Glenn Hefter

Heat capacities of binary aqueous solutions of HNO3, Mg(NO3)2, Ni(NO3)2, and Co(NO3)2 have been measured up to high concentrations using a Picker-type flow calorimeter at 298.15 K and 0.1 MPa. Where comparisons were possible, the present results were mostly in good agreement with literature data. Greater differences in Ni(NO3)2(aq) and Co(NO3)2(aq) may be due to cation hydrolysis. Heat capacities were well fitted with an extended Redlich–Rosenfeld–Meyer-type equation for HNO3(aq), and Pitzer-type equations for the three salts. Ternary solutions HNO3 + M(NO3)2 (M = Mg, Ni, Co) were measured as functions of solution composition at constant ionic strengths of (6.0–12.0, 12.0, and 10.44) mol·kg–1, respectively. In addition, data were obtained at constant molality fractions for Mg(NO3)2 + HNO3 at x(Mg2+) = 0.3331, and for Ni(NO3)2 + HNO3 at x(Ni2+) = 0.2523. It was established that ternary solution heat capacities could be predicted from binary component properties alone, either using Young’s rule (based on molar quantities) or an empirical mixing rule based on massic (“specific”) heat capacities; neither requires information beyond the relevant binary solution quantities, i.e., no additional mixing parameters are needed.

Understanding supramolecular chemistry requires comprehensive knowledge of macrocycle behaviour in solution. This study reports standard molar volumes and compressibilities of the chloride salts of two cationic macrocycles C-methylresorcin[4]arene-tetraminoethylated-hydrochloride (TAE⋅(HCl)4) and their C-ethyl analogue (ETAM⋅(HCl)4) in aqueous and DMSO solutions. These quantities were obtained using density and speed of sound measurements at concentrations (0.005 to 0.08) mol⋅kg−1 and temperatures in the range (278.15 to 308.15) K. NMR techniques were used to study the structures of the macrocycles in solution, while the corresponding compounds were characterised using mass spectrometry, thermogravimetry and differential scanning calorimetry. The partial molar quantities derived from the volumetric and sonometric data are discussed, along with relevant literature information, in terms of solute–solvent interactions. Structural modifications of macrocycles influence the standard molar compressibilities through changes in solvent organization around functional groups, intrinsic compressibility, and water molecules in the resorcinarene cavity. The effect of carbon chain length on the volumes and compressibilities of substituted resorcin[4]arenes is shown to depend on their location in the macrocycle, but not on the solvent. The standard molar volume is shown to be influenced by ionic moieties and intramolecular hydrogen bonds.

Volumetric isobaric heat capacities of aqueous solutions of nitric acid have been measured in a modified commercial differential scanning calorimeter fitted with purpose-built tantalum cells over the temperature range of (325.15–473.45) K, at molalities up to 15.0 mol·kg–1, and at 2.0 MPa pressure. These data were combined with recent density measurements to produce apparent molar isobaric heat capacities. The results so obtained were fitted using a Redlich-Rosenfeld-Meyer-type equation, which gave average and maximum deviations of (0.7 and 5) J·K–1·mol–1, respectively. The present results are in good agreement with the literature data available at lower temperatures and concentrations. More importantly, these data represent a major extension of the thermodynamic database for these industrially vital solutions.

Densities of aqueous solutions of zinc nitrate, Zn(NO3)2, and some of its ternary mixtures with nitric acid have been determined by vibrating tube densimetry at temperatures 293.15 K ≤ T/K ≤ 473.15, molalities 0.02 ≲ m/mol kg–1 ≲ 5, and pressures p = 0.102 or 2.00 MPa. Values of the apparent molar volumes (Vϕ) of Zn(NO3)2(aq) derived from these data easily fitted within the estimated experimental uncertainties. At 298.15 K, the present results agreed well with literature values at m > 1 mol kg–1. However, at lower molalities, the literature results were found to be scattered, with Vϕ sometimes increasing anomalously with decreasing m, probably due to hydrolysis of Zn2+(aq). No meaningful comparisons were possible at other temperatures, but plots of the standard molar volumes, V°(Zn(NO3)2(aq)), against temperature, closely paralleled those of related nitrate salts. Volumes of some ternary mixtures (Zn(NO3)2 + HNO3 + H2O) were well-described by Young’s rule at low to moderate ionic strengths. The present results greatly expand the available volumetric database for both binary and ternary aqueous solutions of this industrially important electrolyte.

Viscosities of aqueous solutions of Mg(NO3)2, HNO3, and their mixtures have been measured by capillary viscometry at temperatures T/K = 298.15, 313.15, and 328.15 at 0.1 MPa pressure. Viscosities of the binary solutions were determined at concentrations up to (4.55 and 15.00) mol·kg–1 for Mg(NO3)2 (aq) and HNO3 (aq) respectively, with an estimated relative experimental uncertainty of 0.009 to 0.013, corresponding to a 68% confidence level (equivalent to one standard deviation). Where comparisons were possible, the present results were generally in good agreement with most literature data. This enabled identification of outliers and inconsistencies in the latter. The binary solution viscosities were well fitted with simple 4-parameter empirical equations which showed limited, but useful, extrapolative capabilities with respect to concentration and, especially, temperature. Viscosities of ternary mixtures [Mg(NO3)2 + HNO3 + H2O] were measured as a function of composition at constant ionic strengths ranging from (3.00 to 12.64) mol·kg–1 and were found to have approximately linear (pro-rata) dependences on composition. This enabled prediction of mixture viscosities with a modest level of accuracy (up to 9% but typically smaller than 5%) throughout the parametrization space without the need for mixing parameters.

Densities of aqueous solutions of nitric acid at concentrations from (0.025 to 36) mol·kg–1 (0.16–70% w/w) have been determined by high precision vibrating-tube densitometry. Measurements over the temperature range of 293.15 ≤ T/K ≤ 343.15 were made at 5 K intervals at atmospheric pressure with a commercial apparatus, using an improved measurement protocol. Special attention was given to the dilute concentration region to determine the (conventional) standard partial volume of the nitrate ion. Measurements at higher temperatures (323.15 ≤ T/K ≤ 473.15) were made at seven temperatures at a pressure of 2.0 MPa using a custom-built high-temperature densimeter. For these measurements, the nitric acid concentration was restricted to 15 mol·kg–1 (50% w/w) to minimize the risk of corrosion. The present results greatly expand density and volumetric databases for the aqueous solutions of this critical reagent.

Aqueous solutions of four heavy-metal nitrate salts (AgNO3, TlNO3, Cd(NO3)2 and Pb(NO3)2) have been studied at 25 °C using broadband dielectric relaxation spectroscopy (DRS) at frequencies 0.27 ≤ ν/GHz ≤ 115 over the approximate concentration range 0.2 ≲ c/mol L–1 ≲ 2.0 (0.08 ≲ c/mol L–1 ≲ 0.4 for the less-soluble TlNO3). The spectra for AgNO3, TlNO3, and Pb(NO3)2 were best described by assuming the presence of three relaxation processes. These consisted of one solute-related Debye mode centered at ∼2 GHz and two higher-frequency solvent-related modes: one an intense Cole–Cole mode centered at ∼18 GHz and the other a small-amplitude Debye mode at ∼500 GHz. These modes can be assigned, respectively, to the rotational diffusion of contact ion pairs (CIPs), the cooperative relaxation of solvent water molecules, and its preceding fast H-bond flip. For Cd(NO3)2 solutions an additional solute-related Debye mode of small-amplitude, centered at ∼0.5 GHz, was required to adequately fit the spectra. This mode was consistent with the presence of small amounts of solvent-shared ion pairs. Detailed analysis of the solvent modes indicated that all the cations are strongly solvated with, at infinite dilution, effective total hydration numbers (Zt0 values) of irrotationally bound water molecules of ∼5 for both Ag+ and Tl+, ∼10 for Pb2+, and ∼20 for Cd2+. These results clearly indicate the presence of a partial second hydration shell for Pb2+(aq) and an almost complete second shell for Cd2+(aq). However, the hydration numbers decline considerably with increasing solute concentration due to ion–ion interactions. Association constants for the formation of contact ion pairs indicated weak complexation that varies in the order: Tl+ < Ag+ < Pb2+ < Cd2+, consistent with the charge/radius ratios of the cations and their Gibbs energies of hydration. Where comparisons were possible the present constants mostly agreed well with the rather uncertain literature values.

Densities of aqueous solutions of MnSO4, CoSO4, NiSO4 and CuSO4 have been measured by vibrating tube densimetry up to near-saturation concentrations over the temperature range 293.15 <= T/K <= 343.15 at 5 K intervals and at 0.1 MPa pressure. Apparent molar volumes, V-Phi , calculated from the densities revealed close similarities among all four electrolytes, with their V-Phi values essentially differing by constant concentration-independent addends for each salt at each temperature. It was found that there is an almost linear rela-tionship between V-Phi(MSO4,aq) and the M-O bond length of the hydrated cations obtained from structural studies. This relationship can be used to predict values of V-Phi(MSO4,aq) for other sulfate salts over wide ranges of concentration and temperature, providing that the requisite structural data are available. Measurements of selected ternary, quaternary and quinary mixtures of these electrolytes at constant total concentrations showed that their V-Phi values almost always exhibit linear mixing (Young's rule) behavior at all temperatures investigated. This finding can be exploited to predict the volumetric properties of solutions containing complex mixtures of bivalent metal sulfates.

This article is the second of three projected IUPAC Technical Reports on reference materials for phase equilibrium studies. The goal of this project was to select reference systems with critically evaluated property values for the verification of instruments and techniques used in phase equilibrium studies of mixtures. This report proposes seven systems for solid–liquid equilibrium studies, covering the four most common categories of binary mixtures: aqueous systems with organic solutes, aqueous systems with inorganic solutes, non-aqueous systems, and systems with low solubility. For each system, the available literature sources, accepted data, smoothing equations, and estimated uncertainties are given.

Aqueous solutions of zinc sulfate, ZnSO4(aq), have been studied at 25°C by dielectric relaxation spectroscopy (DRS) over the frequency range 0.2≤ν/GHz≤89 at concentrations 0.04≲c/mol L−1≲2.4. The spectra obtained could be accounted for mostly by the presence of four Debye processes (a 4D model). These processes consisted of two solvent-related modes at higher frequencies plus two solute-related modes at lower frequencies. At low solute concentrations (c≤0.34molL−1) it was necessary to include an additional small amplitude mode which was thought to be due to ion-cloud relaxation (a (D) + 4D model). While the DR spectra for ZnSO4(aq) were broadly similar to those of other divalent metal sulfate solutions there were also differences. In particular, no evidence was found for the significant formation of double-solvent-separated ion pairs (2SIP). Further, the mode centered around 8 GHz, which for similar systems is usually assigned to the presence of contact ion pairs (CIPs), appears to be an unresolved mixture of contributions from CIPs and dynamically-retarded (‘slow’) water molecules. Analysis of the solvent-related modes indicated that the free ions (Zn2+(aq) and SO42−(aq)) and the ion pairs (SIPs and CIPs) were strongly hydrated. The decline in their effective hydration numbers (Z values) with increasing c is common and is thought to result from ion/IP interactions. The overall association constant for the ion pairs was found to be consistent with literature data.