Ozone is a gas at ambient (22°C) and refrigerated (4 to 7°C) temperatures. This gas is colorless with a pungent odor readily detectable at concentrations as low as 0.02 to 0.05 ppm (v/v). Important properties of ozone are summarized in Table 18.1. Ozone is partially soluble in water (12.9 mg/l ozone can be dissolved in the water when 3% ozone gas flows through water at 20°C), and solubility can be affected by partial pressure, flow rate of ozone, temperature, purity of water, and contact time . Ozone has the unique property of autodecomposition, producing numerous free radical species, the most prominent being the hydroxyl free radical. Ozone reacts with organic and inorganic compounds in aqueous solutions either directly with whole molecular ozone or by its radicals . Ozone is a powerful oxidant with an oxidation potential of —2.07 V. The half-life of molecular ozone in air is relatively long (~12 hours), but in aqueous solutions the half-life can be as short as seconds when organic compounds exist. Decomposition of ozone is so rapid in water that its antimicrobial properties take place mainly at the microbial surface . The PEL of ozone in the workplace is specified by the Occupational Safety and Health Administration (OSHA) at 0.1 ppm in air on an 8 hours/day basis, for a 40-hour work week.
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