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

Everyone who engages in science or simplyinterested 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 the indicators change color in solutions? For what else are applied? About this later in the article.

Definition

Reference literature answers the question thatsuch indicator, the following definition: the indicator is usually an organic chemical compound that is used to determine the parameters of the solution (the concentration of hydrogen ions, the point of equivalence, the determination of the presence of oxidants). In the narrow sense, the word "indicator" means 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 ions+ and R-. Ions H+ have a red color, R- - yellow, therefore in a neutral solution (at pH = 7), this indicator is orange. If the hydrogen ions are larger than R-, the solution becomes red (at pH <7), and yellow, if R-. 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 changesmethyl orange depending on the hydrogen index. This illustration clearly demonstrates what an indicator is in chemistry and what its purpose is.

What is the indicator in chemistry

Examples of indicators

The most common indicators thatthere is in each school, are litmus and phenolphthalein. Litmus in acidic, neutral and alkaline environments has different colors that can not be confused. Paper strips soaked in litmus are placed in a solution and their color changes.

Litmus in acidic and alkaline environment

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

What is an indicator

In the laboratory,less common indicators: 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.

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