Dissociation constant
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In chemistry and biochemistry, a dissociation constant or an ionization constant is a specific type of equilibrium constant used for reversible reactions or processes. That means that it refers to the extent to which a complex, molecule, or salt separates or splits into smaller molecules, ions, or radicals in a reversible manner. The dissociation constant is represented by the symbol Kd.
Given the reaction
- AxBy <=> xA + yB
It is given by the expression
<math>K_d = \frac{[A]^x \times [B]^y}{[A_x B_y]}</math>
Where [A], [B], and [AxBy] indicate the concentrations of A, B, and AxBy, respectively.
pK
The pK value is defined as:
- <math>\!\, pK = -\log{K_d}</math>
where Kd is the dissociation constant.
As a frequently used special case, the dissociation constant of water is often expressed as Kw:
<math>K_w = {[H^+] \cdot [OH^-] \over [H_2O]}</math>
The value of Kw varies with temperature, as shown in the table below. This variation must be taken into account when making precise measurements of quantities such as pH.
| Water temperature | Kd/10-14 | pKd |
|---|---|---|
| 0°C | 0.1 | 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 | 8.0 | 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 |
Acid base reactions
For the deprotonation of acids, K is known as Ka, the acid dissociation constant. Stronger acids, for example sulfuric or phosphoric acid, have larger dissociation constants; weaker acids, like acetic acid, have smaller dissociation constants. A molecule can have several acid dissociation constants. In this regard, that is depending on the number of the protons they can give up, we define monoprotic, diprotic and triprotic acids. The first (e.g. acetic acid or ammonium) have only one dissociable group, the second (carbonic acid, bicarbonate, glycine) have two dissociable groups and the third (e.g. phosphoric acid) have three dissociable groups. In the case of multiple pK values they are designated by indices: pK1, pK2, pK3 and so on. For amino acids, the pK1 constant refers to its carboxyl (-COOH) group, pK2 refers to its amino (-NH3) group and the pK3 is the pK value of its side chain.
<math>H_3 B \Longleftrightarrow\ H ^ + + H_2 B ^ - \qquad K_1 = {[H ^ +] \cdot [H_2 B ^ -] \over [H_3 B]} \qquad pK_1 = - log K_1 </math>
<math>H_2 B ^ - \Longleftrightarrow\ H ^ + + H B ^ {-2} \qquad K_2 = {[H ^ +] \cdot [H B ^{-2} ] \over [H_2 B^ -]} \qquad pK_2 = - log K_2 </math>
<math>H B ^{-2} \Longleftrightarrow\ H ^ + + B ^{-3} \qquad K_3 = {[H ^ +] \cdot [ B ^ {-3}] \over [H B ^ {-2} ]} \qquad pK_3 = - log K_3 </math>
See also
- Acid dissociation constant
- Base dissociation constant
- Receptor (biochemistry)
- Dissociation (chemistry)
- Equilibrium ConstantTemplate:Chem-stub
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