pH is defined as minus the decimal logarithm of the hydrogen ion activity in an aqueous solution. By virtue of its logarithmic nature, pH is a dimensionless quantity. Where aH is the (dimensionless) activity of hydrogen ions.
pH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations. The pH scale is not an absolute scale; it is relative to a set of standard solutions whose pH is established by international agreement (pH and ions )
Pure water is said to be neutral. The pH for pure water at 25 °C (77 °F) is close to 7.0. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are said to be basic or alkaline. pH measurements are important in medicine, biology, chemistry, food science, environmental science, oceanography and many other applications
A solution of a strong acid, such as hydrochloric acid, at concentration 1 mol dm?3 has a pH of 0. A solution of a strong alkali, such as sodium hydroxide, at concentration 1 mol dm?3 has a pH of 14. Thus, measured pH values will mostly lie in the range 0 to 14.
An approximate measure of pH may be obtained by using a pH indicator. A pH indicator is a substance that changes colour around a particular pH value
Some typical pH values
Universal indicator components
Low pH color
Transition pH range
High pH color
Thymol blue (first transition)
Thymol blue (second transition)
A solution whose pH is 7 is said to be neutral, that is, it is neither acidic nor basic. Water is subject to a self-ionisation process.