A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, a pH indicator is a chemical detector for hydronium ions (H3O+) or hydrogen ions (H+) in the Arrhenius model.
pH measurement with indicator paper
Hydrangea in acid soil
Hydrangea in alkaline soil
A gradient of red cabbage extract pH indicator from acidic solution on the left to basic on the right
In chemistry, pH, also referred to as acidity or basicity, historically denotes "potential of hydrogen". It is a logarithmic scale used to specify the acidity or basicity of aqueous solutions. Acidic solutions are measured to have lower pH values than basic or alkaline solutions.
Test tubes containing solutions of pH 1–10 colored with an indicator
Lemon juice tastes sour because it contains 5% to 6% citric acid and has a pH of 2.2 (high acidity).