Editing Dissociation constant (section)

== Acid–base reactions ==
{{Acids and bases}}
{{Main|Acid dissociation constant}}
For the [[deprotonation]] of [[acid]]s, ''K'' is known as ''K''<sub>a</sub>, the  [[acid dissociation constant]].  Strong acids, such as [[sulfuric acid|sulfuric]] or [[phosphoric acid]], have large dissociation constants; weak acids, such as [[acetic acid]], have small dissociation constants.

The symbol ''K''<sub>a</sub>, used for the acid dissociation constant, can lead to confusion with the [[association constant]], and it may be necessary to see the reaction or the equilibrium expression to know which is meant.

Acid dissociation constants are sometimes expressed by p''K''<sub>a</sub>, which is defined by

:<math chem="">
 \text{p}K_\text{a} = -\log_{10}{K_\mathrm{a}}
</math>

This <math chem>\mathrm{p}K</math> notation is seen in other contexts as well; it is mainly used for [[covalent]] dissociations (i.e., reactions in which chemical bonds are made or broken) since such dissociation constants can vary greatly.

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'' [[acid#Polyprotic acids|acids]].  The first (e.g., acetic acid or [[ammonium]]) have only one dissociable group, the second (e.g., [[carbonic acid]], [[bicarbonate]], [[glycine]]) have two dissociable groups and the third (e.g., phosphoric acid) have three dissociable groups. In the case of multiple p''K'' values they are designated by indices: p''K''<sub>1</sub>, p''K''<sub>2</sub>, p''K''<sub>3</sub> and so on. For amino acids, the p''K''<sub>1</sub> constant refers to its [[carboxyl]] (–COOH) group, p''K''<sub>2</sub> refers to its [[amino]] (–NH<sub>2</sub>) group and the p''K''<sub>3</sub> is the p''K'' value of its [[side chain]].

:<math chem="">\begin{align}
      \ce{H3 B} &\ce{{} <=> {H+} + {H2 B^-}}   & K_1 &= \ce{[H+] . [H2 B^-] \over [H3 B]}      & \mathrm{p}K_1 &= -\log K_1 \\
    \ce{H2 B^-} &\ce{{} <=> {H+} + {H B^{2-}}} & K_2 &= \ce{[H+] . [H B ^{2-}] \over [H2 B^-]} & \mathrm{p}K_2 &= -\log K_2 \\
  \ce{H B^{-2}} &\ce{{} <=> {H+} + {B^{3-}}}   & K_3 &= \ce{[H+] . [B^{3-}] \over [H B^{2-}]}  & \mathrm{p}K_3 &= -\log K_3 
\end{align}</math>