Lubomir Hnedkovsky

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.

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.

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.

Heat capacities for single-phase mixtures of the natural gas components methane (1), propane (2) and n-heptane (3) have been determined at temperatures (197 to 421) K and pressures up to 32 MPa using two differential scanning calorimeters with a combined standard uncertainty of (2.0 to 2.6) % (k = 1). In addition, measurements were performed at pressures (0.01 to 4.40) MPa higher than saturation conditions to estimate the heat capacities of the mixtures at their bubble points. The ternary mixture data were compared with three models: the Groupe Européen de Recherches Gazières (GERG) 2008 multi-parameter equation of state (EOS), the Peng-Robinson (PR) EOS, and the Statistical Associating Fluid Theory (SAFT)-γ Mie EOS incorporating group contributions. The relative deviations of the measured heat capacities from the values calculated by the three models show similar, systematic dependences on density, with larger deviations at cryogenic temperatures. The root mean square deviations from the measurements (with ideal gas heat capacity corrected) were 8.2%, 7.0% and 5.9% for the GERG-2008, PR and SAFT-γ Mie EOS, respectively. The presence of n-heptane increased the deviation up to 20% at the lowest temperature. The addition of methane to the binary mixture [0.500 C3H8 + 0.500 C7H16] was found to always increase the heat capacity. This work shows that the SAFT-γ Mie EOS can describe the single-phase measurements at above-ambient temperatures, while none of the models provides reliable predictions for cryogenic single-phase and near-bubble-point measurements.

Densities of aqueous solutions of rubidium triflate (RbTf) and cesium triflate (CsTf), where Tf– is the trifluoromethanesulfonate ion (CF3SO3–), have been measured by vibrating-tube densimetry at temperatures from 293.15 to 343.15 K at 5 K intervals. Concentrations ranged from 0.03 to 5.0 mol·kg–1 for RbTf and from 0.02 to 3.9 mol·kg–1 for CsTf. No volumetric data for either salt appear to have been published previously. Apparent molar volumes (Vϕ) calculated from the densities were well modeled with Pitzer equations. Standard partial molar volumes, Vo, for RbTf(aq) and CsTf(aq) were determined by extrapolation of the Pitzer fits to infinite dilution. Isobaric coefficients of thermal expansion (expansivities), α, derived from the temperature dependence of the densities, increased with concentration, especially at lower T, consistent with the solvent-structure-breaking character of their component ions. Combination of the present Vo values with a common extra-thermodynamic assumption and relevant literature data provided ionic volumes, Vo(Rb+,aq) and Vo(Cs+,aq), that show more realistic temperature dependences than previous estimates.

Densities of fifteen aqueous solutions of sulfuric acid (H2SO4) have been measured by vibrating-tube densimetry at solute molalities (m) from (0.01 to 3.0) mol·kg−1 over the temperature range 293.15 ≤ T/K ≤ 343.15. These data have been used to calculate the corresponding apparent molar volumes Vϕ(H2SO4,aq), which represent a significant expansion of the volumetric database for this industrially-important acid. At 298.15 K the present results agree well with literature data, notably with the century-old values given in the 1926 International Critical Tables. At other temperatures, where comparisons are possible agreement with the present Vϕ values is also very satisfactory. Consistent with earlier studies, Vϕ(H2SO4,aq) was found to exhibit an abnormally-large decrease at low concentrations (m ≤ 0.1 mol·kg−1). This effect is consistent with a change in the chemical speciation of H2SO4(aq), from an essentially 1:1 electrolyte (H+(aq) + HSO4− (aq)) at higher concentrations to a predominantly 1:2 electrolyte (2H+(aq) + SO42− (aq)) in dilute solutions. The Vϕ values were modelled using variants of Young’s rule and the Pitzer formalism. Combination of these results with literature values for the standard volume V°(SO42−,aq) enabled estimation of V°(HSO4−,aq) and the standard volume change, ΔrV°, for the first protonation of the sulfate ion (H+(aq) + SO42−(aq) → HSO4−(aq)) as functions of temperature. It is shown that V°(HSO4−,aq) is sensitive to the value of the first protonation constant and probably cannot be determined to better than ± 0.3 cm3·mol−1 at present.

Densities of up to 16 aqueous solutions of zinc sulfate have been measured at 5 K intervals over the temperature range of 293.15 ≤ T/K ≤ 353.15 at concentrations 0.004 ≤ m/mol·kg–1 ≤ 2.5 and 0.1 MPa pressure using a commercial glass vibrating tube densimeter (vtd). Particular attention was paid to establishing the concentrations and pH values of the solutions. Densities of the same solutions were also measured at 343.15 and 373.15 K at 0.3 MPa pressure in a purpose-built high-temperature Pt/Rh-vtd. The two sets of densities at 343.15 K were in excellent agreement, with an average difference of 0.01%. Apparent molar volumes, Vϕ, calculated from the densities were fitted with an extended Redlich–Rosenfeld–Meyer equation. However, the standard (infinite dilution) molar volumes, Vo(ZnSO4, aq), derived via this equation differed significantly (by up to 2 cm3·mol–1) from the values obtained by ionic additivity using literature data. This difference is probably mostly due to ion-pairing effects.