Activity coefficients of concentrated strong and weak electrolytes by a hydration equilibrium and group contribution model

A new speciation-based group contribution model for activity coefficients is proposed to estimate the equilibrium properties of aqueous solutions containing electrolytes. The chemical part of the model accounts for the hydration equilibrium of water and ions with the formation of ion n-water complexes in a single stage process; the hydration number n and the hydration equilibrium constant K are the two independent parameters in this part. The physical part of the model is the UNIFAC group contribution model for short-range interactions. Each ion is considered as a group. Long-range interactions are accounted for by a Pitzer contribution (Debye–Hückel theory). The model is compared with experimental data at 25 °C including water activity, osmotic coefficients, activity coefficients, and pH of binary diluted and concentrated electrolyte solutions (up to 20 mol kg⁻¹ for NaOH, 16 mol kg⁻¹ for HCl, etc.).     

Re-evaluation of the thermodynamic activity quantities in aqueous alkali metal nitrate solutions at T = 298.15 K

The Hückel equation used in this study to correlate the experimental activities of dilute alkali metal nitrate solutions up to a molality of about 1.5 mol · kg⁻¹ contains two parameters being dependent on the electrolyte: B [that is related closely to the ion-size parameter (a∗) in the Debye–Hückel equation] and b₁ (this parameter is the coefficient of the linear term with respect to the molality and this coefficient is related to hydration numbers of the ions of the electrolyte). In more concentrated solutions up to a molality of 7 mol · kg⁻¹, an extended Hückel equation was used, and it contains additionally a quadratic term with respect to the molality and the coefficient of this term is parameter b₂. All parameter values for the Hückel equations of LiNO₃, NaNO₃, and KNO₃ were determined from the isopiestic data measured by Robinson for solutions of these salts against KCl solutions [J. Am. Chem. Soc. 57 (1935) 1165]. In these estimations, the Hückel parameters determined recently for KCl solutions [J. Chem. Eng. Data 54 (2009) 208] were used. The Hückel parameters for RbNO₃ and CsNO₃ were determined from the reported osmotic coefficients of Robinson [J. Am. Chem. Soc. 59 (1937) 84]. The resulting parameter values were tested with the vapour pressure and isopiestic data existing in the literature for alkali metal nitrate solutions. These data support well the recommended Hückel parameters up to a molality of 7.0 mol · kg⁻¹ for LiNO₃ and NaNO₃, up to 4.5 mol · kg⁻¹ for RbNO₃, up to 3.5 mol · kg⁻¹ for KNO₃, and up to 1.4 mol · kg⁻¹ for CsNO₃ solutions. Reliable activity and osmotic coefficients of alkali metal nitrate solutions can, therefore, be calculated by using the new Hückel equations, and they have been tabulated at rounded molalities. The activity and osmotic coefficients obtained from these equations were compared to the values suggested by Robinson and Stokes [Electrolyte Solutions, second ed., Butterworths Scientific Publications, London, 1959], to those calculated by using the Pitzer equations with the parameter values of Pitzer and Mayorga [J. Phys. Chem. 77 (1973) 2300], and to those calculated by using the extended Hückel equation of Hamer and Wu [J. Phys. Chem. Ref. Data 1 (1972) 1047].     

Standard enthalpy of formation of aqueous titanyl chloride, TiOCl2(aq), at T = 298.15 K

Enthalpies of solution of TiCl₄(l) in aqueous perchloric acid have been measured in an isothermal calorimeter at T = 298.15 K at ionic strengths of (1.964, 3.002, and 4.062) mol · kg⁻¹. These results were extrapolated to zero ionic strength using an extended Debye-Hückel equation, to yield the standard enthalpy of solution ; from which the standard partial molar enthalpy of formation of the titanyl ion was derived: .     

Activity Coefficient Determinations of KCl in the KCl + Formamide + Water Mixed Solvent System Based on Potentiometric Measurements at 298.2 K

In this work mean activity coefficient measurements for KCl in the KCl + formamide + water system, using the potentiometric method, are reported. The electromotive force measurements were performed on a galvanic cell of the type Ag | AgCl | KCl (m), formamide (w%), H₂O (1−w)% | K-ISE, in solvent mixtures containing w=(0,10,20,30, and 40)% mass percent of formamide over ionic strengths ranging from 0.0010 to 3.9578 mol⋅kg⁻¹. Modeling of the activity coefficients of this ternary system was based on an extended Debye–Hückel equation and the Pitzer ion-interaction model. The resulting values of the mean activity coefficients, the osmotic coefficients and the excess Gibbs energy, together with Pitzer ion-interaction parameters (β ⁽⁰⁾, β ⁽¹⁾ and C ^ϕ ) and Debye–Hückel parameters (a, c and d), are reported for the investigated system.     

Syntheses and investigation of the effects of position and nature of substituent on the spectral,electrochemical and spectroelectrochemical properties of new cobalt phthalocyanine complexes

The syntheses of new cobalt phthalocyanine (CoPc) complexes, tetra-substituted with diethylaminoethanethio at the peripheral (complex 3a) and non-peripheral (complex 3b) positions, and with benzylmercapto at the non-peripheral position (complex 5), are reported. The effects of the nature and position of substituent on the spectral, electrochemical and spectroelectrochemical properties of these complexes are investigated. Solution electrochemistry of complex 3a showed three distinctly resolved redox processes attributed to Co^IIIPc⁻²/Co^IIPc⁻² (E_½ = +0.64 V versus Ag|AgCl), Co^IIPc⁻²/Co^IPc⁻² (E_½ = −0.24 V versus Ag|AgCl) and Co^IPc⁻²/Co^IPc⁻³ (E_½ = −1.26 V versus Ag|AgCl) species. No ring oxidation was observed in complex 3a. Complex 3b showed both ring-based oxidation, attributed to Co^IIIPc⁻¹/Co^IIIPc⁻² species (E_p = +0.86 V versus Ag|AgCl), and ring-based reduction associated with Co^IPc⁻²/Co^IPc⁻³ species (E_½ = −1.46 V versus Ag|AgCl), with the normal metal-based redox processes in CoPc complexes: Co^IIIPc⁻²/Co^IIPc⁻² (E_p = +0.41 V versus Ag|AgCl) and Co^IIPc⁻²/Co^IPc⁻² (E_½ = −0.38 V versus Ag|AgCl). Solution electrochemistry of complex 5 showed the same type and number of species observed in complex 3a: Co^IIIPc⁻²/Co^IIPc⁻² (E_p = +0.59 V versus Ag|AgCl), Co^IIPc⁻²/Co^IPc⁻² (E_½ = −0.26 V versus Ag|AgCl) and Co^IPc⁻²/Co^IPc⁻³ (E_½ = −1.39 V versus Ag|AgCl) species. These processes were confirmed using spectroelectrochemistry.     

Activity coefficients of NaCl in the NaCl + NaBF4 + H2O ternary system at 298.15 K

The activity coefficients of sodium chloride in the NaCl + NaBF₄ + H₂O ternary system were experimentally determined at 298.15 K, at ionic strengths of 0.3. 0.5, 1, 2 and 3 mol kg⁻¹ from emf from the bi-ISE cell without liquid junction:

ISE-Na|NaCl(m_A), NaBF₄(m_B)|ISE-Cl

Determination of adrenaline by using inhibited Ru(bpy)3 electrochemiluminescence

Adrenaline was found to inhibit strongly the electrochemiluminescence (ECL) from the Ru(bpy)₃²⁺/tripropylamine system when a working Pt electrode was maintained at 1.05 V (versus Ag/AgCl) in pH 8.0 phosphate buffer. On this basis, a flow injection (FI) procedure with inhibited electrochemiluminescence detection has been developed for determination of adrenaline. The method exhibited a good reproducibility, sensitivity, and stability with a detection limit (signal-to-noise ratio = 3) of 7.0×10⁻⁹ mol l⁻¹ and dynamic concentration range of 2×10⁻⁸ to 1×10⁻⁴ mol l⁻¹. The relative standard deviation was 2.2% for 1.0×10⁻⁶ mol l⁻¹ adrenaline (n=11). The method was successfully applied to the determination of adrenaline in pharmaceutical samples. Moreover, ECL emission spectra, UV-Vis absorption spectra and cyclic voltammograms of Ru(bpy)₃²⁺/tripropylamine/adrenaline were studied. The inhibition mechanism has been proposed as the interaction of electrogenerated Ru(bpy)₃^2+* and the o-benzoquinone derivatives, adrenochrome and adrenalinequinone, at the electrode surface.     

Activity coefficients of KCl in PEG 4000 + water mixtures at 288.15, 298.15 and 308.15 K

The electromotive force of the cell containing two ion-selective electrodes (ISE), K-ISE|KCl(m), PEG 4000(Y), H₂O(100 − Y)|Cl-ISE has been measured at temperatures of 288.15, 298.15, and 308.15 K as a function of the weight percentage Y of PEG 4000 in a mixed solvent. Y was varied between 0 and 25 wt.% in five-unit steps and the molality of the electrolyte (m) was between ca. 0.05 mol kg⁻¹ and almost saturation. The values of the standard electromotive force were calculated using routine methods of extrapolation together with extended Debye-Hückel and Pitzer equations. The results obtained produced good internal consistency for all the temperatures studied. Once the standard electromotive force was determined, mean ionic activity coefficients for KCl, Gibbs energy of transfer from the water to PEG 4000 + water mixtures, interaction parameters (g_EN, h_EN, s_EN, c_p,EN), salting constants, and the KCl primary hydration number were estimated and comparatively discussed in terms of a model of structural and electrostatic interactions with those of the LiCl and NaCl previously obtained in similar mixtures.     

Re-evaluation of the Thermodynamic Activity Quantities in Pure Aqueous Solutions of Chlorate, Perchlorate, and Bromate Salts of Lithium, Sodium or Potassium Ions at 298.15 K

The Hückel equation used to correlate the experimental activities of dilute alkali metal chlorate, perchlorate or bromate solutions up to a molality of about 1.5 mol⋅kg⁻¹ contains two electrolyte-dependent parameters: B {that is related closely to the ion-size parameter (a ^∗) in the Debye–Hückel equation} and b ₁ (this parameter is the coefficient of the linear term with respect to the molality, which is related to hydration numbers of the ions of the electrolyte). In more concentrated solutions up to 7 mol⋅kg⁻¹, an extended Hückel equation was used, it contains additionally a quadratic term with respect to the molality, and the coefficient of this term is the parameter b ₂. Parameters for the Hückel equations were evaluated from isopiestic data for LiClO₃, LiBrO₃, LiClO₄, NaClO₃, NaBrO₃, NaClO₄, KClO₃, and KBrO₃. In these estimations, the Hückel parameters determined recently for NaCl solutions were used. The resulting parameter values were tested with the vapor pressure and isopiestic data available in the literature for solutions of these salts. Most of these data were reproduced within experimental error by means of the Hückel or extended Hückel equations, at least up to a molality of 3.0 mol⋅kg⁻¹, for all salts considered. Reliable activity and osmotic coefficients for solutions of these salts can, therefore, be calculated by using the new Hückel equations, and they are tabulated here at rounded molalities. The activity and osmotic coefficients obtained from these equations were compared to the values reported in several previous tabulations.     

Determination of noradrenaline and dopamine in pharmaceutical injection samples by inhibition flow injection electrochemiluminescence of ruthenium complexes

Two maximal potential-resolved flow injection-electrochemiluminescent (FI-ECL) peaks were observed for Ru(bpy)₃²⁺/TPrA system at 0.90 and 1.05 V, and for Ru(phen)₃²⁺/TPrA at 1.01 and 1.25 V (vs. Ag/AgCl) in pH 8.0 phosphate buffer solutions. Sensitive ECL inhibition effects were observed in the presence of noradrenaline and dopamine for both of these systems. Therefore, an FI-ECL inhibition method for determination of noradrenaline and dopamine has been developed. Under optimal conditions, linear responses between logarithm of ECL intensity changes and logarithm of sample concentration were found for noradrenaline in the linear range (LR) of 4×10⁻⁸-1×10⁻⁵ mol l⁻¹ with theoretical detection limit (DL) of 2.5×10⁻⁸ mol l⁻¹ for Ru(bpy)₃²⁺/TPrA system, and in LR of 2×10⁻⁸-2×10⁻⁵ mol l⁻¹ with DL of 7.1×10⁻⁹ mol l⁻¹ for Ru(phen)₃²⁺/TPrA system; and for dopamine in LR of 8×10⁻⁸-2×10⁻⁵ mol l⁻¹ with DL of 5.2×10⁻⁸ mol l⁻¹ for Ru(bpy)₃²⁺/TPrA system, in LR of 4×10⁻⁸-2×10⁻⁵ mol l⁻¹ with DL of 1.5×10⁻⁸ mol l⁻¹ for Ru(phen)₃²⁺/TPrA system. It was applied for determination of commercial pharmaceutical injection samples with satisfied results. The mechanism of the inhibition effects was proposed in the preliminary way.     

Biosensor for chlorogenic acid based on an ionic liquid containing iridium nanoparticles and polyphenol oxidase

A biosensor based on the ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate containing dispersed iridium nanoparticles (Ir-BMI.PF₆) and polyphenol oxidase was constructed. This enzyme was obtained from the sugar apple (Annona squamosa), immobilized in chitosan ionically crosslinked with oxalate. The biosensor was used for determination of chlorogenic acid by square wave voltammetry. The polyphenol oxidase catalyzes the oxidation of chlorogenic acid to the corresponding o-quinone, which is electrochemically reduced back to this substance at +0.25 V vs. Ag/AgCl. Under optimized operational conditions the chlorogenic acid concentration was linear in the range of 3.48 × 10⁻⁶ to 4.95 × 10⁻⁵ mol L⁻¹ with a detection limit of 9.15 × 10⁻⁷ mol L⁻¹. The biosensor was applied in the determination of chlorogenic acid in organic and decaffeinated coffee and the results compared with those obtained using the capillary electrophoresis method. The recovery study for chlorogenic acid in these samples gave values of 93.2-105.7%.     

Electrochemiluminescent behaviors of alkaloids and tris(2,2'-bipyridine) ruthenium in organically modified silicate film

Electrogenerated chemiluminescences (ECLs) of alkaloids, such as berberine, trigonelline, allantoin and betaine, were studied in an aqueous alkaline buffer solution (pH 9.5), based on tris(2,2′-bipyridine)ruthenium(II) [Ru(bpy)₃²⁺] immobilized in organically modified silicates (ORMOSILs) film on a glassy carbon electrode (GCE). The immobilized Ru(bpy)₃²⁺ showed good electrochemical and photochemical activities. In a flow system, the eluted alkaloids were oxidized on the modified GCE, and reacted with immobilized Ru(bpy)₃²⁺ at the potential of +1.50 V (versus Ag/AgCl). The luminescence with λ_max 610 nm was caused by a reaction of electrolytically formed Ru(bpy)₃³⁺ with an oxidized amine group to generate Ru(bpy)₃^2+*. The determination limit was 5 × 10⁻⁶ mol L⁻¹, 8 × 10⁻⁶ mol L⁻¹, 2.0 × 10⁻⁵ mol L⁻¹ and 5.0 × 10⁻⁵ mol L⁻¹ for berberine, trigonelline, allantoin and betaine at S/N 3, respectively. In addition, the factors affecting the determination of the four alkaloids were also studied.     

A solution-reaction isoperibol calorimeter and standard molar enthalpies of formation of Ln(hq)2Ac (Ln = La, Pr)

An on-line solution-reaction isoperibol calorimeter has been constructed. The performance of the apparatus was evaluated by measuring the molar enthalpy of solution of KCl in water at 298.15 K. The uncertainty and the inaccurary of the experimental results were within ±0.3% compared with the recommended reference data. Using the calorimeter, the molar enthalpies of reaction for the following two reactions: LaCl₃·7H₂O(s)+2Hhq(s)+NaAc(s)=La(hq)₂Ac(s)+NaCl(s)+2HCl(g)+7H₂O(l) and PrCl₃·6H₂O(s)+2Hhq(s)+NaAc(s)=Pr(hq)₂Ac(s)+NaCl(s)+2HCl(g)+6H₂O(l), were determined at T=298.15 K, as −(78.3±0.6) and −(97.3±0.5) kJ mol^−l, respectively. From the above molar enthalpies of reaction and other auxiliary thermodynamic quantities, the standard molar enthalpies of formation of La(hq)₂Ac and Pr(hq)₂Ac, at T=298.15 K, have been derived to be −(1535.5±0.7) and −(1536.7±0.6) kJ mol^−l, respectively.     

Chronoamperometric determination of lead ions using PEDOT:PSS modified carbon electrodes

A new simple chronoamperometry methodology was developed for the ultrasensitive determination of lead ions using a PEDOT:PSS coated graphite carbon electrode. The polymer was directly coated on a graphite carbon electrode and characterized using simple cycle voltammetric measurements. The presence of lead ions induced a cathodic peak starting at −550 ± 10 mV vs. Ag/AgCl, and an anodic peak starting at −360 ± 10 mV vs. Ag/AgCl. Electroaccumulation of lead ions onto the PEDOT:PSS modified electrode was performed at −650 mV vs. Ag/AgCl for 30 s in a pH 2.2 hydrochloric acid solution. Chronoamperometry measurements were carried out at −350 mV vs. Ag/AgCl allowing the oxidation of accumulated lead. Using this method, lead ions were detected for concentrations ranging between 2.0 nmol L⁻¹ and 0.1 μmol L⁻¹ (R² = 0.999). The detection limit was calculated to be 0.19 nmol L⁻¹ and the quantification limit of 0.63 nmol L⁻¹. The method was shown to be highly precise and sensitive, negligible interference was detected from other metal ions. The proposed method was successfully applied for the detection of lead ions in vegetables.     

Biosensor based on laccase and an ionic liquid for determination of rosmarinic acid in plant extracts

Novel biosensors based on laccase from Aspergillus oryzae and the ionic liquids (ILs) 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆) and 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIBF₄) were constructed for determination of rosmarinic acid by square-wave voltammetry. The laccase catalyzes the oxidation of rosmarinic acid to the corresponding o-quinone, which is electrochemically reduced back to rosmarinic acid at +0.2 V vs. Ag/AgCl. The biosensor based on BMIPF₆ showed a better performance than that based on BMIBF₄. The best performance was obtained with 50:20:15:15% (w/w/w/w) of the graphite powder:laccase:Nujol:BMIPF₆ composition in 0.1 mol L⁻¹ acetate buffer solution (pH 5.0). The rosmarinic acid concentration was linear in the range of 9.99 × 10⁻⁷ to 6.54 × 10⁻⁵ mol L⁻¹ (r = 0.9996) with a detection limit of 1.88 × 10⁻⁷ mol L⁻¹. The recovery study for rosmarinic acid in plant extract samples gave values from 96.1 to 105.0% and the concentrations determined were in agreement with those obtained using capillary electrophoresis at the 95% confidence level. The BMIPF₆-biosensor demonstrated long-term stability (300 days; 920 determinations) and reproducibility, with a relative standard deviation of 0.56%.     

Adsorptive stripping voltammetric determination of trace concentrations of molybdenum at an in situ plated lead film electrode

An in situ plated lead film electrode has been applied for adsorptive stripping voltammetric determination of trace concentrations of molybdenum in the presence of Alizarin S. The procedure is based on the preconcentration of the molybdenum-Alizarin S complex at an in situ plated lead film electrode held at −0.6 V (versus Ag/AgCl), followed by a negatively sweeping square wave voltammetric scan. The peak current is proportional to the concentration of molybdenum over the range 2 × 10⁻⁹ to 5 × 10⁻⁸ mol L⁻¹, with a 3σ detection limit of 9 × 10⁻¹⁰ mol L⁻¹ with an accumulation time of 60 s. The measurements were carried out from underaerated solutions. The proposed procedure was validated in the course of Mo(VI) determination in water certified reference materials.     

Preparation of cobalt-tetraphenylporphyrin/reduced graphene oxide nanocomposite and its application on hydrogen peroxide biosensor

A novel cobalt-tetraphenylporphyrin/reduced graphene oxide (CoTPP/RGO) nanocomposite was prepared by a π–π stacking interaction and characterized by ultraviolet–visible absorption spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS). The CoTPP/RGO nanocomposite exhibited high electrocatalytic activity both for oxidation and reduction of H₂O₂. The current response was linear to H₂O₂ concentration with the concentration range from 1.0 × 10⁻⁷ to 2.4 × 10⁻³ mol L⁻¹ (R = 0.998) at the reductive potential of −0.20 V and from 1.0 × 10⁻⁷ to 4.6 × 10⁻⁴ mol L⁻¹ (R = 0.996) at the oxidative potential of +0.50 V. The H₂O₂ biosensor showed good anti-interfering ability towards oxidative interferences at the oxidative potential of +0.50 V and good anti-interfering ability towards reductive interferences at the reductive potential of −0.20 V.     

Thin Pd–23%Ag/stainless steel composite membranes: Long-term stability,life-time estimation and post-process characterisation

The long-term stability of Pd–23%Ag/stainless steel composite membranes has been examined in H₂/N₂ mixtures as a function of both temperature and feed pressure. During continuous operation, the membrane shows a good stability at 400 °C while the N₂ leakage increases very slowly at a temperature of 450 °C (P_feed = 10 bar). After 100 days of operation (P_feed = 5–20 bar, T = 350–450 °C), the N₂ permeance equals 7.0 × 10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹, which indicates that the H₂/N₂ permselectivity still lies around 500, based on a H₂ permeance equal to 3.0 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹. Despite the generation of small pinholes, a membrane life-time of several (2–3) years (T ≤ 425 °C) is estimated for the experimental conditions employed based on long-term stability tests over 100 days. Post-process characterisation shows a considerable grain growth and micro-strain relaxation in the Pd–23%Ag membrane after the prolonged permeation experiment. Changes in surface area are relatively small. In addition, segregation of Ag to the membrane surfaces is observed. The formation of pinholes is identified as the main source for the increased N₂ leakage during testing at higher temperature.