Temperature dependence of neutral pH

Definition

As water warms, self-ionization increases, Kw rises, and the neutral pH drops below 7 even though the water remains neutral. Neutral always means hydrogen and hydroxide are equal, not pH 7.

Splitting water costs energy, but higher temperature makes that cost easier to pay, so the ionized state becomes less thermodynamically expensive relative to neutral water. Both ion concentrations rise together, and because hydrogen is larger the pH falls. Anchor points include neutral pH near 7.47 at 0 degrees Celsius, 7.00 at 25, 6.63 at 50, and about 6.14 at 100 degrees Celsius.

Related concepts

Neutral pH · Self-ionization constant of water · Temperature compensation

---

Part of The Blueprint of Water · 🔥 thermodynamics