Anyone who is engaged in science or just interested in chemistry will be interested to know what an indicator is. Many people came across this concept in chemistry lessons, but school teachers did not give comprehensive explanations about the principle of action of such substances. So what is an indicator? Why do indicators change color in solutions? What else are they used for? More on this later in the article.
Definition
Reference literature answers the question of what an indicator is, by the following definition: an indicator is usually an organic chemical compound that is used to determine the parameters of a solution (concentration of hydrogen ions, equivalence point, determination of the presence of oxidizing agents). In a narrow sense, the word indicator means a substance that allows you to determine the pH of the medium.
Operating principle
To better understand what an indicator is, consider the principle of its action. Take methyl orange as an example. This indicator is a weak acid, and its general formula is HR. This acid in an aqueous solution dissociates into H + and R - ions. H + ions are red, R - - yellow, therefore in a neutral solution (at pH = 7) this indicator is orange. If there are more hydrogen ions than R - , the solution turns red (at pH <7), and yellow if R - ions predominate. Indicators can be either acids, or salts or bases. Their principle of operation is based on simple primary and secondary electrolytic dissociation.
The photo below shows how the color of methyl orange changes depending on the hydrogen index. This illustration clearly demonstrates what an indicator is in chemistry and what is its purpose.
Indicator Examples
The most common indicators that every school has are litmus and phenolphthalein. Litmus in an acidic, neutral and alkaline environment has different colors that cannot be confused. Paper strips soaked in litmus are placed in a solution, and their color changes.
Phenolphthalein acquires color only in an alkaline environment and becomes raspberry. The available methyl orange indicator is also used.
In laboratory conditions, less common indicators may be used: methyl violet, methyl red, tenolphthalein. Most indicators are used only in a narrow pH range, but there are universal indicators that do not lose their properties at any value of the hydrogen index.