Indicators | Litmus, Methyl orange, Phenolphthalein

Definition of Indicators

An indicator tells us whether the substance we are testing is acidic or basic by a change in its colour. In other words, Those substances which change their colour in different types of substances are known as indicators. There are many natural substances like red onion peels, red cabbage leaves, beetroot extract, coloured petals of some flowers like rose, Petunia, Hydrangea and Geranium, Bougainvillea, turmeric which indicate the presence of sour substances (acids) and bitter substances (bases). They are known as indicators because they indicate the presence of acid or base by showing the change in colour.

Three are three common indicators:

(i) Litmus

(ii) Methyl orange

(iii) Phenolphthalein

Litmus

Litmus can be used in the form of litmus paper or litmus solution. It is a purple dye that is extracted from a type of plant known as lichen.

• Litmus is a natural indicator.
• It is used as an acid-base indicator
• Litmus is of two types: (i) Blue litmus and (ii) Red litmus.
• The natural colour of litmus paper is purple.
• Litmus paper is made by the leaf of the lichen plant.
• An acid turns blue litmus to red.
• A base turns red litmus to blue.

Lichen: It is an air pollution indicator plant. It is a type of thallophyta plant.

  Indicator paper with sour testing                           Indicator paper with alkaline testing                                                                                                                                                                                                                                                              

  Methyl orange

The natural colour of methyl orange is orange.

•  In the acid solution, Methyl orange indicator gives a red colour.
•  In a basic solution, Methyl orange indicator gives a yellow colour.

Phenolphthalein

The natural colour of phenolphthalein is colourless.

•  In acid solution, Phenolphthalein indicator gives colourless.
•  In a basic solution, the Phenolphthalein indicator gives pink colour.

Turmeric indicator: It is also a natural indicator. The natural colour of turmeric = yellow.

•  In the acid solution, the Turmeric indicator is a yellow colour.
•  In the basic solution, the Turmeric indicator gives a red colour.

Olfactory indicator

Those substances whose smell changes in acidic or basic solution are called olfactory indicators. It is a substance whose smell varies depending on whether it is mixed with an acidic or basic solution. These types of indicators are used in the laboratory to check whether a given solution is a basic or an acidic solution by a process known as olfactory titration.

Acid-Base Indicator

It is a substance that possesses one colour in acid solution and an altogether different colour in an alkaline medium or the substance which shows colour change with change in pH. The point where the indicator shows a sudden change in colour during the titration is known as the endpoint.

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