Buffers and Ionic Salts: Densities and Solubilities of Aqueous and Electrolyte Solutions of Tris(hydroxymethyl)aminomethane and N-Tris[hydroxymethyl]-4-amino-butanesulfonic Acid

摘要

The densities of aqueous solutions of tris(hydroxymethyl)aminomethane (TRIS) and tris[hydroxymethyl]-4-amino-butanesulfonic acid (TABS), useful biological buffers within the pH range of 7.0 to 9.0 for TRIS and 8.2 to 9.5 for TABS, have been measured by a high-precision vibrating-tube digital densitometer in aqueous and in aqueous electrolyte solutions from (298.15 to 328.15) K under atmospheric pressure. This study was undertaken to investigate the interactions between these compounds and electrolytes of potassium acetate (KAc), potassium bromide (KBr), potassium chloride (KCl), and sodium chloride (NaCl). In this series of measurements, the aqueous samples were prepared with various concentrations of the buffers, up to saturated conditions, and over salt concentrations from (1 to 4) mol·dm~(-3). The experimental densities were correlated as a function of the concentration of the buffers and ionic salts. The solubilities of buffers at 298.15 K in aqueous and in aqueous electrolyte solutions have also been determined from the experimental results of density measurements. It was found that the solubilities of TRIS and TABS in aqueous solution decrease with increasing concentration of salts (salting-out effect). The solubility data were further used to estimate the apparent free energy of transfer (AG!,) of buffers from water to aqueous electrolyte solutions. The contribution of TABS residue (-CH2CH2CH2CH2SO_3~-) from water to aqueous electrolyte solutions was predicted from the ΔG_(tr)' results. The measured densities served to evaluate the apparent molar volumes, V_φ(m, T), and fitted them to an equation that describes the surface (V_φ against T against m). The apparent molar volumes of buffers at infinite dilution (V_φ~o) have been determined from the solubility data. The trends of transfer volumes (Δ_(tr)V_φ~o) have been interpreted in terms of solute-cosolute interactions on the basis of a cosphere overlap model.