pH Definition
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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.  |
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
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.
Summary
| • 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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Hydrogen Ion Activity
I want to add a slight clarification of hydrogen activity versus hydrogen concentration. Hydrogen activity can be thought of as applying a correction factor ...
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