The Acidic Environment > 3. Acids >
Describe the use of the pH scale in comparing acids and bases
- Pure water contains very small amounts of hydrogen and hydroxide ions due to self-ionisation:
- At 25°C, the concentration of hydrogen ions is 1.0 x 10-7 mol L-1.
- When an acid is added to water, the above equilibrium shifts in the reverse direction, causing a decrease in the hydroxide concentration.
- When a base is added to water, the above equilibrium shifts in the reverse direction, causing a decrease in hydrogen ion concentration.
- Thus, acidic solutions have a higher concentration of hydrogen ions than 1.0 x 10-7 mol L-1, while basic solutions have a lower concentration of hydrogen ions than 1.0 x 10-7 mol L-1.
- The pH scale is used to compare the concentration of hydrogen ions in solutions of acids and bases.
- It is a logarithmic scale, as the concentration of hydrogen ions would require a very large linear scale.
THE PH SCALE