Chlorine and bromine are widely used as disinfectants, primarily for water sanitation in swimming pools, spas, and municipal systems. While they serve the identical function of keeping water safe by neutralizing harmful microorganisms, they are distinct chemical elements with practical differences in how they perform this job. Understanding their relationship requires looking at their fundamental chemistry and how each reacts within a water environment. This article will clarify the unique properties and behaviors of these two common sanitizers.
Chemical Foundations of Halogens
Chlorine (Cl) and bromine (Br) are separate elements on the Periodic Table, but they belong to the same column, Group 17, known as the halogens. This shared classification means they exhibit similar chemical reactivity; all halogens possess seven electrons in their outermost shell, making them highly reactive oxidizing agents that readily seek to gain one electron to achieve stability. Chlorine (atomic number 17) is the lighter element, while bromine (atomic number 35) is heavier, which influences their physical states and reactivity.
At room temperature, chlorine exists as a greenish-yellow gas, whereas bromine is a reddish-brown liquid that easily vaporizes into a dense fume. Despite bromine being heavier, chlorine is generally considered the more reactive of the two halogens. These differences in atomic structure and weight lead to the practical distinctions observed in water treatment.
The Mechanism of Disinfection
Both chlorine and bromine disinfect water through oxidation, attacking and destroying pathogens like bacteria and viruses. When added to water, both form an active, sanitizing acid: chlorine forms hypochlorous acid (HOCl), and bromine forms hypobromous acid (HOBr). These active molecules are collectively referred to as the “free available” disinfectant, which is the concentration measured to determine water safety.
The hypohalous acids are strong oxidizers that destroy microorganisms by penetrating the cell wall and interfering with the internal enzymes and cellular structures necessary for the microbe to function. This fundamental killing mechanism is the same for both halogens, as their effectiveness depends on the quantity of this free available form present in the water system.
Stability and Environmental Factors
A major difference arises from how chlorine and bromine react to environmental conditions, particularly the water’s pH level. Hypochlorous acid, the active form of chlorine, loses its sanitizing power significantly as the pH rises above 7.5; at a pH of 8.5, chlorine retains only about 10% of its potent form. Conversely, hypobromous acid remains much more effective across a wider range of alkalinity, with nearly 60% of its active form still present at a pH of 8.5. This high pH tolerance is an advantage for bromine in systems where maintaining a neutral pH is difficult.
Another divergence is their reaction with nitrogen-containing contaminants, such as those found in sweat and urine, which create combined halogen compounds. Chlorine reacts with these compounds to form chloramines, which are often gaseous and evaporate, leading to the irritating “chlorine smell” and reducing active sanitizer. Bromine reacts to form bromamines, which, unlike chloramines, are stable and remain in the water as effective disinfectants. Bromamines are also less volatile and do not gas off, resulting in less odor and irritation.
Chlorine is highly susceptible to degradation from ultraviolet (UV) light, such as direct sunlight, which rapidly breaks down the active hypochlorous acid. In outdoor pools, this requires the use of a stabilizing agent, like cyanuric acid, to shield the chlorine from UV radiation. Bromine is also degraded by UV light, but its common use in indoor environments, hot tubs, and spas—where UV exposure is minimal—often makes stabilization less of a concern.
Common Applications and Suitability
The practical differences in stability and byproducts lead to distinct preferred applications for each halogen. Chlorine is typically the preferred choice for large outdoor swimming pools and municipal water treatment because of its lower cost and its ability to be stabilized against UV degradation with cyanuric acid. It is also fast-acting, making it suitable for pools with high bather loads requiring quick disinfection.
Bromine is best suited for high-temperature, low-volume systems like spas and hot tubs. Its effectiveness is maintained at higher temperatures and across a broader, more alkaline pH range, which are common characteristics of hot tub water. The fact that bromamines remain active disinfectants and produce less irritating odor also makes bromine a more comfortable choice for indoor pools and enclosed spas.