Hypochlorous acid (HOCl) is not the same as bleach, although both are chlorine-based disinfectants. Household bleach is primarily composed of sodium hypochlorite (NaOCl), a distinct chemical compound. The confusion arises because HOCl is the active, germ-killing component produced when bleach is mixed with water. This article clarifies the fundamental differences in chemical identity, stability, pH, and practical application.
Defining Hypochlorous Acid and Sodium Hypochlorite
Hypochlorous acid (HOCl) and sodium hypochlorite (NaOCl) are chemically distinct molecules, though both are chlorine oxidizers. HOCl is a weak acid with a simple molecular structure (one hydrogen, one oxygen, and one chlorine atom) that makes it a highly effective disinfectant. This molecule is naturally produced by human immune cells as part of the body’s defense against pathogens.
Common household bleach is a solution of sodium hypochlorite (NaOCl), which is a salt or base formed when chlorine reacts with sodium hydroxide. While both are potent oxidizers, HOCl is a neutral molecule while NaOCl is an ionic compound. The high alkalinity of sodium hypochlorite gives bleach its traditional harsh and corrosive properties.
Differences in Stability and pH
The primary difference between these two chlorine species is their stability in water and the pH at which they exist. Sodium hypochlorite solutions, the main ingredient in bleach, are highly alkaline with a typical pH ranging from 11 to 13. This high pH stabilizes the hypochlorite ion (\(ClO^-\)), which is the dominant species in alkaline conditions, giving bleach its long shelf life.
In contrast, hypochlorous acid is most effective and stable at a near-neutral or slightly acidic pH, generally between 5 and 7. This narrow pH range ensures the chlorine exists predominantly as the neutral HOCl molecule. HOCl is up to 100 times more effective at penetrating cell walls than the negatively charged hypochlorite ion.
Maintaining the optimal pH for HOCl is challenging, making these solutions inherently unstable. They often have a short shelf life due to their tendency to break down quickly when exposed to light, heat, or air. The high alkalinity of sodium hypochlorite is responsible for its corrosive nature, while HOCl is non-irritating and gentler on biological tissues because it is stable near the body’s natural pH.
How Bleach Produces Hypochlorous Acid
The confusion between the two compounds stems from the chemical equilibrium that occurs when sodium hypochlorite is dissolved in water. When NaOCl is added to water, a dynamic, pH-dependent reaction generates both the hypochlorite ion (\(ClO^-\)) and hypochlorous acid (HOCl). A bleach solution thus contains both species, with the ratio determined by the solution’s pH.
Highly concentrated household bleach solutions have a strongly alkaline pH (above 11), meaning they contain a large majority of the less effective hypochlorite ion. Only when the bleach solution is significantly diluted and buffered to a lower, more neutral pH does the chemical equilibrium shift. This shift favors the production of the more potent HOCl molecule, which is the ultimate source of bleach’s disinfectant power.
Divergent Applications and Safety Profiles
The differences in pH and stability lead to completely separate application profiles for the two products. Sodium hypochlorite (bleach) is utilized for its aggressive oxidizing power, long shelf life, and low cost. Its primary uses are in high-level disinfection, large-scale water treatment, industrial sanitation, and strong household cleaning. Users must handle concentrated bleach with caution, ensuring proper ventilation and avoiding contact with skin to prevent corrosive damage.
Hypochlorous acid is valued for its superior germ-killing power at low concentrations and its exceptional safety profile. It is commonly used in gentle, medical-grade applications such as wound care, eye washes, and dermatology because it is non-toxic and non-irritating to human tissue. HOCl is also used for food surface sanitation and as a mild household sanitizer, as its rapid breakdown leaves no harmful residue.