Comparison of free and bound iodine and iodide species as a function of the dilution of three commercial povidone–iodine formulations and their microbicidal activity

Equilibrium dialysis on povidone–iodine-solutions (Braunol®, standardized Betadine® and non-standardized iso-Betadine®) reveal that the amount of available iodine, free iodine, iodide and triiodide varies significantly both in the undiluted and diluted forms. These differences are reflected in the different bactericidal activity against Staphyloccus aureus as determined by the standard quantitative in vitro suspension test. The amount of available iodine is not an appropriate measure for an assessment of the microbicidal activity. For this, the free iodine has to be determined by means of equilibrium dialysis. The free iodine concentration in the Braunol® concentrate was found to be 22 mg/L, in the standardized Betadine® 9.7 mg/L and in the non-standardized Betadine® concentrate only 2.1 mg/L. Because of the atypical behaviour of iodophores and the increase of free iodine at dilution and because of a bactericidal level of free iodine of 5 mg/L, Braunol® and standardized Betadine® can be employed as disinfectant as such, iso-Betadine® has to be diluted before use. Summarizing all results, it can be stated that Braunol® is superior to standardized Betadine and unstandardized iso-Betadine® both as to the release of free iodine in the undiluted and in the diluted forms as in the killing rate of S. aureus.