pKw

pKw, short for the negative base-10 logarithm of the ionic product of water, is a term used in chemistry to describe the autoprotolysis of water. Kw is defined as the product of the activities of hydrogen ions and hydroxide ions in water: Kw = a(H+) a(OH−). In dilute aqueous solutions, activities are well approximated by concentrations, giving Kw ≈ [H+][OH−]. The quantity pKw is then pKw = −log10(Kw).

At 25°C in pure water, Kw ≈ 1.0 × 10−14 and pKw ≈ 14.0. Because [H+] = [OH−] in neutral water, pH = pOH = 7 and pKw = pH + pOH.

pKw is temperature dependent. Kw increases with rising temperature, which means pKw decreases as temperature increases. As a result, the neutral pH of pure water shifts below 7 at higher temperatures.

In practice, Kw and pKw depend on temperature and, at higher ionic strength, on activity coefficients, so the tabulated 14.0 is a reference for pure water at 25°C and infinite dilution. The relation pH + pOH = pKw is a convenient way to convert between acidic and basic measures in water chemistry.

Applications of pKw include calculating pH and pOH, assessing acidity in environmental systems, and informing water-treatment and biological processes. The concept underpins many pH-based calculations in chemistry and environmental science.