1774 · Chlorine is isolated
Carl Wilhelm Scheele produced and described the gas later named chlorine. Work on chlorine and water chemistry laid the foundations for understanding its reaction products.
1834 · HOCl is prepared
French chemist Antoine Jérôme Balard is credited with preparing hypochlorous acid in laboratory chemistry. HOCl is formed when chlorine dissolves in water and reacts with it.
19th century · Chlorine chemistry enters sanitation
Chlorine-based methods became part of water disinfection and public-health practice. In water, chlorine exists in a pH-dependent balance between hypochlorous acid and hypochlorite ion.
20th century · The immune role becomes clear
Research established that myeloperoxidase in activated neutrophils uses hydrogen peroxide and chloride to generate HOCl as part of the innate immune response.
Today · A broad research field
Studies examine HOCl in microbiology, water treatment, medicine, food systems, agriculture and materials. Results depend on the formulation and test conditions; evidence from one context cannot automatically be transferred to another.
Further reading: A dermatology review notes Balard’s 1834 synthesis, and the National Academies’ water-chemistry text describes HOCl formation when chlorine reacts with water. Read the review · Read the National Academies chapter.