REACTIVE & FUNCTIONAL POLYMERS ELSEVIER Reactive & Functional Polymers 36 (1998) 217-220 Thermodynamics of proton dissociation of acridinium ion in aqueous solution Maria P. Ros *, Jesus Thomas, Guillermo Crovetto, Juan Llor Departamento de Quimica-Fhica, Universidad de Granada, 18071 Granada, Spain Received 15 December 1996; revised version received 20 January 1997; accepted 6 May 1997 Abstract Standard thermodynamic function changes (AGO”, AHe’, A&” and Acre”) are reported for proton dissociation of acridinium cation in aqueous solution. These values have been determined from a series of precise and accurate acid equilibrium constants, K,. The equilibrium constants have been obtained spectrophotometrically over a range of temperatures from 2 to 66°C at several ionic strengths (I = 0.005, 0.007, 0.01, 0.05 and 0.25 M). Although the ionic strength does not affect in general the Gibbs free-energy values, the results suggest a possible influence on entropy and enthalpy changes. 0 1998 Elsevier Science B.V. All rights reserved. Keywords: Acridine; Thermodynamic functions; Proton dissociation 1. Introduction The current interest of the acridine and its derivatives in pharmaceutical use [l], as labels in biological assays [2] and DNA probe-based assays [3] makes determination of ionization constants and its corresponding thermodynamic properties useful. The dissociation equilibrium constant of acri- dinium ion (K,) at 20°C and ionic strength 0.01 M has been reported by Albert and Serjeant [4]. In fact, that we found nothing in the literature studies either about the effect of the temperature and ionic strength or about the standard thermo- dynamic function changes in this process proves that there is, in general, little reliable information * Corresponding author. Fax: +34 58 272879. available about the thermodynamic parameters of the dissociation process. For this reason, most discussions of the relative acid strengths have been based only on p K data. Because the magnitude of a pK value is de- termined by the relative magnitudes of AH” and AS” values, a knowledge of these quantities be- comes important for an understanding of acid dissociation. In the same way, relatively little work [5-71 has been done on the effect of the ionic strength. In this article a systematic study of the in- fluence of the ionic strength on the parameters of the dissociation reaction has been carried out. The pK, values of acridinium ion in aqueous solutions have been determined as a function of ionic strength (ranging from 0.005 to 0.25 M) and temperature (2 to 66°C). From these values, the standard thermodynamic function changes of 1381-5148/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PIZ Sl381-5148(97)00084-9