J.P. Shukla

THERMODYNAMICS OF PROTON DISSOCIATION OF ACETYLACETONE AND 3-METHYLACETYLACETONE IN DIOXANE-WATER MIXTURES

Abstract Thermodynamic ionization constants ( T p K a ) of acetylacetone and 3-methylacetylacetone have been determined potentiometrically in aqueous and aqueous-dioxane media at 25 and 35 ± 0.01°C applying an empirical pH correction for mixed aqueous media. The T p K a values do not vary linearly with the reciprocal of the dielectric constant of the medium, but a plot of T p K a vs. the mole fraction of dioxane is linear at a given temperature. Temperature as well as medium effects are briefly discussed. Values of Δ G 0 , Δ H 0 , and ΔS 0 are tabulated.

The effect of temperature on the extraction of plutonium(VI) from nitric acid into dodecane by di(2-ethylhexyl)sulphoxide

Abstract The effect of temperature on the extraction of plutonium(VI) into dodecane by di(2-eth-ylhexyl)sulphoxide (DEHSO) has been investigated as a function of temperature in the range 25–50°C using a wide range of concentrations of both nitric acid as well as DEHSO. The results show that the distribution ratios of plutonium(VI) considerably decrease with increasing temperature. From the variation of the distribution ratio (Dpu) with temperature, the thermodynamic parameters such as the free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) changes associated with the formation of PuO2(NO3)2·2DEHSO have been evaluated following a least squares method. Extraction of plutonium(VI) from nitric acid by DEHSO is invariably exothermic in nature and is mainly enthalpy controlled, the entropy change counteracting in all cases. Values of ΔH are not much affected by increase in nitric acid concentration. Negative values of ΔS are indicative of a smaller entropy gain due to loss of hydration water molecules compared to entropy loss due to complex formation.

Influence of temperature on the extraction of uranium(VI) and plutonium(IV) from nitric acid into n-dodecane by di(2-ethylhexyl)sulphoxide

Abstract The influence of temperature on the extraction of uranium(VI) and plutonium(IV) from 2 M HNO 3 medium into n-dodecane by di(2-ethylhexyl)sulphoxide (DESO) was investigated as a function of temperature in the range 25–50°C. The results show that the distribution ratios of both U(VI) and Pu(IV) considerably decrease with increasing temperature. From the distribution measurements, thermodynamic parameters such as the free energy (ΔG), enthalpy (Δ H ) and entropy (Δ S ) changes associated with the formation of UO 2 (NO 3 ) 2 ·2DESO and Pu(NO 3 ) 4 · 2DESO were evaluated. Extraction of both U(VI) and Pu(IV) from nitric acid by DESO is exothermic in nature and is mainly enthalpy controlled, with negative entropy changes for the relatively less hydrated UO 2 2+ species and a small positive value for the more hydrated Pu 4+ species.

Thermodynamics of proton-ligand dissociation equilibria in mixed solvents. acid dissociation of 2-isopropyltropolone in mixtures of water and dioxane

Abstract Thermodynamic acid dissociation constants, T p K a , of 2-isopropyltropolone (HIPT) have been determined by pH titration in aqueous and aqueous dioxane media at 25 and 35 ± 0.01°C applying an empirical pH correction for mixed aqueous media. HIPT is a very weak acid with T p K a between 7 and 9. The T p K a values do not vary linearly with the reciprocal of the dielectric constant of the medium, but a plot of T p K a vs. the mole fraction of dioxane is linear at a given temperature. The “true constant”, K′ a , and solvation number, n , in the expression (H + )(A − )/(HA) = K′ a (S) n = K * a for this reagent have been evaluated. Mean ionic radii, r ±, for the tropolonate ion, being in the region of 2.0–2.4 A, slightly increases with dioxane percentages. Values of Δ G 0 , Δ H 0 and Δ S 0 have been evaluated. The effects of temperature as well as medium effects are briefly discussed.

Thermodynamic proton-ligand dissociation constants of substituted 3-hydroxy-4-pyrones in dioxane-water mixtures

Abstract Thermodynamic proton-ligand dissociation constants Tp K a were determined pH-metrically in 10–50 vol.% dioxane-water mixtures at 25 and 35 ± 0.1° C for two substituted 3-hydroxy-4-pyrones (3-hydroxy-2-methyl-4-pyrone (maltol) and 3-hydroxy-5-(hydroxymethyl)-4-pyrone (kojic acid). Extrapolation and least-squares methods were used to obtain Tp K a in pure water (0%). Both of the 3-hydroxy-4-pyrones are monoprotic and they are very weak acids. The Tp K a values do not vary linearly with the reciprocal of the dielectric constant of the medium, but a plot dioxane of mole fraction n 2 vs. Tp K a is linear at a given temperature. The “true constant” Tp K a t and solvation number n (in the expression (H +)(A − )/(HA = Tp K a t (S) n = Tp K a ∗ for these compounds were evaluated by analysing the experimental data mathematically. Values of the changes in standard free energy, enthalpy and entropy associated with the dissociation were calculated. The effects of temperature, medium and substituent are briefly discussed.

Thermodynamics of the complexing of uranyl ions with 1-phenyl-3-methyl-4-benzoylpyrazolone-5 in aqueous dioxane

Abstract The thermodynamics of the complexing between hexavalent U and 1-phenyl-3-methyl-4-benzoylpyrazolone-5 (PMBP) have been studied in 70 vol% aqueous—dioxane medium at 25 and 35±0.1°C following the Bjerrum—Calvin pH titration technique, as applied by Van Uitert and Haas. The ligand is mono-protonic. The refinement of results of formation constants has been accomplished by the method of least squares treatment after an algebraic transformation. The formation of 1:1, 1:2 and 1:3 complexes has been observed, the order of stability being log K 1 > log K 2 > log K 3 . The stability invariably increases with an increase in temperature both in aqueous as well as aqueous dioxane media. The changes in Δ G 0 , Δ H 0 and Δ S 0 at 25 and 35°C for the overall equilibrium constants have also been evaluated. Uranyl complexes of PMBP are entropy stabilized, the values of enthalpy changes being positive. Other factors which affect chelate stability are briefly discussed.

Thermodynamics of proton-ligand dissociation of some fluorinated β-diketones in dioxane-water mixtures

Abstract The thermodynamic proton-ligand dissociation constants (Tp K a ) of the three fluorinated β-diketones namely 1,1,1-trifluoro-2,4-hexanedione, 1,1,1-trifluoro-5-methyl-2,4-hexanedione and 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione have been determined pH-metrically in various dioxane-water mixtures at 25 and 35±0.1°C by applying an empirical pH correction for mixed organo-aqueous media. Both extrapolation and the least-squares methods were used to obtain Tp K a in pure water (0%). All the compounds investigated are weak monoprotic acids. The Tp K a values do not vary linearly with the reciprocal of the dielectric constant of the medium whereas a plot of dioxane mole fraction ( n 2 ) versus Tp K a is linear at a given temperature. An expression relating the solvation number and ‘true ionization constant’ has been mathematically evaluated. The values of changes in the standard free energy, enthalpy and entropy associated with their proton-ligand dissociation have been calculated. Temperature and medium as well as substituent effects are briefly discussed.

Potentiometric studies on the complex formation between hexavalent uranium and tropolones

Abstract Thermodynamics of the complexation of uranium(VI) with tropolone (HTP) as well as β-isopropyltropolone (HIPT) have been studied in 50 vol.% water-dioxane medium at 25 and 35±0.01°C adopting the Bjerrum-Calvin pH-titration technique as applied by Van Uitert and Haas. Both the ligands are weak monoprotic acids. Uranium(VI) forms 1:1 and 1:2 chelates with both ligands; the order of stability being log K 1 > log K 2 The stability invariably increases with an increase in temperature in both aqueous and water-dioxane media. The changes in Δ G 0 , Δ H 0 and Δ S 0 at 25 and 35°C for the first stepwise formation constants have also been computed. Uranyl tropolonates are entropy stabilized, the enthalpy change values being positive. Factors affecting chelate stability are briefly discussed.

Thermodynamic proton—ligand stability constants of tropolone in mixtures of water and dioxane

Abstract Thermodynamic proton—ligand stability constants, TpKa, of tropolone have been determined pH-metrically in 0–50 vol.% dioxane—water mixtures at 25 and 35±0.01°C applying an empirical pH correction for mixed aqueous media. Tropolone is a very weak acid with a TpKa value between 6.9 and 8.8. TpKa values do not vary linearly with the reciprocal of the dielectric constant of the medium, but a plot of TpKa vs. the mole fraction of dioxane is linear at a given temperature. Values of ΔG0 and ΔH0 and ΔS0 are also computed. Temperature as well as medium effects are briefly discussed.

Thermodynamics of the interaction of lanthanum(III) with 1-phenyl-3-methyl-4-benzoylpyrazolone-5 in aqueous dioxane

Abstract The thermodynamics of the interaction between lanthanum(III) with 1-phenyl-3-methyl-4-benzoylpyrazolone-5 (PMBP) have been investigated in 70% (v/v) aqueous dioxane medium at 25 and 35±0.01°C, adopting the Bjerrum-Calvin pH-titration technique, as applied by Van Uitert and Hass. The refinement of the results of formation constants has been accomplished by the method of least squares after an algebraic transformation. The formation of 1:1, 1:2 and 1:3 complexes has been observed, the order of stability being log K 1 ⪢ log K 2 ⪢ log K 3 . The changes in Δ G 0 , Δ H 0 and Δ S 0 for the complexation reactions have also been evaluated. Factors affecting chelate stability are briefly discussed.