Talanta 66 (2005) 653–658 Multiwavelength spectrophotometric determination of protolytic constants of 4-(2-pyridylazo) resorcinol (PAR) in binary DMF–water mixtures A. Rouhollahi a , F.M. Kiaie a , J. Ghasemi b,∗ a ChemistryDepartment,FacultyofSciences,K.N.ToosiUniversityofTechnology,Tehran,Iran b ChemistryDepartment,FacultyofSciences,RaziUniversity,Kermanshah,Iran Received 5 June 2004; received in revised form 8 September 2004; accepted 1 December 2004 Available online 6 January 2005 Abstract A multiwavelength spectrophotometric titration method was applied to study the protolytic constants of 4-(2-pyridylazo) resorcinol(PAR), in binary DMF + water mixtures. UV–vis absorption spectra of PAR solution were recorded in the course of pH-metric titration of acidic solutions of PAR with standard base solution. The protolytic equilibrium constants, spectral profiles, concentration diagrams and also the number of components have been calculated from the fitting of the pH-spectral titration data with appropriate mass balance equations by a home written program according to an established target factor analysis. To precise determination of number of absorptive components a recently developed statistical indicator function (IND function) was used. A glass electrode calibration procedure based on a four-parameter equation pH = α + Sp c H + J H + [H + ] + J OH - K w /[H + ] based on the Gran’s plots was used to obtain pH readings in the concentration scale (p c H). It has been observed that there is an inverse relationship between second and third protolytic constants and mole fraction of DMF. The effect of the solvent on the protolytic constants was discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: 4-(2-Pyridylazo)resorcinol; PAR; Acidity constants; Multiwavelength spectrophotometry; DMF + water mixture; Target factor analysis; IND function 1. Introduction Acid dissociation constants are useful physico-chemical properties describing the extent of ionization of functional groups with respect to pH. These parameters are important in research areas such as acid–base titration, solvent extrac- tion, complex formation, and ion transport. It has been shown that the acid–base properties affect the toxicity [1], chromato- graphic retention behavior, and pharmaceutical properties of organic acids and bases. Much of the theoretical foundation of modern organic chemistry is based on the observation of the effects on acid–base equilibrium of changing molecular structure [2,3]. It has been shown that the spectrophotometric methods are suitable for determining protolytic constants of compounds ∗ Corresponding author. Tel.: +98 831 835 6077; fax: +98 831 836 9572. E-mailaddress: jahan.ghasemi@tataa.com (J. Ghasemi). which are contains chromophore(s) and ionizable center(s) such that the protonated and deprotonated forms [4]. The spectroscopic instrumentation used today, however, almost invariably has the capacity to collect data in a full spectral range. Using a single or a few wavelengths discards most of the information in the collected spectra and requires both the presence of and knowledge of such suitable wave- lengths. However, in many cases, the spectral responses of components overlap and analysis is no longer straightforward [5,6]. The predefined model, known as hard-modeling anal- ysis, cannot be applied if crucial information is missing. Soft modeling or model free approaches are based on much more general prerequisites, such as positive molar absorbance, pos- itive concentration of all species, unimodality of concentra- tion profiles, and closure (concentration of all species are the same for all solutions). Naturally, if the strengths of hard- and soft-modeling methodologies are combined, a much more powerful method of data analysis can be expected [7–9]. The 0039-9140/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2004.12.001