ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
Bromothymol blue indicator being compared in acid (left) and alkaline (right) solutions. Changes in colour (here yellow to blue) are a common way to test the pH of a solution. Alkali and acid are the two parts of the pH scale, indicating the hydrogen ion concentration in a solution relative to that found in water. Alkalis contain less free hydrogen ions than water and acids contain more of these hydrogen ions. Water is considered neutral. Bromothymol blue (C21.H15.Na.Br2.O3.S) is yellow, but the ion that forms with decreasing acidity is blue. The colour change occurs at around pH 7.
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