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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 ↔ H3 O + + OH¯          [1]

and an equlibrium constant can be defined as

K eq = a(H3 O + ) * 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

K w = [H3 O + ] [OH¯]            [3]

where [H3 O + ] and [OH¯] represents the concentration of H3 O + and OH - ions in water.

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

From the chemical equation [1] above, it can be seen that H3 O + and OH¯ concentrations are in the molar ratio of one-to-one. This means that [H3 O + ] = [OH¯]. Therefore the values of [H3 O + ] and [OH¯] can be determined by taking the square root of K w .

Heavy water, D2O, self-ionizes less than normal water, which gives larger pK w 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 weight , Dynamic and kinematic viscosity , Enthalpy and entropy , Heat of vaporization , Melting points at high pressure , Prandtl number , Properties 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 .

Water - Ionization Constant - pK w - of Normal and Heavy Water
Temperature Ionization constant, pK w
[°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

Water - Ionization Constant - pK w - of Normal and Heavy Water
Temperature Ionization constant, pK w
[°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

• The 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 Month Year].

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7.8.15

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