Hydrogen ion is bacteriostatic at pH ~3–6 and bactericidal at pH <3. Strong mineral acids (HCl, H2SO4, etc) in concentrations of 0.1–1 N have been used as disinfectants; however, their corrosive action limits their usefulness. Un-ionized weak organic acids can readily penetrate and disrupt bacterial cell membranes. Acids are used as food preservatives (eg, benzoic acid), antiseptics (eg, boric acid, acetic acid), fungicides (eg, salicylic acid, benzoic acid), spermatocides (eg, acetic acid, lactic acid), and cauterizing agents (strong mineral acids).
Acetic acid, 1%, can be used in surgical dressings. At 5%, it is bactericidal to many bacteria and has been used to treat otitis externa produced by Pseudomonas, Candida, Malassezia, or Aspergillus spp. Skin and hides that have been contaminated by anthrax spores can be disinfected with 2.5% HCl.
Hydroxyl ion also exerts antimicrobial activity. At a pH >9, it inhibits most bacteria and many viruses. Hydroxides of sodium and calcium are used as disinfectants. Their irritant or caustic property usually precludes their application on tissues.
A 2% solution of soda lye (contains 94% sodium hydroxide [NaOH]) in hot water is used as a disinfectant against many common pathogens, such as those causing fowl cholera and pullorum disease. It is a potent caustic and must be handled with care.
Calcium oxide (CaO), ie, lime (hydrated or air-slaked lime), soaked in water produces Ca(OH)2. Aqueous suspensions of slaked lime are used to disinfect premises.