What is Role of pH in Chlorination Effectiveness?

Chlorine Species and pH

In water chlorination, pH plays a significant role in determining the concentration of available chlorine species. The dissociation of hypochlorous acid (HOCl) into hypochlorite ion (OCl-) is directly influenced by pH. At a pH of 7 (neutral), 99% of the available chlorine is in the form of HOCl, while at a pH of 9 (alkaline), only 1% remains as HOCl. This decrease in HOCl concentration significantly reduces the water’s ability to inactivate pathogens.

Optimal pH Ranges for Chlorination

The optimal pH range for effective chlorine disinfection is between 6.5 and 8.0. Within this range, the concentration of HOCl remains high, and the water’s ability to inactivate pathogens is maximized. At pH levels below 6.5, the concentration of HOCl decreases, reducing the effectiveness of chlorine disinfection. Similarly, at pH levels above 8.0, the concentration of OCl- increases, further reducing the effectiveness of chlorine. As a general guideline, water chlorination systems should aim to maintain a pH level between 7.0 and 7.5 for optimal chlorine effectiveness.

pH Adjustment Techniques

To maintain the optimal pH range for effective chlorine disinfection, water treatment systems can implement pH adjustment techniques. These techniques may include the addition of acidic or alkaline substances, such as sulfuric acid or sodium hydroxide, to adjust the pH level of the water. Additionally, water treatment systems can employ pH-stabilizing agents, such as buffers, to maintain a stable pH level within the optimal range. By controlling the pH level of the water, water treatment systems can ensure effective chlorine disinfection and maximize the inactivation of pathogens.