Liquid chlorine, commonly used for sanitizing swimming pools and water systems, is a solution of sodium hypochlorite. This solution is highly alkaline, which is necessary for its long-term stability. However, this high alkalinity presents a challenge when the solution is used for disinfection. Understanding the pH of this chemical is fundamental for ensuring both safety and effective sanitation.
Defining Liquid Chlorine and Its pH Value
Commercial liquid chlorine is a concentrated aqueous solution of sodium hypochlorite (NaOCl). It is produced by combining chlorine gas with a sodium hydroxide solution. The strength varies, typically ranging from 10% to 12.5% available chlorine for pool use. The pH of concentrated sodium hypochlorite solutions is high, generally falling between 11 and 13, classifying it as strongly alkaline. Maintaining this high pH is required for the product’s shelf life, as it prevents the hypochlorite from rapidly decomposing.
Why Liquid Chlorine is Highly Alkaline
The high pH of liquid chlorine results from its chemical makeup and behavior in water. When sodium hypochlorite (NaOCl) dissolves, it dissociates into sodium ions (Na+) and hypochlorite ions (OCl-). The hypochlorite ion (OCl-) then reacts with water molecules in a process called hydrolysis. This reaction forms hypochlorous acid (HOCl) and releases hydroxide ions (OH-). The presence of these excess hydroxide ions is the direct chemical cause of the high alkalinity and the resulting high pH value.
The Impact of High pH on Sanitizing Power
Chlorine’s effectiveness depends on the ratio of two forms in water: hypochlorous acid (HOCl) and the hypochlorite ion (OCl-). HOCl is the superior, fast-acting disinfectant, up to 100 times more effective than OCl-. The equilibrium between these two species is determined entirely by the water’s pH. When the pH is high, the equilibrium shifts toward the less effective OCl- form; for instance, at a pH of 8.0, about 80% exists as OCl-. Adding highly alkaline liquid chlorine raises the water’s pH, reducing immediate sanitizing power. For optimal disinfection, water systems aim for a pH range of 7.2 to 7.8, where a higher percentage of potent HOCl is present.
Practical Steps for pH Management in Water Systems
Because liquid chlorine introduces high alkalinity, users must regularly manage the water system’s pH to maintain disinfection efficacy. This requires consistent testing, aiming for a pH range of 7.2 to 7.6 for optimal chlorine performance. If the pH rises above this target, it must be reduced using acidic compounds like muriatic acid (hydrochloric acid) or sodium bisulfate. Muriatic acid is a highly effective liquid but requires careful handling, while sodium bisulfate is a safer, granular alternative. These acidic additives neutralize the excess alkalinity, shifting the HOCl/OCl- balance back toward the more effective hypochlorous acid.