Free Available Chlorine

Definition - What does Free Available Chlorine mean?

Free available chlorine is the part of the total chlorine measurement that has not yet reacted with contaminants. Therefore, it is called available or free.

The total chlorine in this case is the sum of combined available chlorine and free available chlorine, also called total residual chlorine.

Corrosionpedia explains Free Available Chlorine

Chlorine is usually added to water in different forms such as:

  • Chlorine gas
  • Sodium hypochlorite or liquid bleach
  • Lithium hypochlorite
  • Stabilizers

When any of these chlorine forms are added to water, a highly potent bactericide known as hypochlorous acid is created. This is a type of weak acid that can be dissociated into hypochlorite ions.

Hypochlorous acid and hypochlorite ions, when combined, is known as free chlorine. Both of these establish equilibrium and are both temperature and pH dependent at 7.5 pH and 77°F (25°C). Hypochlorous acid is more potent than hypochlorite ions in terms of disinfection. Through proper control of pH, hypochlorous acid remains dominant in the solution of free available chlorine to ensure its effectiveness as a bactericide.

Free available chlorine readily reacts with compounds such as ammonia to produce chloramines, or industrially known as combined chlorine. Although chloramines also have antibacterial properties, these are about 80 times less effective than free chlorine.

In the formation of free chlorine and chloramines, optimum temperature, pH as well as mixing conditions should be taken into consideration.

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