Stability of Weakly Acidic Hypochlorous Acid Solution with Microbicidal Activity

Hypochlorous acid (HOCI) solution (200 ppm, pH 6) was prepared and evaluated for their stabilities and microbicidal activities. We demonstrated that HOCI is unstable against ultraviolet (UV) light, sunshine, contact with air, and elevated temperature (>= 25 degrees C). Furthermore, in the HOCI solution, the presence of excess NH2- or CHO-containing organic compounds such as proteins and carbohydrates, or of inorganic ions such as NO2-, SO3-, PO3-, Fe2+, Cu2+, and CuS, resulted in the rapid consumption of HOCI by oxidation reactions, and significantly decreased the microbicidal activity of the HOCI solution against coliform bacteria and total viable cell count. Thus, production of stable HOCI solution requires formulation in pure water harboring concentrations as low as possible of various compounds and ions, as well as storage in dark and cool conditions (<10 degrees C) to maintain the concentration of HOCI molecules and microbicidal activity.