Temperature Dependence Of Ionization Research Articles

Thermodynamic properties of aqueous dimethylamine and dimethylammonium chloride at temperatures from 283 K to 523 K : Apparent molar volumes, heat capacities, and temperature dependence of ionization

Data for the apparent molar volumes of aqueous dimethylamine and dimethylammonium chloride have been determined with platinum vibrating tube densimeters at temperatures 283.15 K ≤ T ≤ 523.15 K and at different pressures. Apparent molar heat capacities were measured with a Picker flow microcalorimeter over the temperature range 283.15 K ≤T ≤ 343.15 K at 1 bar. At high temperatures and steam saturation pressures, the standard partial molar volumes $$V_2^ \circ $$ of dimethylamine and dimethylammonium chloride deviate towards positive and negative discontinuities at the critical temperature and pressure, as is typical for many neutral and ionic species. The revised Helgeson-Kirkham-Flowers (HKF) model and fitting equations based on the appropriate derivatives of solvent density have been used to represent the temperature and pressure dependence of the standard partial molar properties. The standard partial molar heat capacities of dimethylamine ionization $$\Delta _{{\text{ion}}} C_{{\text{p,2}}}^ \circ $$ , calculated from both models, are consistent with literature data obtained by calorimetric measurements at T ≤ 398 K to within experimental error. At temperatures below 523 K, the standard partial molar volumes of dimethylamine ionization $$\Delta _{{\text{ion }}} V_2^ \circ $$ agree with those of morpholine to within 12 cm3-mol-1, suggesting that the ionization of secondary amine groups in each molecule is very similar. The extrapolated value for $$\Delta _{{\text{ion }}} V_2^ \circ $$ of dimethylamine above 523 K is very different from the values measured for morpholine at higher temperature. The difference is undoubtedly due to the lower critical temperature and pressure of (CH3)2NH(aq).

Thermodynamic Properties of Aqueous Morpholine and Morpholinium Chloride at Temperatures from 10 to 300 °C: Apparent Molar Volumes, Heat Capacities, and Temperature Dependence of Ionization

Apparent molar heat capacities Cp,φ of aqueous morpholine and morpholinium chloride were determined with a Picker flow microcalorimeter at temperatures from 10 to 55 °C. The apparent molar volumes Vφ have been determined with platinum vibrating tube densitometers at temperatures from 10 to 300 °C and pressures in excess of steam saturation. Values of Vφ for morpholine approach large positive values at elevated temperatures, consistent with a lowering of the critical temperature in the solutions relative to water. The effect in aqueous morpholinium chloride is reversed, confirming the profound effect of ionic charge on the high-temperature thermodynamic properties of aqueous solutes, even for large organic molecules. Standard partial molar heat capacity functions were estimated from the high-temperature Vφ data and low-temperature values of Cp,φ using an empirical model based on the appropriate solvent density derivatives and the revised Helgeson−Kirkham−Flowers model. The results from both models are consistent with literature values for the heat capacity of ionization determined from high-temperature potentiometric measurements to within the combined experimental uncertainties. The results show that the effects of solvent expansion by the neutral species is significant at elevated temperatures. The effective Born radius of ions containing organic groups could be significantly larger than the radius calculated from the formula for simple cations because of this effect.

Thermodynamics of aqueous carbon dioxide and sulfur dioxide: heat capacities, volumes, and the temperature dependence of ionization

A flow microcalorimeter and vibrating tube densimeter were used at 25 °C to obtain apparent molar heat capacities and volumes of aqueous NaHCO3, KHCO3, NaHSO3, and KHSO3, from 0.1 to 1.0 mol kg−1, aqueous CO2 from 0.01 to 0.10 mol kg−1 and aqueous SO2 from 0.045 to 2.0 mol kg−1. The contribution of "chemical relaxation" (changes in equilibrium state and enthalpy due to change in temperature) to the experimental heat capacities of aqueous SO2 required special attention, leading to the derivation of a new equation for calculating this effect. Standard state values for the heat capacities and volumes of aqueous CO2, SO2, HCO3−, and HSO3− were obtained from the apparent molar properties by extrapolation to infinite dilution. Combining these results with other thermodynamic data from the literature gave estimates of log K1b the equilibrium constant for the first neutralization of CO2 and SO2, at high temperatures. The results for CO2 reproduce very accurate literature values to within 0.2 at 200 °C. The expression for the reaction [Formula: see text] log K1b = 22.771 + 2776.0/T–8.058 log T, is consistent with the sparse and limited experimental data.