Enthalpy of solution of α-cyclodextrin in water and in formamide at 298.15 K

A new dissolution microcalorimeter that can measure enthalpies of dissolution of slightly soluble solids was developed by Ingemar Wadsö at Lund University, and the prototype as well as the commercial vessel were tested in our laboratory. Recently we did report the testing of the prototype and we are now extending the previous investigation to measurements with an organic solvent in the commercial vessel. The instrument performance was found to be as good with formamide as with water.The vessel was calibrated chemically (dissolution of KCl in water) and electrically, by means of a permanent and an insertion heater. The results obtained from the three methods are compared and discussed.The enthalpy of dissolution of α-cyclodextrin, dry and hydrated with six water molecules, was determined in water and in formamide. The results are discussed in terms of the difference between water and formamide as regarding dissolution and binding to the cyclodextrin molecule.     

Thermodynamic properties of the gaseous barium silicates BaSiO2 and BaSiO3

In the present study, the stability of gaseous barium silicates was confirmed by the high temperature mass spectrometry. On the basis of equilibrium constants measured for gas-phase reactions, the standard formation enthalpies were determined for gaseous barium silicates as (−510 ± 15) kJ · mol⁻¹ and (−884 ± 18) kJ · mol⁻¹ at 298 K; standard atomization enthalpies as (1637 ± 17) kJ · mol⁻¹ and (2318 ± 20) kJ · mol⁻¹ at 298 K for BaSiO₂ and BaSiO₃, respectively. Based on the results obtained the critical analysis of the literature data was carried out.     

Enthalpies of solution of paracetamol and sodium diclofenac in phosphate buffer and in DMSO at 298.15 K

We have been using a heat conduction microcalorimeter to measure enthalpies of dissolution of slightly soluble solids. Recently we did report the testing of the prototype and we are now extending the previous investigation to measurements with organic solvents.In the present work we use DMSO due to its importance in the pharmaceutical industry. The importance of the applications of enthalpies of solution in organic solvents calls for the need to find a substance that can be used in the future for chemical calibration in these solvents. Attempts for proposing such a substance have also been a goal for our research group.The enthalpies of solution of paracetamol and sodium diclofenac were measured in DMSO and in phosphate buffer (pH 6.91) at 298.15 K. We show that it is possible to obtain very good results with DMSO as solvent for these two drugs on our dissolution microcalorimeter. Nevertheless, due to their slow and low enthalpy of dissolution in this solvent, they cannot be seen as good candidates for test substances.     

Sequestration of Cu(II), Ni(II), and Co(II) by ethyleneimine immobilized on silica

Thermodynamic data on interaction of Cu(II), Ni(II), and Co(II) with silica modified with ethyleneimine are obtained by calorimetric titration. The amount of ethyleneimine anchored on silica surface was estimated to be 0.70 mmol g⁻¹. The enthalpies of binding Ni(II), Cu(II) and Co(II), are −3.59 ± 0.001, −4.88 ± 0.001, and −7.75 ± 0.003 kJ mol⁻¹, respectively.     

Enthalpies of formation of adducts of antimony(III) iodide with pyridine and methyl-pyridines

Solid adducts of formula SbI₃·L (L = pyridine or 2-, 3- or 4-methylpyridine abbreviated as Py, 2MPy, 3MPy or 4MPy) were synthesized and characterized by elemental analysis, IR spectroscopy and thermal analysis. IR data showed that coordination to antimony is through nitrogen. Thermal degradation of adducts starts at 431, 423, 413 and 411 K for Py, 2MPy, 3MPy and 4MPy, respectively. Reaction-solution calorimetry was used to evaluate the enthalpy change of reaction: SbI₃(cr) + L(l) = SbI₃·L(cr), 61.13 ± 1.75, −82.60 ± 1.55, −67.50 ± 0.97 and −74.10 ± 1.19 kJ mol⁻¹, respectively. Enthalpy change values for decomposition of adducts, lattice enthalpies and enthalpies of the Lewis acid-base reaction in the gas phase were calculated through appropriate thermochemical cycles. Mean SbN bond enthalpies were estimated as 134 ± 3, 154 ± 3, 140 ± 3 and 147 ± 4 kJ mol⁻¹, for Py, 2MPy, 3MPy and 4MPy, respectively.     

Kleppa type calorimeter for the study of high temperature processes

A Kleppa type microcalorimeter was designed and constructed for the application to measure the thermal change in a reaction tube at T>1300 K. The details of new calorimeter are described. In order to verify the performance of the new calorimeter, the standard enthalpies of formation for NiSi and Ni₃Al were measured by direct synthesis in the calorimeter at T=1323 K. The enthalpies of formation obtained were −(79.3 ± 1.8) and −(189.3 ± 6.3) kJ · mol⁻¹ for NiSi and Ni₃Al, respectively. A comparison of results in the present work with available reported values has been made.     

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.     

Determination of the rate of production and dissolution of biosilica in marine waters by thermal ionisation mass spectrometry

A new method is described for a precise and simultaneous determination of the rate of production and dissolution of biosilica in marine waters, using isotopic dilution technique. No HF or F₂ is required for chemical preparations as the change in isotopic composition is measured on silica producing SiO₂⁻ ions. The seawater sample flask is spiked with ³⁰Si(OH)₄ (<10% of increase in situ concentration) and incubated in in situ conditions. At the end of incubation, changes of the ³⁰Si:²⁸Si ratios in particulate and liquid phases are measured by using a thermal ionisation mass spectrometer Finnigan THQ. The relative analytical precision of the isotopic ratio measurements is <0.5%. The limit of detection of the change in isotopic ratio during incubation is 0.02 atom%. The overall repeatability determined on eight subsamples (average production: 0.23 μM day⁻¹; average dissolution: 0.07 μM day⁻¹) is ±0.02 and ±0.01 μM day⁻¹ for production and dissolution, respectively. Using mass and isotopic balances of the particulate and dissolved phases in the incubation flask, the best estimates for production and dissolution rates are calculated iteratively. This method was applied to 112 samples of marine waters (production, range: 0.00-2.38 μM day⁻¹; dissolution, range: 0.00-1.18 μM day⁻¹).     

Thermochemical study of two anhydrous polymorphs of caffeine

The mean values of the standard massic energy of combustion of caffeine in phase I (or alpha) and in phase II (or beta) measured by static-bomb combustion calorimetry in oxygen, at T = 298.15 K, are Δ_cu^° (C₈H₁₀O₂N₄, I) = −(21823.27 ± 0.68) J · g⁻¹ and Δ_cu^° (C₈H₁₀O₂N₄, II) = −(21799.96 ± 1.08) J · g⁻¹, respectively.The standard (p^° = 0.1 MPa) molar enthalpy of formation in condensed phase for each form was derived from the corresponding standard molar enthalpies of combustion as, and .The difference between the standard enthalpy of formation of the two polymorphs in condensed phase was also evaluated by using reaction-solution calorimetry. The obtained result, 2.04 ± 0.25 kJ · mol⁻¹, is in agreement, within the uncertainty, with the difference between the molar enthalpies of formation obtained from combustion experiments (4.5 ± 3.2) kJ · mol⁻¹, which can be considered as an internal test for consistency of the results.A value for the standard enthalpy of formation of caffeine in the gaseous state was proposed: , estimated from the values of the standard enthalpies of formation of both crystalline forms obtained in this work, and the data on standard enthalpies of sublimation collected from the literature.     

Synthesis and thermochemistry of SrB2O4·4H2O and SrB2O4

Two pure strontium borates SrB₂O₄·4H₂O and SrB₂O₄ have been synthesized and characterized by means of chemical analysis and XRD, FT-IR, DTA-TG techniques. The molar enthalpies of solution of SrB₂O₄·4H₂O and SrB₂O₄ in 1 mol dm⁻³ HCl(aq) were measured to be −(9.92 ± 0.20) kJ mol⁻¹ and −(81.27 ± 0.30) kJ mol⁻¹, respectively. The molar enthalpy of solution of Sr(OH)₂·8H₂O in (HCl + H₃BO₃)(aq) were determined to be −(51.69 ± 0.15) kJ mol⁻¹. With the use of the enthalpy of solution of H₃BO₃ in 1 mol dm⁻³ HCl(aq), and the standard molar enthalpies of formation for Sr(OH)₂·8H₂O(s), H₃BO₃(s), and H₂O(l), the standard molar enthalpies of formation of −(3253.1 ± 1.7) kJ mol⁻¹ for SrB₂O₄·4H₂O, and of −(2038.4 ± 1.7) kJ mol⁻¹ for SrB₂O₄ were obtained.     

Nuclear magnetic resonance and optosensing properties of di-2-thienyl ketone p-nitrophenylhydrazone (DSKNPH) in non-aqueous media

The compound di-2-thienyl ketone p-nitrophenylhydrazone (DSKNPH) melting point 168-170 °C was isolated in good yield from the reaction between di-2-thienyl ketone (DSK) and p-nitrophenylhydrazine in refluxing ethanol containing a few drop of concentrated HCl. Nuclear magnetic resonance studies on DSKNPH in non-aqueous solvents revealed strong solvent and temperature dependence due to solvent-solute interactions. Optical measurements on DSKNPH in DMSO in the presence and absence of KPF₆ gave extinction coefficients of 83,300±2000 and 25,600±2000 M⁻¹ cm⁻¹ at 612 and 427 nm at 295 K. In CH₂Cl₂, extinction coefficient of 34,000±2000 M⁻¹ cm⁻¹ was calculated at 422 nm. When DMSO solutions of DSKNPH were allowed to interact with DMSO solutions of NaBH₄ the low energy electronic state becomes favorable and when DMSO solutions of DSPKNPH where allowed to interact with DMSO solutions of KPF₆ or NaBF₄, the high energy electronic state becomes favorable. The reversible BH₄⁻/BF₄⁻ interconversion points to physical interactions between these species and DSKNPH and hints to the possible use of DSKNPH as a spectrophotometric sensor for a variety of physical and chemical stimuli. Thermo-optical measurements on DSKNPH in DMSO confirmed the reversible interconversion between the high and low energy electronic states of DSKNPH and allowed for the calculations of the thermodynamic activation parameters of DSKNPH. Changes in enthalpy (ΔH^∅) of +57.67±4.20; 27.15±0.90 kJ mol⁻¹, entropy (ΔS^∅) of +160±12.88; 83±2.91 J mol⁻¹ and free energy (ΔG^∅) of −8.52±0.40; 2.66±0.25 kJ mol⁻¹ were calculated at 295 K in the absence and presence of NaBH₄, respectively. Manipulation of the equilibrium distribution of the high and low energy electronic states of DSKNPH allowed for the use of these systems (DSKNPH and surrounding solvent molecules) as molecular sensors for group I and II metal ions. Group I and II metal ions in concentrations as low as 1.00×10⁻⁵ M can be detected and determined using DSKNPH in DMSO.     

A novel fused-silica capillary dropping mercury electrode with long drop-time and its application on determination of critical micelle concentration

A novel long drop time mercury electrode has been constructed from common fused-silica capillary (50 μm I.D., 360 μm E.D.). Proposed device provides reproducible mercury drops with typical lifetime 40-120 s. The electrode was used for a set of electrocapillary measurements aimed at determination of critical micelle concentration of anionic surfactants by a convection controlled drop-time technique. A critical micelle concentration of sodium dodecyl sulfate 5.6 ± 0.4 mmol L⁻¹ and 4.3 ± 0.4 mmol L⁻¹ were obtained in 1 mmol L⁻¹ and 5 mmol L⁻¹ phosphate buffer (pH 7.0), respectively. The values were comparable to those obtained from conductometric measurements under the same conditions (7.0 ± 0.1 mmol L⁻¹ and 5.2 ± 0.1 mmol L⁻¹, respectively) and the difference was explained in accordance with theory of hemi-micelle formation.     

Kinetics and retention characteristics of some nitrophenols onto polyurethane foams

The kinetics of sorption of the nitrophenols by the unloaded polyurethane foams (PUFs) were found fast, reached equilibrium in few minutes and followed a first-order rate equation with an overall rate constant k in the range (0.16-0.21)±0.01 min⁻¹. The retention of the tested nitrophenols by the unloaded foams is consistent with the “solvent extraction” mechanism. However, the sorption also followed Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherms. The mean free sorption energy of the nitrophenols onto the PUF was found equal to 7.5±0.4 kJ/mol, which reflects physical sorption. Thus, a dual-mode involves both absorption related to solvent extraction and an added component for surface adsorption seems a more likely sorption mechanism model. While a dual-mode sorption model explains the observed retention behavior, the data suggest that, solvent extraction plays a much larger role than the added component for surface adsorption. The sorption and recovery percentages of the nitrophenols from fresh, natural and industrial wastewater by the proposed unloaded foam columns were quantitatively achieved. The height equivalent to theoretical plates (HETP), N, the breakthrough capacity and the critical capacity for the unloaded foam columns were found in the range of (0.8-1.1)±0.6 mm, (94-132)±3, 3.2-4.02 and 1.5-2.67 mg/g, respectively. The method was successfully applied for the retention and recovery of the tested nitrophenols spiked to fresh and industrial wastewaters.     

Thermochemistry of Li, Na, K, Rb and Cs alkylated phenoxides

Lattice energies and thermochemical radii of the anions OR⁻ (R = 2-Me; 2,6-Me₂; 2,4,6-Me₃; 2,6-t-Bu₂) in alkali metal phenoxides, MOR (M = Li, Na, K, Rb and Cs) were investigated based on the corresponding standard molar enthalpies of formation determined by reaction-solution calorimetry. The results obtained at 298.15 K were as follows: (MOR, cr)/kJ mol⁻¹ = −398.4 ± 1.1 (LiO-2-MePh), −423.4 ± 1.6 (LiO-2,6-Me₂Ph), −457.3 ± 7.1 (LiO-2,4,6-Me₃Ph), −346.6 ± 1.4 (NaO-2-MePh), −399.1 ± 1.5 (NaO-2,6-Me₂Ph), −422.4 ± 7.1 (NaO-2,4,6-Me₃Ph), −496.6 ± 7.1(NaO-2,6-t-Bu₂Ph), −367.8 ± 1.2 (KO-2-MePh), −399.1 ± 1.4 (KO-2,6-Me₂Ph), −368.8 ± 1.2 (RbO-2-MePh), −403.6 ± 1.3 (RbO-2,6-Me₂Ph), −387.0 ± 1.6 (CsO-2-MePh) and −413.6 ± 1.3 (CsO-2,6-Me₂Ph). Estimates of thermochemical raddi, lattice energies and standard enthalpies of formation of not experimentally measured alkali metal phenoxides was successfully done with a model based on the Kapustinskii equation and the set of values experimentally determined.     

Molar excess enthalpies of ethyl acetate + alkanols at T = 298.15 K, p = 10.0 MPa

A commercial flow-mixing isothermal calorimeter was tested by measuring heat of mixing curves for exothermic, endothermic, S-shaped and double minimum molar excess enthalpy mixtures at high pressure. The results show this calorimeter is able to produce good quality data. Molar excess enthalpies for ethyl acetate mixed with a series of simple alkanols were measured at T = 298.15 K and p = 10 MPa.     

Application of correlation-gas chromatography to evaluate the vaporization enthalpy of a component in an equilibrium mixture

The vaporization enthalpies and vapor pressures of acetoin, ethyl 3-hydroxybutyrate and ethyl 3-hydroxyhexanoate, found in a variety of foods and flavors, are evaluated at T = 298.15 K using correlation-gas chromatography; values of (48.7 ± 0.4), (55.9 ± 0.6) and (61.9 ± 0.6) kJ mol⁻¹, respectively, were obtained. These values are in good agreement with estimated values. Vapor pressures of the standards as a function of temperature were also used to calculate vapor pressures of the target compounds and all resulting data were fit to second order polynomials. These polynomials were then used to predict boiling temperatures of both standards and target substances. Agreement with experimental boiling temperatures was generally within 10 K suggesting that vapor pressures are accurate to within a factor of two. Acetoin exists as an equilibrium mixture of monomer and dimer. This report provides an example of the utility of using correlation-gas chromatography to obtain thermochemical data on an impure material.     

Synthesis and solvent dependence of the photophysical properties of [60]fullerene-sugar conjugates

A method for the synthesis of optically pure C₆₀ derivatives containing one or two d-galactose or d-glucose units is described. It involves the synthesis of sugar-malonate derivatives followed by a cyclopropanation reaction with C₆₀. The solvent dependence of the photophysical properties of the methano[60]fullerene-sugar derivatives was studied using nanosecond laser flash photolysis coupled with kinetic UV-vis absorption spectroscopy and time-resolved singlet oxygen luminescence measurements. The triplet properties of these fullerenes, including transient absorption spectra, molar absorption coefficients and quantum yield for the photosensitised production of ¹O₂ were determined in toluene, benzonitrile and acetonitrile solutions. The transient absorption spectral profiles are solvent independent although small differences are observed in the transient absorption maximum: 720±5 nm for toluene, 710±5 nm for benzonitrile and 700±5 nm for acetonitrile. Triplet state molar absorption coefficients (ε_T) of C₆₀ derivatives vary from 9456±2090 M⁻¹ cm⁻¹, for compound 10 in toluene, and 15,272±4462 M⁻¹ cm⁻¹, for compound 6 in acetonitrile. Triplet state lifetimes (τ_T) for methano[60]fullerene-sugar derivatives, under our experimental conditions, are similar in toluene or benzonitrile solutions (47.5±1.1 μs≤τ_T≤51.4±2.0 μs) but are lower in acetonitrile solutions (31.8±0.6 μs≤τ_T≤43.0±1.1 μs). Toluene and benzonitrile solutions of C₆₀ derivatives have Φ_Δ close to unity.     

Determination of As, Sb, Se, Te and Bi in milk by slurry sampling hydride generation atomic fluorescence spectrometry

A simple and fast analytical procedure has been developed for the determination of As, Sb, Se, Te and Bi in milk samples by hydride generation atomic fluorescence spectrometry (HG-AFS). Samples were treated with aqua regia for 10 min in an ultrasound water bath and pre-reduced with KBr for total Se and Te determination or with KI and ascorbic acid for total As and Sb, the determination of Bi being possible in all with or without pre-reduction. Slurries of samples, in the presence of antifoam A, were treated with NaBH₄ in HCl medium to obtain the corresponding hydrides, and AFS measurements were processed in front of external calibrations prepared and measured in the same way as samples. Results obtained by the developed procedure compare well with those found after microwave-assisted complete digestion of samples. The proposed method is simple and fast, and only 1 ml of milk is needed. The values obtained for detection limit are 2.5, 1.6, 3, 6 and 7 ng l⁻¹ for As, Sb, Se, Te and Bi respectively in the diluted samples, with average relative standard deviation values of 3.8, 3.1, 1.9, 6.4 and 1.2% for three independent analysis of a series of commercially available samples of different origin. Data found in Spanish market samples varied from 3.2±0.3 to 11.3±0.2 ng g⁻¹ As, from 3.1±0.2 to 11.6±0.4 ng g⁻¹ Sb, from 10.7±0.5 to 25.5±0.4 ng g⁻¹ Se, from 0.9±0.2 to 9.4±0.6 ng g⁻¹ Te and from 11.5±0.1 to 27.7±0.4 ng g⁻¹ Bi.     

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.     

Humic acid-divalent cation interactions

The adsorption behavior of divalent cations M²⁺ (Cu, Ni, Co and Zn) with commercial humic acid (HAAl) and also with an extracted fraction of peat soil (HAPs) was followed in aqueous solution. The series of adsorption isotherms were fitted to a modified Langmuir equation. The maximum number of moles adsorbed gave: 0.55±0.02, 0.66±0.02, 0.54±0.02, 0.40±0.02 mmol per gram for HAAl and 0.63±0.03, 0.61±0.06, 0.55±0.02, 0.54±0.03 mmol/g for solid HAPs, for copper, nickel, zinc and cobalt, respectively. The same interaction followed calorimetrically gave endothermic values: 2.4±1.0, 8.4±0.9, 18.3±0.9, 10.6±0.9 kJ mol⁻¹ and 18.4±1.2, 15.9±1.4, 15.4±1.2, 15.0±1.2 kJ mol⁻¹ for HAAl and HAPs, respectively, for the same sequence. Because all Gibbs free energies were negative. Complexation must be accompanied by an increase in entropy.