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# Water - Ionization Constant, pKw, of normal and heavy water

## Ionization constant (= dissociation constant = self-ionization constant = ion product = autoprotolysis constant ) of water and heavy water, given as function of temperature (°C and °F) in figures and tables

Autoprotolysis describes the reaction of water with itself:

2 H2O ↔ H3O+ + OH¯          [1]

and an equlibrium constant can be defined as

Keq = a(H3O+) * a(OH-) / a(H2O)2           [2]

where a is the chemical activity of the water ions.

Because most acid–base solutions are typically very dilute, the activity of water is generally approximated as being equal to unity. In dilute aqueous solutions, the activities of the solute particles are approximately equal to their concentrations.

Then, and an autoprotolysis constant or water ionization constant can be defined as

Kw = [H3O+] [OH¯]            [3]

where [H3O+] and [OH¯] represents the concentration of H3O+ and OH- ions in water.

Often, a general value of 1.00 x 10¯14 is used. However, pKw varies with the temperature as shown in the figures and tables below.

From the chemical equation [1] above, it can be seen that H3O+ and OH¯ concentrations are in the molar ratio of one-to-one. This means that [H3O+] = [OH¯]. Therefore the values of [H3O+] and [OH¯] can be determined by taking the square root of Kw.

Heavy water, D2O, self-ionizes less than normal water, which gives larger pKw values for D2O compared to H2O.

See Water and Heavy Water  for thermodynamic properties at standard condtions.
See also other properties of Water at varying temperature and pressure: Boiling points at high pressure, Boiling points at vacuum pressure, Density and specific weightDynamic and kinematic viscosity, Enthalpy and entropy, Heat of vaporization, Melting points at high pressure, Prandtl numberProperties at Gas-Liquid Equilibrium Conditions, Saturation pressure, Specific gravity, Specific heat (heat capacity), Specific volume, Thermal conductivity, Thermal diffusivity and Vapour pressure at gas-liquid equilibrium.

 Temperature Ionization constant, pKw [°C] [°F] H2O D2O 0 32 14.947 15.972 5 41 14.734 15.743 10 50 14.534 15.527 15 59 14.344 15.324 20 68 14.165 15.132 25 77 13.995 14.951 30 86 13.833 14.779 35 95 13.680 14.616 40 104 13.535 14.462 45 113 13.396 14.316 50 122 13.265 14.176 55 131 13.140 14.044 60 140 13.020 13.918 65 149 12.907 13.798 70 158 12.799 13.683 75 167 12.696 13.574 80 176 12.598 13.470 85 185 12.505 13.371 90 194 12.417 13.276 95 203 12.332 13.186 100 212 12.252 13.120 150 302 11.64 200 393 11.31 250 482 11.20 300 572 11.34 350 662 11.92

 Temperature Ionization constant, pKw [°F] [°C] H2O D2O 32 0.0 14.947 15.972 40 4.4 14.758 15.768 50 10.0 14.534 15.528 60 15.6 14.324 15.302 70 21.1 14.126 15.090 80 26.7 13.940 14.891 90 32.2 13.764 14.705 100 37.8 13.598 14.529 110 43.3 13.442 14.364 120 48.9 13.293 14.208 130 54.4 13.153 14.060 140 60.0 13.020 13.918 150 65.6 12.895 13.785 160 71.1 12.776 13.658 170 76.7 12.663 13.537 180 82.2 12.556 13.423 190 87.8 12.455 13.317 200 93.3 12.360 13.219 210 98.9 12.270 13.13 212 100.0 12.252 13.12 250 121.1 11.97 300 148.9 11.65 400 204.4 11.29 500 260.0 11.20 600 315.6 11.49 650 343.3 11.84

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## Citation

• Engineering ToolBox, (2017). Water - Ionization Constant, pKw, of normal and heavy water. [online] Available at: https://www.engineeringtoolbox.com/ionization-dissociation-autoprotolysis-constant-pKw-water-heavy-deuterium-oxide-d_2004.html [Accessed Day Mo. Year].

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