ISSN:
1572-8927
Keywords:
Ions
;
solvent mixtures
;
Gibbs energies of transfer
;
acetic acid-water mixtures
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract The photo-absorbing, basic sensor, 4-nitroaniline, has been used to determine theequilibrium constant for solvent reorganization around the proton in mixtures ofvarying composition of water with acetic acid. In all the mixtures used, theself-ionization of the acetic acid was suppressed. In contrast to mixtures of waterwith the related ethanol or acetone, this equilibrium is shifted more toward thewater-solvated species as the mole fraction x 2 of the cosolvent increases. TheGibbs energy of transfer of protons from water into the mixture ΔG o t (H+) can bederived with the aid of this equilibrium constant for the solvent reorganization.Using ΔG o t (H+), ΔG o t (i) for i denoting anions and other cations can be evaluated.In comparison the ΔG o t (i) for cations have lower negative values than when eitherethanol or acetone is added to water. Correspondingly, for halide anions, thepositive ΔG o t (i) with added acetic acid are rather less than is found with eitherethanol or acetone added. The influence on the ion-solvent interaction of bothelectron withdrawing hydroxy and carbonyl groups in acetic acid may beresponsible for this. Although ΔG o t (i) for C10− 4 and Re0− 4 are also positive, both picrateions and OH− give negative values with acetic acid added to water. With picrateions, the hydrophobic effect of the carbon ring produces stabilization in themixture relative to water. With OH−, complete conversion to acetate anionsoccurs. As is found with other cosolvents, the contribution of the charge onacetate anion to ΔG o t (CH3COO−) is found to increase as x 2 rises. The aciddissociation constant K a for acetic acid is found to decrease slowly as x 2 rises to0.5, followed by a rapid decrease for x 2 greater than 0.7 where dimerization ofacetic acid occurs.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1005102517584
Permalink
