepentamine) are quite low and the main contribution to the stability of these complexes arises from the entropic term, as expected for electrostatic interactions. When j = 0, the formation entropy is linearly dependent on i according to the simple equation TS° = 13.4 i kJ-mol^–1.     

Transfer free energies of some ions from water to dimethylsulfoxide-water and urea-water mixtures

The standard free energies of transfer (G _t ^o ) of some electrolytes from water to aqueous mixtures of dimethylsulfoxide (DMSO) and of urea have been split into the contribution from individual ions by use of the reference electrolyte Ph ₄ AsBPh ₄ (RE), where Ph=phenyl. For each of the solvents, G _t ^o (Ph ₄ AsBPh ₄ ) was determined from the solubility products of the salts KBPh ₄ , Ph ₄ AsPi, and KPi, where Pi=picrate ion. The observed G _t ^o (i) values for the individual ions are strikingly different from the corresponding values obtained by the simultaneous extrapolation (SE) procedure reported earlier.     

Thermochemistry of aqueous solutions of alkylated nucleic acid bases V. Enthalpies of hydration of N-methylated adenines

Enthalpies of solution in water, H _sol ^o , and of sublimation, H _subl ^o , were determined experimentally for a number of crystalline N-methyl adenines: m⁶Ade, m ₂ ^6,6 Ade, m⁹Ade, m ₂ ^6,9 Ade, and m ₃ ^6,6,9 Ade. Derived standard enthalpies of hydration H _hydr ^o , were corrected for the calculated cavity terms H _cav ^o to yield enthalpies of interaction H _int ^o of the solutes with their hydration shells. The increments of H _int ^o per unit area of the water-accessible molecular surface S _B , H _int ^o (CH₃)/S _B (CH₃), for the particular methyl groups: is considered to be the net effect of the gain in the energy resulting from van der Waals' interactions and of the loss in the energy due to polar interactions upon methyl substitution. It proved to vary somewhat numerically in agreement with the theoretically predicted hydration schemes of adenine. Comparison of H _int ^o /S _B value for adenine with those previously determined for uracil and thymine indicates that the aminopurine moiety is less hydrated than the diketopyrimidine ring.     

The position of plutonium/III/ in the lanthanide series in respect to thermodynamic functions of complex formation with nitrates and thiocyanates

The position of Pu/III/ within lanthanides in respect to G⁰, H⁰ and S⁰ of complex formation with nitrate and thiocyanate ligands was determined by the extraction method. It was found that in respect to G⁰, Pu/III/ is a light pseudolanthanide for nitrate ligands and a heavy pseudolanthanide for thiocyanate ligands. A comparison of the positions of Pu/III/ and Am/III/ in respect to G⁰, H⁰ and S⁰ shows that the radius of plutonium is greater than that of americium in the An/NO₃/ ₅ ^2– complex and smaller in the An/NCS/₃/TBP/_n complex. The increase in the radii between plutonium and americium in the thiocyanate complex points out to a contribution from 5f orbitals to bonding.     

Thermodynamics of ionization of benzoic acid in water at 298°K

The standard heat of ionization of aqueous benzoic acid has been determined by solution calorimetry. The value obtained for H ^o of ionization, 0.11±0.04, is in good agreement with H ^o from other calorimetric values; 0.10±0.05 kcal-mole ^–1 is suggested to be the best value for this ionization at 298° K.     

Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XXXVIII. Correlation analysis of solvent effects on the activation parameters of heterolysis of <Emphasis Type="Italic">t</Emphasis>-BuBr and <Emphasis Type="Italic">t</Emphasis>-BuI

Heterolysis of t-BuBr and t-BuI in aprotic solvents involves a H - S compensation effect. The G of t-BuBr heterolysis in aprotic solvents decreases with increasing solvent polarity and cohesion, whereas the respective value for t-BuI heterolysis decreases with increasing solvent polarity, nucleophilicity, and polarizability. In protic solvents, a negative effect of nucleophilic solvation is observed.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 9, 2004, pp. 1476–1483.Original Russian Text Copyright © 2004 by Ponomarev, Zaliznyi, Dvorko.This revised version was published online in April 2005 with a corrected cover date.     

Thermodynamic parameters for the interaction of adenosine 5′-diphosphate,and adenosine 5′-triphosphate with Mg2+ from 323.15 to 398.15 K

The interaction of adenosine 5-diphosphate (ADP) and adenosine 5-triphosphate (ATP) with Mg²⁺ in water has been studied calorimetrically at 323.15, 348.15, 373.15, and 398.15 K for ATP and at 348.15 and 373.15 K for ADP. The enthalpies of reaction of Mg²⁺ with ADP and ATP were obtained from the heats of mixing of aqueous solutions of tetramethylammonium salts of ADP and ATP with MgCl₂ solutions in an isothermal flow calorimeter. Equilibrium constant (K), enthalpy change (H°), entropy change (S°), and heat capacity change (Cp°) values were calculated for the interaction: Mg²⁺+L^n–=MgL^2–n and Mg²⁺+MgL^2–n=Mg₂L^4–n, where n=4 for L=ATP and n=3 for L=ADP. The results are consistent with those at lower temperatures. For the two nucleotides studied, the above two reactions are endothermic and entropy-driven in the temperature range studied. Large Cp° values for the interaction of Mg²⁺ with ADP with ATP indicate the involvement of phosphate groups of nucleotides in the coordination of Mg²⁺. The coordination of the first and second Mg²⁺ ions involves the phosphate chain in both ADP and ATP. No evidence was found for the involvement of the adenine ring or the ribose moiety in the coordination of Mg²⁺ with these nucleotides. Approximate values of logK, H°, and S°, and Cp° for the self-association of ADP and ATP in the presence of Mg²⁺ are also given.     

Conductimetric determination of ion-association constants for calcium,cobalt, zinc,and cadmium sulfates in aqueous solutions at various temperatures between 0°C and 45°C

Thermodynamic ion-association constants for calcium, cobalt, zinc, and cadmium sulfates in aqueous solutions were determined by means of conductivity measurements at various temperatures between 0°C and 45°C. The standard Gibbs energy, enthalpy, and entropy for the reaction M ²⁺ +SO ₄ ^2– M ²⁺ ·SO ₄ ^2– (M=Ca, Co, Zn, and Cd) were calculated from the temperature dependence of the ion-association constants. The values obtained are as follows: G ₂₉₈ ^o =–12.42 kJ-mole ^–1 , H ^o =6.11 kJ-mole ^–1 , and S ₂₉₈ ^o =62.1 J- ^o K ^–1 -mole ^–1 for Ca ²⁺ ·SO ₄ ^2– ; G ₂₉₈ ^o =–12.84 kJ-mole ^–1 , H ^o =5.00 kJ-mole ^–1 , and S ₂₉₈ ^o =59.8 J- ^o K ^–1 -mole^–1 for Co ²⁺ ·SO ₄ ^2– ; G ₂₉₈ ^o =–12.65 kJ-mole ^–1 , H ^o =8.65 kJ-mole ^–1 , and S ₂₉₈ ^o =71.4 J- ^o K ^–1 -mole ^–1 for Zn ²⁺ ·SO ₄ ^2– ; G ₂₉₈ ^o =–13.28 kJ-mole ^–1 , H ^o =8.39 kJ-mole ^–1 , and S ₂₉₈ ^o =72.7 J- ^o K ^–1 -mole ^–1 for Cd ²⁺ ·SO ₄ ^2– .     

Complexation,solubilities and thermodynamic functions for cerium(III) fluoride-water system

The solubility, solubility product and the thermodynamic functions for the CeF₃–H₂O system have been measured using the radiometric, conductometric and potentiometric techniques. The radiometric values for the solubility and solubility product, the lowest and more acceptable for reasons cited in previous papers, are 3.14·10^–5 M and 2.17·10^–17 respectively. The enthalpy change measured by the conductometric method is almost twice as that obtained by potentiometric method due to abnormal conductances registered at higher temperatures. The average values for H^o and G^o and S^o at 298 K are 53.0±17.4, 91.7±4.0 and –129.7±58.2 KJ·mol^–1 respectively. The positive values for H^o and G^o and the negative value for S^o are indicative of the low solubility of this salt in water. The stability constants for the mono- and difluoride complexes of Ce(III) have been determined potentiometrically using unsaturated solution mixtures of Ce(III) and F^–. These values for CeF⁺ and CeF ₂ ⁺ are 997±98 and (1.03±0.44)·10⁵, respectively. Studies on pH dependence of the solubility shows that the solubility reaches a minimum value at a pH of about 3.2.