Acid-base indicator (pH indicator)

Weak acids or bases with an intense inherent colour are used to indicate the value of a substance as they change their colours analogous to the concentration of the tested substance. The perceptible colour change is termed colour transition range (transition pH). Each indicator covers a different colour transition range on the pH scale. Thus, particularly for testing unknown substances, universal indicators (a mix of several indicators) are used. Indicators are available as solution or paper strips. The pH value is determined with the colour scale appropriate to the indicator. In the field of construction acid-base indicators are used e.g. to prove the alkaline pH value of concrete. Some acid-base indicators are: phenolphthalein, thymolphthalein, methyl orange, litmus, thymol blue etc.