What is the indicator in chemistry: the definition, examples, the principle of action

Everyone who is engaged in science or just interested in chemistry, it will be interesting to know what an indicator is. Many people encountered this concept in chemistry lessons, but school teachers did not give exhaustive explanations about the principle of action of such substances. So what is an indicator? Why do indicators change color in solutions? For what else are applied? About this later in the article.

Definition

References answer the question, what is an indicator, the following definition: an indicator is usually an organic chemical compound that is used to determine the parameters of a solution (hydrogen ion concentration, equivalence point, determination of the presence of oxidants). In the narrow sense, the word "indicator" refers to a substance that makes it possible to determine the pH of the medium.

Operating principle

To better understand what an indicator is, consider the principle of its operation. As an example, take the methyl orange. 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 have a red color, R ^- - yellow, therefore in a neutral solution (at pH = 7), this indicator is orange. If the hydrogen ion is larger than R ^- , the solution becomes red (at pH <7), and yellow, if the 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 the methyl orange varies depending on the hydrogen index. This illustration clearly demonstrates what an indicator is in chemistry and what its purpose is.

Examples of indicators

The most common indicators that exist in each school are litmus and phenolphthalein. Litmus in an acidic, neutral and alkaline medium has different colors that can not be confused. Paper strips impregnated with litmus are placed in a solution, and their color changes.

Phenolphthalein acquires color only in alkaline medium and becomes crimson. An available orange methyl indicator is also used.

In laboratory conditions, less common indicators can 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 for any values of the hydrogen index.