pH Definition
- or what is pH?

The pH definition is based on the amount of hydrogen ions available in a solution. So what is a hydrogen ion (H+)? First, pH is a measurement related to an aqueous solution (a solution in water). So, let us take a closer look at what are the components of water.

Autodissociation of water

water molecule

All substances are made up of tiny particles called atoms. These atoms form small groups called molecules. In water, for example, each molecule is made up of two hydrogen atoms and one oxygen atom.

Water molecules are in continuous motion. When two water molecules collide, a hydrogen and hydroxide ion are created. This process in which water itself dissociates (splits) into hydrogen and hydroxide ions is called autodissociation or self ionization.

This is an equilibrium reaction, which means that under ordinary conditions water consists of both H2O molecules, H+ and OH- ions. How many H2O dissociate? Autodissociation increases for increasing temperature (higher internal energy of the water). Temperature is an important factor in pH measurement.

Hydrogen (H+) ions are always present . .

Every water solution contains H+ (hydrogen) and OH- (hydroxide) ions, either from water autodissociation or from introduced acids and bases.

Acidity or alkalinity of a solution

So why are the hydrogen (H+) ions of interest?

The hydrogen ions (H+) are what determines if a solution is acidic, alkaline or neutral. If we can measure the hydrogen ion content of a solution, we can determine the acidity or alkalinity of that solution.

Calculating pH

ph definition

The pH definition we use today is the negative logarithm of the hydrogen (H+) ion activity in a given solution. Read "What does pH stand for".

Note I say hydrogen (H+) ion activity, not concentration. Read more.


• Every water solution contains H+ (hydrogen) ions.
•The activity of hydrogen ions in solution determines the acidity or alkalinity of a solution.
• The pH value is a measure of the H+ activity in a give solution.

or if we combine the two previous statements:

• The pH value is a measure of the acidity or alkalinity of a solution.
• The range of the pH scale is from 0 to 14.
• The pH defintion: the negative logarithm of the H+ activity in a given solution.

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Download the new release ISO 14001:2015