Determination of the dissociation constants of the cephalosporins cefepime and cefpirome using UV spectrometry and pH potentiometry Vassilis Evagelou a , Anna Tsantili-Kakoulidou a, *, Michael Koupparis b a Department of Pharmaceutical Chemistry, School of Pharmacy, University of Athens, Panepistimiopolis, Zografou, Athens 15771, Greece b Laboratory of Analytical Chemistry, Department of Chemistry, University of Athens, Panepistimiopolis, Zografou, Athens 15771, Greece Received 2 August 2002; received in revised form 6 December 2002; accepted 6 December 2002 Abstract UV spectrometry and pH potentiometry were used for the determination and direct characterization of the dissociation constants of cefepime (Cef) and cefpirome. The absorbance/pH profiles at two analytical wavelengths and different conditions were assessed and found to conform to those of diprotic acids. The titration curves indicated a triprotic acid profile with two overlapping dissociation constants. The comparison of the results of both techniques permitted the direct attribution of the three dissociation constants to the carboxylic group at position 4 of the D-3 cephem nucleus, the aminothiazole group and the amide group at position 7 of the D-3 cephem nucleus. Stability studies of Cef in alkaline solutions were also performed in order to evaluate the accuracy of the measurements carried out for the determination of the third pK a value. The experimental pK a values were compared to the corresponding predicted values derived by PALLAS/PKALC and Advanced Chemical Development (ACD) software packages. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Cefepime; Cefpirome; Dissociation constants; UV Spectrometry; Potentiometry; pK a prediction 1. Introduction The important role of the degree of ionization in the biological behavior of chemical substances, as well as in their ability to passive transcellular diffusion and/or in their suitability as substrates for active transport is well established [1]. Several experimental approaches have been employed for the determination of the dissociation constants, including potentiometry, UV spectrometry, HPLC, phase solubility and recently capillary zone electrophoresis techniques [2 /5]. In addition, relevant software have been developed for the rapid estimation of pK a values based on the chemical structure [6,7], the reliability of which * Corresponding author. Tel.:  /30-2107274530; fax:  /30- 2107274747. E-mail address: tsantili@pharm.uoa.gr (A. Tsantili- Kakoulidou). Journal of Pharmaceutical and Biomedical Analysis 31 (2003) 1119  /1128 www.elsevier.com/locate/jpba 0731-7085/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0731-7085(02)00653-2