Stability constants and thermodynamic parameters of cadmium complexes with sulfonamides and cephapirin

Stability constant (log ) and thermodynamic parameters of Cd complexes with sulfonamide and cephapirin were determined by Polarographic technique at pH = 7.30 ± 0.01 and μ = 1.0 M KNO3 at 250°C. The sulfonamides were sulfadiazine, sulfisoxazole, sulfamethaxazole, sulfamethazine, sulfathiazole, sulfacetamide and sulfanilamide used as primary ligands and cephapirin as secondary ligand. Cd formed 1:1:1, 1:2:1 and 1:1:2 complexes. The nature of electrode processes were reversible and diffusion controlled. The stability constants and thermodynamic parameters ( G, H and S) were determined. The formation of the metal complexes has been found to be spontaneous, exothermic in nature, and entropically unfavourable at higher temperature. Keyword: polarography; stability constant; sulfonamide; cephapirin; thermodynamics. pH measurements of the analytes were made on a Elico pH meter (LI – 10) using glass and calomel electrodes and fixed at 7.30 ± 0.01 which was adjusted with dilute solutions of HNO3 or NaOH as required. Electrochemical Analysis was performed using a Polarographic Analyzer (Elico, Hyderabad Model CL 362). The Polarographic capillary was 5.0 cm. long with diameter 0.06 mm with dropping mercury electrode (DME) characteristics m2/3t1/6 = 2.04 mg2/3s-1/2. All the analytes were deaerated by pure nitrogen gas before recording the current voltage data. Potassium dihydrogen phosphate – sodium hydroxide buffer was added with the analyte to stabilize its pH. Results and discussion A well defined two electron [13] reversible reduction and diffusion controlled wave Cd2+ was observed in 1.0 M KNO3 at pH = 7.30 to 8.50 [14], but pH = 7.30 was selected to study the complex formation in human blood pH. The value of E1/2 for Cd was 586 mV vs SCE. The nature of current voltage curve of Cd2+ complexes with sulfonamide and cephapirin was also reversible and diffusion controlled. Stability constant of [Cd – sulfonamide – cephapirin] complexes In this system, the concentration of Cd2+, KNO3 and gelatin were 0.50 mM, 1.0 M and 0.001% respectively. Neither cephapirin nor sulfonamide gave their current voltage curves in 1.0 M KNO3 at pH = 7.30 ± 0.01 at 25 0C. When [Cd2+] was added with either of the drugs, complex formation was taken place and their current voltage curves were obtained. The concentration of sulfonamide in the analyte varied from 0.50 mM to 30.0 mM at 0.025 M to 0.05 M constant concentration of cephapirin. The half wave potential E1/2 values become more negative with the addition of cephapirin to the binary complex [Cd – sulfonamide] confirmed the [Cd – sulfonamide – cephapirin] complex formation. The stability constant of ternary complexes were determined by using Schaap and McMaster [15] 30 Ecl. Quím., São Paulo, 33(1): 29-34, 2008 Figure 1. [Cd sulfadiazine cephapirin] complex. method which confirmed the formation of 1:1:1, 1:2:1 and 1:1:2 metal ligand complexes. The values of stability constant of complexes were given in (Table 1). The data and plots between Fij [X, Y] vs [X] for [Cd – sulfadiazine – cephapirin] complex {where X and Y are sulfonamide and cephapirin and i and j are the stoichiometric numbers for primary and secondary ligands respectively} were given in (Table 2) and (Fig. 1) respectively. The polarograms of [Cd sulfadiazine – cephapirin] at [cephapirin] = 0.025 M were given in (Fig. 2). It is clear from the polarograms that E1/2 values of [Cd – sulfadiazine – cephapirin] increased with increased of the concentration of cephapirin confirmed the ternary complex formation. These ligands offered bonding to metal ion through the sulfonamido nitrogen atom and sulfonyl oxygen atom of SO2 group [16, 17]. In case of cephapirin, N of the – lactam ring and O of the carboxylic group might take part in bond formation with Cd making 5 membered ring [18].