Comment on the “Thermodynamic Dissociation Constant of the Bisulfate Ion from Raman and Ion Interaction Modeling Studies of Aqueous Sulfuric Acid at Low Temperatures”

Comment on the “Thermodynamic Dissociation Constant of the Bisulfate Ion from Raman and Ion Interaction Modeling Studies of Aqueous Sulfuric Acid at Low Temperatures

Improvement of the Zdanovskii−Stokes−Robinson Model for Mixtures Containing Solutes of Different Charge Types

The Zdanovskii−Stokes−Robinson (ZSR) relationship [Stokes and Robinson (J. Phys. Chem. 1966, 70, 2126−2130)] enables the solvent content of a liquid mixture to be estimated, for a specified solvent activity, from data for pure solutions of each of the individual solutes. There is an analogous relationship for the activity coefficients of the solutes. The method has been shown to be exact, in the limit of extreme dilution, only for mixtures containing either all uncharged (neutral) solutes or electrolytes all of the same charge type, and in practice it is found to be most accurate for such mixtures. Here we derive an addition to the ZSR equations which removes this limitation by incorporating simple Debye−Hückel terms into the equations for solvent mass and solute activity coefficients. This addition, in its simplest form, does not involve any new fitted parameters or require any further thermodynamic information. The relationship is general, and not limited to particular Debye−Hückel expressions. Application of the revised model to activity and osmotic coefficient data for the system NaCl−Na2SO4−H2O at 298.15 K shows that errors are reduced, compared to predictions of the standard model, by up to a factor of 2. Solubilities of NaCl(cr), Na2SO4·10H2O(cr), and Na2SO4(cr) in that system are similarly better predicted. Activity coefficients of uncharged solutes in salt solutions calculated using the revised model are now largely consistent with the empirically observed Setchenow relationship

Ion Interaction Models and Measurements of Eu³⁺ Complexation: DTPA in Aqueous Solutions at 25 °C Containing 1:1 Na⁺ Salts and Malonate pH Buffer

The separation of lanthanides from actinides in the TALSPEAK liquid–liquid distribution process is accomplished using an aminopolycarboxylate complexing agent, for example diethylenetriamine-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-pentaacetic acid (DTPA, CAS Reg. No. 67-43-6), in a low pH buffered aqueous phase in contact with an organic phase containing an extractant such as di­(2-ethylhexyl)­phosphoric acid (HDEHP, CAS Reg. No. 298-07-7). Literature measurements show that the partitioning of lanthanides to the organic phase falls with rising pH whereas thermodynamic equilibrium models suggest that, at pH above approximately 3.5, the partitioning should increase. In this study, the partitioning of Eu³⁺ between an aqueous phase (with NaNO₃ background electrolyte, malonate buffer, and DTPA complexing agent), and an organic phase (HDEHP in <i>n</i>-dodecane) is measured from pH 2 to 4.5 and for ionic strengths from 0.25 to 1.0 mol kg^–1. The measurements include systems with reduced (by 10×) concentrations of buffer, DTPA, and Eu³⁺. A Pitzer activity coefficient model of the aqueous mixture is developed based upon available osmotic and activity coefficient data, and stoichiometric equilibrium constants in different 1:1 electrolyte media over a range of ionic strengths. This enables the DTPA and buffer speciation, and complexation of Eu³⁺ by both DTPA and malonate, to be calculated for different solution compositions and pH. The measured distribution coefficients are consistent with model predictions up to pH 3.5 and, below this pH, vary little with ionic strength. At higher pH, the distribution coefficients at different ionic strengths deviate both from the model and each other, consistent with other reactions occurring in the organic phase than the simple exchange of lanthanide and H⁺ embodied in the TALSPEAK phase transfer reaction

Ion Interaction Models and Measurements of Eu³⁺ Complexation: HEDTA in Aqueous Solutions at 25 °C Containing 1:1 Na⁺ Salts and Citrate pH Buffer

In the TALSPEAK liquid–liquid distribution process, dissolved lanthanides can be separated from actinides using a complexing agent such as <i>N</i>-(2-hydroxyethyl)­ethylenediamine-<i>N</i>,<i>N</i>′,<i>N</i>′-triacetic acid (HEDTA, CAS Reg. No. 150-39-0) in a low pH buffered aqueous phase in contact with an organic phase containing a suitable extractant. This study focuses on the chemical speciation of HEDTA, citrate pH buffer, and Eu³⁺ in aqueous solutions of 1:1 Na⁺ salts (mainly NaNO₃) as a function of ionic strength and pH. New measurements of stoichiometric protonation constants of HEDTA, and the HEDTA complex of Eu³⁺, in aqueous NaNO₃ are reported for ionic strengths from 0.5 to 4.0 M at 25 °C. A Pitzer activity coefficient model of the aqueous mixture has been developed based upon these measurements, available osmotic and activity coefficient data, and stoichiometric equilibrium constants in different 1:1 electrolyte media over a range of ionic strengths. This enables the HEDTA and buffer speciation, and complexation of Eu³⁺ by both HEDTA and citrate, to be calculated for different solution compositions and pH values. The model of the citrate buffer, which is based on an extensive range of data for NaCl and NaNO₃ media, should also be useful in other practical applications

Water Activities and Osmotic Coefficients of Aqueous Solutions of Five Alkylaminium Sulfates and Their Mixtures with H₂SO₄ at 25^oC

<div><p>Alkylaminium sulfates are frequently detected in ambient aerosols, and are believed to be important in the nucleation of new particles in the atmosphere, despite the comparatively low gas phase concentrations of amines. In this study, water activities and osmotic coefficients have been measured, using a chilled mirror dew point technique, of aqueous mixtures of sulfuric acid and the following alkylaminium sulfates: methylaminium, ethylaminium, dimethylaminium, diethylaminium, and trimethylaminium sulf-ate. The samples were prepared by mixing solutions of the five corresponding amines and aqueous sulfuric acid and determining the exact aminium to sulfate molar ratios by ion chromatography. The results were correlated using an extended Zdanovskii–Stokes–Robinson equation to enable concentration/water activity rela-tionships to be calculated over the entire composition range from pure aqueous sulfuric acid to pure aqueous aminium sulfate. Water activities and osmotic coefficients for aminium:sulfate ratios of 1:1 (the bisulfate salts) and lower showed great similarity with ammonium bisulfate, but osmotic coefficients for the 2:1 ratio (the aminium sulfates) were significantly larger (and water activities lower) than for ammonium sulfate. These results differ from those obtained in Clegg et al.'s (2013) study. The relative values of the osmotic coefficients, in concentrated solutions, suggest that the numbers of methyl or ethyl groups in the aminium ion may have a stronger lowering effect on water activity than the alkyl chain length.</p><p>Copyright 2015 American Association for Aerosol Research</p></div

Studies of Single Aerosol Particles Containing Malonic Acid, Glutaric Acid, and Their Mixtures with Sodium Chloride. I. Hygroscopic Growth

We describe a newly constructed electrodynamic balance with which to measure the relative mass of single aerosol particles at varying relative humidity. Measurements of changing mass with respect to the relative humidity allow mass (m) growth factors (maqueous/mdry) and diameter (d) growth factors (daqueous/ddry) of the aerosol to be determined. Four aerosol types were investigated: malonic acid, glutaric acid, mixtures of malonic acid and sodium chloride, and mixtures of glutaric acid and sodium chloride. The mass growth factors of the malonic acid and glutaric acid aqueous phase aerosols, at 85% relative humidity, were 2.11 ± 0.08 and 1.73 ± 0.19, respectively. The mass growth factors of the mixed organic/inorganic aerosols are dependent upon the molar fraction of the individual components. Results are compared with previous laboratory determinations and theoretical predictions