Heavy water
The physical properties of water and heavy water differ in several respects. Heavy water is less dissociated than light water at given temperature, and the true concentration of D+ ions is less than ions would be for a light water sample at the same temperature. The same is true of OD− vs. ions. For heavy water Kw D2O (25.0 °C) = 1.35 × 10−15, and [D+] must equal [OD−] for neutral water. Thus pKw D2O = p[OD−] + p[D+] = 7.44 + 7.44 = 14.87 (25.0 °C), and the p[D+] of neutral heavy water at 25.0 °C is 7.44.