What is the pKa of protonated hypochlorous acid?

The pKa of protonated hypochlorous acid, which refers to hypochlorous acid (HOCl) itself, is 7.53.

Understanding the Acidity of Hypochlorous Acid

Hypochlorous acid (HOCl) is a significant chemical species renowned for its powerful oxidizing and chlorinating properties, making it widely utilized as a disinfectant and in various industrial and biological processes. Its pKa value is a fundamental measure that indicates its acidic strength and how readily it dissociates to release a proton in solution.

The Exact pKa Value of HOCl

The pKa value is defined as the negative logarithm of the acid dissociation constant (Ka). This constant quantifies the extent to which an acid dissociates into its conjugate base and a proton. For hypochlorous acid, the pKa value is 7.53. This value is considered relatively high, classifying HOCl as a weak acid when compared to strong mineral acids such as hydrochloric acid (pKa ≈ -7) or sulfuric acid.

What "Protonated Hypochlorous Acid" Signifies

In the context of pKa, asking for the "pKa of protonated hypochlorous acid" refers to the dissociation of the uncharged, acidic form of the molecule. Hypochlorous acid (HOCl) is the protonated species that can donate a proton. Its pKa of 7.53 specifically describes the following equilibrium in aqueous solution:

HOCl (aq) ⇌ H⁺ (aq) + OCl⁻ (aq)

This equilibrium demonstrates that hypochlorous acid (HOCl) loses a proton (H⁺) to form its conjugate base, the hypochlorite ion (OCl⁻). The pKa value indicates that at a pH of 7.53, half of the HOCl molecules will have dissociated into H⁺ and OCl⁻. At pH values below 7.53, HOCl is the predominant species, while at pH values above 7.53, the hypochlorite ion (OCl⁻) becomes more prevalent.

Factors Influencing HOCl's Acidity

The acidity of HOCl stems from the polarization of the O-H bond, which is influenced by the electronegative oxygen and chlorine atoms. While the pKa of HOCl is significantly lower than that of water (pKa ≈ 15.7), indicating it is a much stronger acid than water, its value of 7.53 confirms its classification as a weak acid.

Table: Comparative Acid Strengths

Note: pKa values are approximate and can vary slightly with temperature and ionic strength.

Practical Implications and Applications

The pKa of hypochlorous acid is a critical parameter that dictates its effectiveness and behavior in various practical applications:

• Disinfection: HOCl is known to be a far more effective disinfectant than its conjugate base, the hypochlorite ion (OCl⁻), which is the active ingredient in common household bleach. Due to its pKa of 7.53, at the near-neutral pH levels often found in swimming pools (e.g., pH 7.2-7.8) or water treatment systems, a substantial portion of the free chlorine remains in the highly effective HOCl form, ensuring potent germicidal action.
• Water Treatment: In municipal water treatment, understanding HOCl's pKa allows operators to adjust pH levels to maximize the concentration of the more potent HOCl for optimal disinfection, while minimizing the formation of less effective or potentially harmful byproducts.
• Biological Activity: Hypochlorous acid is naturally produced by neutrophils, a type of white blood cell, as part of the human immune response. Its moderate acidity enables it to function effectively as an antimicrobial agent within physiological pH ranges without causing undue damage to host cells, making it a crucial component of innate immunity.
• Wound Care: HOCl solutions are increasingly used in wound care due to their broad-spectrum antimicrobial activity and biocompatibility, properties directly linked to the specific pKa that allows for stable and effective formulations.

By precisely controlling the pH of a solution containing chlorine, it is possible to manage the proportion of HOCl relative to OCl⁻, thereby optimizing its properties for specific applications, ranging from powerful sterilization to gentle wound disinfection.