Table of Contents
Water does not only act as a passive solvent in living organisms; it also participates actively in chemical reactions. One particularly important process is the ability of pure water to react with itself, forming ions. This self-ionization is called autoprotolysis (or autoionization) of water.
What Autoprotolysis Means
The word “autoprotolysis” describes a proton transfer (H⁺ transfer) between two identical molecules. In the case of water, one water molecule donates a proton, and another water molecule accepts it.
The reaction can be written as:
$$
2\,\mathrm{H_2O} \rightleftharpoons \mathrm{H_3O^+} + \mathrm{OH^-}
$$
H₂O= neutral water moleculeH₃O⁺= hydronium ion (water that has accepted a proton)OH⁻= hydroxide ion (water that has lost a proton)
In simplified form, you will also see:
$$
\mathrm{H_2O} \rightleftharpoons \mathrm{H^+} + \mathrm{OH^-}
$$
Biology often uses H⁺ as a shorthand, but in liquid water the proton is actually bound to water as H₃O⁺.
The Ionic Product of Water (Kw)
Although only a tiny fraction of water molecules are ionized at any moment, this ionization is measurable and temperature-dependent. The equilibrium of the autoprotolysis reaction is described by the ionic product of water.
For the equilibrium
$$
2\,\mathrm{H_2O} \rightleftharpoons \mathrm{H_3O^+} + \mathrm{OH^-}
$$
an equilibrium constant can be written. Because the concentration of liquid water is essentially constant, it is combined into a single constant:
$$
K_\mathrm{w} = [\mathrm{H_3O^+}]\,[\mathrm{OH^-}]
$$
This is the ionic product of water.
Square brackets [...] mean “concentration of …” in moles per liter (mol/L).
At 25 °C, in pure water:
- $[\mathrm{H_3O^+}] = [\mathrm{OH^-}] = 1.0 \times 10^{-7}\,\mathrm{mol/L}$
- Therefore, it follows for the constant $K_\mathrm{w}$:
$$
K_\mathrm{w} = 1.0 \times 10^{-7} \cdot 1.0 \times 10^{-7}
= 1.0 \times 10^{-14}
$$
This numerical value is specific for 25 °C; at other temperatures, Kₙw changes.
Neutral, Acidic, and Basic Solutions
Autoprotolysis explains why even pure water contains H₃O⁺ and OH⁻ ions and thus provides a reference point for acidity and basicity.
- Neutral solution:
$[\mathrm{H_3O^+}] = [\mathrm{OH^-}] = 1.0 \times 10^{-7}\,\mathrm{mol/L}$
The product obeys: $K_\mathrm{w} = 10^{-14}$ - Acidic solution:
Added acid increases $[\mathrm{H_3O^+}]$, so $[\mathrm{OH^-}]$ must decrease so that the product stays close to $K_\mathrm{w}$.
Thus: $[\mathrm{H_3O^+}] > 10^{-7}\,\mathrm{mol/L}$ and $[\mathrm{OH^-}] < 10^{-7}\,\mathrm{mol/L}$. - Basic (alkaline) solution:
Added base increases $[\mathrm{OH^-}]$, so $[\mathrm{H_3O^+}]$ must decrease.
Thus: $[\mathrm{OH^-}] > 10^{-7}\,\mathrm{mol/L}$ and $[\mathrm{H_3O^+}] < 10^{-7}\,\mathrm{mol/L}$$
In all cases at a given temperature:
$$
[\mathrm{H_3O^+}]\,[\mathrm{OH^-}] = K_\mathrm{w}
$$
This relationship is the basis for defining pH and pOH, which are used throughout biology to describe the acidity of solutions (details of pH are treated elsewhere).
Why Autoprotolysis Matters in Biology
Autoprotolysis is not just a chemical curiosity; it has several biological consequences:
- Basis for pH-dependent processes
Because water always containsH₃O⁺andOH⁻, even in the absence of added acids or bases, all aqueous biological systems have some inherent level of acidity. Many biological molecules, especially proteins and nucleic acids, change structure and function depending onH⁺concentration. - Buffering and pH stability
Biological fluids contain buffers that interact withH₃O⁺andOH⁻. The constant relationship given byKₙwdefines how these buffers respond when acids or bases are added or removed. - Enzyme activity
Enzymes often require a narrow pH range to function. Changes inH₃O⁺concentration, governed in part by autoprotolysis and its equilibrium, can alter enzyme charge states and shapes, thereby changing reaction rates. - Membrane transport and energy conversion
Many transport processes use proton gradients (H⁺differences) across membranes. Although these gradients are actively built by cells, they exist on top of, and are measured relative to, the backgroundH₃O⁺level originating from water autoprotolysis. - Consistency across environments
Because water is the main medium in cells and body fluids, the same fundamental autoprotolysis equilibrium applies in cytoplasm, blood, and other biological compartments, providing a universal chemical framework for acid–base reactions.
Temperature Dependence and Biological Ranges
Kₙw increases with temperature, meaning that warmer water contains slightly higher concentrations of H₃O⁺ and OH⁻ than colder water, while their product still equals Kₙw for that temperature.
For organisms:
- Small temperature changes within a biological range cause only modest shifts in
Kₙwand thus in the neutralH₃O⁺concentration. - Nevertheless, many organisms regulate both temperature and pH, because both together influence the degree of ionization of water and of dissolved biological molecules.
Understanding autoprotolysis is therefore essential for grasping how water, beyond its physical properties, controls the chemical environment in which all life processes occur.