Study of solution equilibria between iron(III) ion and ciprofloxacin in pure nitrate ionic medium and micellar medium

Abstract

Prototropic equilibria (protonation, hydrolytic and complexation) in iron(III) ion+ciprofloxacin (Hcipx) solutions were studied in 0.1 and 0.5 mol dm −3 (Na)NO 3 ionic media in the absence and in the presence of ionic surfactants: sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB). Experiments were carried out at 298±0.2 K using glass electrode potentiometric and UV–Vis spectrophotometric measurements. The concentration of SDS in Fe III–ciprofloxacin solutions was 10 mmol dm −3 while that of CTAB was 8 mmol dm −3. The hydrolysis of the iron(III) ion was studied separately within the concentration range of iron 1–5 mmol dm −3 and pH 1.6–3.0. Non-linear least-squares treatment of the data indicates that in both ionic media studied the main hydrolytic products were Fe(OH) 2 + and Fe 2(OH) 2 4+. The effect of the concentration of the nitrate ion on the stability constants of these complexes is much more pronounced than the effect of the addition of either SDS or CTAB. The successive protonation constants of ciprofloxacin anion, cipx −, relating to the formation of Hcipx and H 2cipx + species, were influenced to a significant extent by the presence of SDS and, to a considerably lesser extent, by the presence of CTAB in both studied concentrations of NO 3 − ionic media. The complexation between iron(III) and ciprofloxacin was studied in the concentration range of iron(III) 0.15–0.58 mmol dm −3, with ligand to metal concentration ratios ranging from 3:1 to 10:1 in the pH interval 2.0–6.0. The experimental data obtained could be explained by the formation of mononuclear complexes: Fe(cipx) 2 +, Fe(cipx) 2+ and Fe(OH)cipx. In the presence of either SDS or CTAB the same complexes were formed; however, much higher formation constants were obtained with SDS, while the effect of CTAB was sluggish. The structure and mechanism of formation of iron(III)–ciprofloxacin complexes in pure ionic and in micellar media are discussed.