All of the following are used to disinfect water EXCEPT?

Fluorine is not commonly used for disinfecting water. While chlorine, bromine, and iodine are well-established disinfectants used in various drinking water treatment processes, fluorine does not serve this purpose. Chlorine is widely utilized due to its effectiveness in killing bacteria and viruses and providing residual disinfection. Bromine is also used, particularly in certain pool applications and some water treatment situations, offering a less common alternative to chlorine.

Iodine, similarly, has been employed in smaller-scale applications, especially in emergency situations or for portable water purification. It is recognized for its disinfectant properties and can effectively kill pathogens.

Fluorine, on the other hand, is primarily associated with water fluoridation to prevent dental cavities and is not effective as a disinfectant. The chemical properties of fluorine do not support its use in microbial inactivation, which is the primary goal of water disinfection. Thus, fluorine stands out as the correct answer to this question, as it does not align with the function of water disinfection like the other listed substances do.