Ozone reacts with chlorine in water primarily when the chlorine is present as hypochlorite ion (OCl−), leading to the formation of chloride ions (Cl−) and some chlorate ions (ClO−3).
Here's a breakdown of the reaction:
Reaction Details
When ozone (O3) is introduced to water containing chlorine, the following occurs:
- Reactant Preference: Ozone preferentially reacts with the hypochlorite ion (OCl−), which is the dissociated form of hypochlorous acid (HOCl), rather than with HOCl itself. This is due to the higher reactivity of OCl−.
- Rate Constant: The reaction between ozone and hypochlorite ion proceeds with a second-order rate constant of approximately 120 ± 15 M−1 s−1 at 20°C. This signifies how quickly the reaction occurs.
- Product Distribution: The reaction results in approximately 77% of the chlorine being reduced to chloride ions (Cl−), while about 23% is oxidized to chlorate ions (ClO−3).
Chemical Equations
While the specific intermediate steps can be complex, the overall reactions can be summarized as follows:
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Primary Reaction (major): O3 + OCl− → Cl− + Products (e.g., oxygen)
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Secondary Reaction (minor): O3 + OCl− → ClO3− + Products (e.g., oxygen)
Factors Influencing the Reaction
- pH: The pH of the water significantly impacts the equilibrium between HOCl and OCl−. Higher pH values favor the formation of OCl−, thus accelerating the reaction with ozone. Lower pH favors HOCl and reduces the reaction rate.
- Temperature: As with most chemical reactions, higher temperatures generally increase the reaction rate.
- Chlorine Concentration: The concentration of chlorine, and specifically OCl−, will affect the rate of the reaction.
- Ozone Concentration: Higher ozone concentrations will drive the reaction forward.
Implications
- Water Treatment: Ozone is sometimes used in water treatment to remove chlorine and its disinfection by-products. The reaction with chlorine effectively neutralizes the chlorine. However, it's crucial to control the process to minimize the formation of chlorate, which is a regulated contaminant in drinking water.
In summary, ozone's reaction with chlorine in water is a complex process primarily involving the hypochlorite ion, leading to the formation of chloride and chlorate ions, with the reaction rate influenced by factors such as pH and temperature.
