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
Indicator
Low pH color
Transition pH range
High pH color
Thymol blue (first transition)
red
1.2–2.8
orange
Methyl red
red
4.4–6.2
yellow
Bromothymol blue
yellow
6.0–7.6
blue
Thymol blue (second transition)
yellow
8.0–9.6
blue
Phenolphthalein
colorless
8.3–10.0
purple
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.
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