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PHYSICAL SIGNIFICANCE OF ACTIVITY COEFFICIENTS OF INDIVIDUAL IONS

Journal/Book: Reprinted from RECUEIL DES TRAVAUX CHIMIQUES DES PAYS-BAS edited by the Koninklijke Nederlandse Chemische Vereniging (The Hague Holland) T. 86 - No. 10 - October 1967 N.V. UITGEVERIJ W. P. VAN STOCKUM & ZOON - 'S-GRAVENHAGE. 1967;

Abstract: The interaction of ions of organic acids and bases with the solvent and the influence of substitution by alkyl and phenyl groups* Laboratory for Analytical Chemistry ** State University of Utrecht The Netherlands * This work has been taken from the thesis of M. Alfenaar Utrecht (1966). ** Address: Croesestraat 77a Utrecht. A study has been made of the influence of substitution by alkyl and phenyl groups in organic acids and bases an the interaction of the ions of these substances with the solvent. Knowledge of single ion activity coefficients is essential for the interpretation of the different effects. 1. Introduction For the ionization equilibrium of a weak acid HA: HA ? H+ + A- in a solvent S it holds that: K = aHaA/aHA = a*Ha*A/a*HA·°H°A/°HA = K*·°H°A/°HA (1) or: pK* - pK - log °H = log °A - log °HA (1a) in which: a = activity referred to the solvent water (a = 1 for a -hypothetical - ideal 1 molal solution in water) a* = activity referred to the solvent S (a* = 1 for a - hypothetical - ideal 1 molal solution in S) K = ionization constant referred to the solvent water (numerically equal to the ionization constant in water) K* = ionization constant referred to the solvent S ° = activity coefficient accounting for the transfer from water to the solvent S (RTIn° is the chemical potential difference associated with this transfer). RTln° for an ion is composed of an "electrical" part and a "neutral" part (see ref. 1 § 2.2.). The latter is closely related to the reversible work needed for the transfer of an uncharged particle of the same physical properties (size polarizability) as the ion RTln°neutr. For very large ions it holds that: log ° = log °el + log °neutr (2) in which log °el represents the electrostatic interaction of the ionic charge with the solvent. For moderately large ions (2) will be a reasonable approximation. As the particles A- and HA have nearly identical dimensions and polarizabilities log °HA ˜ log °neutr. A so that: pK* - pK - log °H ˜ log °A - log °neutr.A ˜ log °el.A (3) ... ___MH


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