INDICATORS

 INDICATORS 

The substances which are neither acidic nor basic and are chemically inert in nature. Touching or tasting of acid or base is very harmful and hence typical indicators are used to recognise them. The substances which change their colours in presence of acid or base are called ‘Indicators’. 

you might have seen that the spot of turmeric on cloth becomes red when soap is applied on it. what do you think has happened? Tumeric has acted as an indicator of base present in soap. There are many substances that shows one colour in an acidic medium and another colour in a basic medium. such substances are called acid base indicator. 

litums is a natural dye founded in certain lichens. It was the earliest indicator to be used. it shows red colour in acidic medium and blue colour in basic solutions . phenolphthalein and methyl orange are some other indicators. The colour of these indicator in acidic, neutral and basic solutions are given below in the table.

Indicators in Laboratory :

In the laboratory, litmus paper is mainly used to test alkali or base. This paper is made by an extract of licane plant. It is red or blue coloured. Blue litmus paper turns red on dipping in acid and red litmus turns blue by alkali. Similarly phenolphthalein, methyl orange and methyl red are used in laboratory in solution form. Methyl orange indicator becomes pink in acid and yellow in alkali. Phenolphthalein remains colourless in acid and becomes pink in alkali. Universal indicator which are in liquid state change their colour in the presence of acid and base.

Domestic Indicator : 

On non availability of laboratory indicator, ‘natural indicators’ can be made by using several domestic substances. You must have seen yellow food stain turning red after washing with soap. This colour change is a result of chemical reaction between turmeric and alkaline mateiral of soap. Here turmeric acts as on indicator. Natural indicators can also be prepared from red cabbage, radish, tomato and similarly from hibiscus and rose.

Indicators and their colours in acid and base 

Litmus 

Litmus is a purple dye which is extracted from 'lichen' a plant belonging to a variety Thallophyta. It can also be applied on paper in the form of strips and is available as blue and red strips. A blue litmus strip when dipped in an acid solution acquires red colour. Similarly, a red litmus strip when dipped in a base solution becomes blue. 

Phenolphthalein 

It is also an organic dye. In neutral or acidic solution, it is colourless while in the basic solution, the colour of Indicator changes to pink.

Methyl orange. Methyl orange is an orange coloured dye and keeps this colour in the neutral medium. In the acidic medium, the colour of Indicator becomes red and in the basic medium. it changes to yellow. An indicator has a certain colour in the acidic medium which undergoes a change when the medium becomes basic or alkaline. The indicators listed above and their characteristic colours in different media are given in a tabular form.

Some other Indicators

The acid base Indicators mentioned above are very commonly used in the laboratory. Apart from these, red cabbage juice and turmeric juice can also act as acid base indicators.

Red cabbage juice  

It is purple in colour in neutral medium and turns red or pink in the acidic medium. In the basic or alkaline medium, its colour changes to green

Turmeric juice 

It is yellow in colour and remains as such both in the neutral and acidic medium. In the basic medium, its colour becomes reddish or deep brown. 

Apart from these, coloured petals of some flowers like petunias, hydrangea and geranium can also be used to identify the presence of acid or base in a solution.

There are certain substances which give different smell in acidic and basic solutions. These are known as Olfactory indicators. For example, onion, vanilla and clove oil etc. All of them have characteristic odours and we are all quite familiar with them. These change when some acid or base solution is brought in their contact and can be identified. However, the results in this case are not quite satisfactory. 

Only some guess can be made about the nature of the substance whether acid or base. Acids are identified by their sour taste. 

1.Souring of milk is due to the presence of lactic acid. 

2.The acid present in the stomach is hydrochloric acid and causes acidity. 

3.Acids obtained from rocks and minerals are called mineral acids. 

4.Acids obtained from plants and animals are known as organic acids.

5. Bases are identified by their bitter taste and soapy touch. 

6.Sodium hydroxide (NaOH) and potassium hydroxide (KOH) have strong corrosive action on skin. These are called caustic alkalies. 

7.In aqueous solution, an acid gives H⁺  ions while a base releases  OH^-    ions. 

8.The main acid-base indicators are litmus solution, phenolphthalein and methyl orange. 

9.The acid-base indicators are mostly organic dyes with characteristic colours. 10.Litmus is the extract of the plant lichen. Phenolphthalein and methyl orange are synthesised in the laboratory. 

11.The indicators impart different colours to the acid/base solutions and help in their identification.