What Is the pKa of Water? The Definitive Answer

Water autoionizes, meaning two water molecules can react to form a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻). The equilibrium constant for that reaction is the ion product of water, K_w, which is 1.0 × 10⁻¹⁴ at 25 °C. The pK_a of water is defined from the acid dissociation constant of the hydronium ion, and the accepted value is –1.74. That number is a direct answer to what is the pKa of water, though the term can also refer to water acting as an acid, which gives a pK_a of 15.74. The value you need depends on whether you are treating water as the acid or as the conjugate acid of hydroxide.

Why There Are Two pKa Values for Water

Water is amphoteric — it can donate a proton (act as an acid) or accept a proton (act as a base). In the Brønsted–Lowry framework, every acid has a conjugate base, and every base has a conjugate acid. This duality produces two distinct pK_a numbers for water.

When water acts as an acid, it donates a proton to form hydroxide:

H₂O ⇌ H⁺ + OH⁻

The equilibrium constant for that reaction is K_a = [H⁺][OH⁻] / [H₂O]. Because [H₂O] is about 55.5 M in pure water and K_w = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, the K_a calculates to roughly 1.8 × 10⁻¹⁶. Taking the negative logarithm gives a pK_a of 15.74. This is the value you use when water is the acid.

When water acts as a base, it accepts a proton to form hydronium:

H₂O + H⁺ ⇌ H₃O⁺

Here, H₃O⁺ is the conjugate acid of water. The pK_a of H₃O⁺ is –1.74. That number comes from the equilibrium between hydronium and water in concentrated acid solutions. It is the value most often cited in textbooks and by organizations such as IUPAC when discussing the acidity of the hydronium ion.

Which Value Should You Use?

If you are working with water as a solvent and need its acidity constant for a calculation, the 15.74 value applies. If you are treating hydronium as an acid (for example, in a strong acid titration), use –1.74. The two numbers are not interchangeable; they describe different chemical species.

The Derivation of pKa = 15.74

The pK_a of water as an acid is derived directly from the autoionization constant. Pure water at 25 °C has a hydronium ion concentration of 1.0 × 10⁻⁷ M. The concentration of water itself is 55.5 M (1000 g/L divided by 18.015 g/mol). The acid dissociation constant is:

K_a = (1.0 × 10⁻⁷)(1.0 × 10⁻⁷) / 55.5 ≈ 1.8 × 10⁻¹⁶

The negative logarithm of 1.8 × 10⁻¹⁶ is 15.74. This value is consistent with the fact that water is a weaker acid than many common weak acids like acetic acid (pK_a 4.76). It also explains why hydroxide is a strong base — its conjugate acid (water) has a high pK_a.

The Derivation of pKa = –1.74

The pK_a of hydronium is a thermodynamic value that comes from the standard state of H₃O⁺ in aqueous solution. In pure water, the concentration of H₃O⁺ is very low, but the equilibrium constant for the reaction H₃O⁺ ⇌ H₂O + H⁺ is defined relative to the standard state of 1 M. The K_a for hydronium is about 55, which corresponds to a pK_a of –1.74.

This negative pK_a means hydronium is a very strong acid — stronger than nitric acid (pK_a –1.3) and comparable to perchloric acid. In practice, any acid stronger than H₃O⁺ will protonate water completely, which is why strong acids like HCl are considered fully dissociated in water. The PubChem entry for hydronium lists it as a strong acid with a pK_a near –1.7.

A Short Worked Example

Suppose you need to compare the acidity of two species in water. Acetic acid has pK_a 4.76, and water as an acid has pK_a 15.74. Since a lower pK_a means a stronger acid, acetic acid is much stronger than water. That is why acetate (the conjugate base of acetic acid) is a weaker base than hydroxide. Similarly, comparing hydronium (pK_a –1.74) to acetic acid shows that hydronium is far stronger, so acetic acid does not protonate water to any significant extent.

Common Misconceptions About Water’s pKa

A frequent error is to state that the pK_a of water is 14. That number comes from the pK_w (the negative log of K_w), not from any acid dissociation constant. pK_w is 14.0 at 25 °C, but it is not a pK_a. Another mistake is to treat water as having a single pK_a value without specifying whether it is the acid or the conjugate acid. If you see a table listing water’s pK_a as 15.74, it refers to the reaction H₂O → H⁺ + OH⁻. If you see –1.74, it refers to H₃O⁺ → H⁺ + H₂O.

Some sources also confuse the pK_a of water with the pH of pure water, which is 7.0 at 25 °C. pH is a measure of hydronium ion concentration, not acidity constant. Water is neutral (pH 7) even though its pK_a as an acid is 15.74.

Key Takeaways

• The pK_a of water depends on which reaction you are describing: 15.74 when water is the acid, and –1.74 when hydronium is the acid.
• The value 15.74 comes from the autoionization constant K_w and the concentration of water (55.5 M).
• The value –1.74 comes from the equilibrium constant for hydronium dissociation and indicates that H₃O⁺ is a very strong acid.
• The pK_w of water (14.0) is not a pK_a; it is the negative log of the ion product constant.
• Always specify the chemical species when citing a pK_a for water to avoid confusion.