_______________ available chlorine is very effective in killing bacteria.

• A. Chlorinated
• B. Total
• C. Combined
• D. Free

Answer: (D)

Disinfection strength comes from the form of chlorine that’s available to react with microbes. Free available chlorine consists of the active species, mainly hypochlorous acid and hypochlorite ion, that are not bound to ammonia or organic compounds. These forms readily penetrate bacterial cell walls and disrupt vital enzymes and proteins, making them very effective at killing bacteria. When chlorine combines with ammonia or organics, it forms chloramines, which are weaker, slower-acting disinfectants, though they can still provide some residual. Total chlorine is simply the sum of free and combined forms. Because the active, unbound chlorine does the immediate work of sanitizing, free available chlorine is the form described as very effective for killing bacteria. The effectiveness is also influenced by pH, with HOCl being more prevalent (and potent) at lower pH.