Original scientific paper

Acidity at a Liquid-Liquid Interface

Kim-Hung Chow ; Chemical Dynamics Laboratory, Department of Chemistry, University of Minnesota, 207 Pleasant Street, S.E., Minneapolis, MN 55455
Maurice M. Kreevoy ; Chemical Dynamics Laboratory, Department of Chemistry, University of Minnesota, 207 Pleasant Street, S.E., Minneapolis, MN 55455

Abstract

By using porous polypropylene as a support, we have measured the contact angle of buffered aqueous droplets on surfaces which were over half bis(2-ethylhexyl)hydrogen phosphate, HA. From the variation of the contact angle with the pH of the aqueous solution the degree of dissociation, a, of HA has been evaluated as a function of pH. By defining the standard states of the interfacial HA and its conjugate base at the 50% ionized aqueous- organic interface, a pKR is obtained for the interfacial acid, equal to the pH of the aqueous solution which is in equilibrium with the 50% ionized interfacial acid.
Following Katchalsky and Spitnik14 the ratio of the activity coefficient of the conjugate base yA- to the activity coefficient of the acid, yHA is equated to [a/(l - afn l> ] for a values near 0.5. This permits the evaluation of the empirical parameter, n, and leads to a relation between pH and a.
pH = pKa-n log[(1 -a)/a]
This relation is found to hold from a > 0.1 to a < 0.9, although it cannot be valid for either a = 0.0 or a = 1.0.

Keywords

Hrčak ID:

136768

URI

https://hrcak.srce.hr/136768

Publication date:

1.12.1994.

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