Dissociation constant
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In chemistry and biochemistry the dissociation constant, abbreviated K_d, is a measure of the extent to which a dissociation reaction

AB ↔ A + B

proceeds at equilibrium.

It is given quantitatively by the expression

K_d = [A]×[B]×[AB]^-1

Where [A], [B], and [AB] indicate the concentrations of A, B, and AB, respectively.

See also hydrogen bond.

The dissociation constant is a specific type of equilibrium constant.

The dissociation constant of water (Ke) varies in function of the temperature. The value of pH for neutrality is not the same at different temperatures.

water	Ke	pKe
0°C	0.1e-14	14.92
10°C	0.3	14.52
18°C	0.7	14.16
25°C	1.2	13.92
30°C	1.8	13.75
50°C	8	13.10
60°C	12.6	12.90
70°C	21.2	12.67
80°C	35	12.46
90°C	53	12.28
100°C	73	12.14

The original article can be found at: http://en.wikipedia.org/wiki/Dissociation_constant.
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