Any of various substances that show the presence, absence, or concentration of a substance.

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Whenever we visit a laboratory, we can find a number of chemicals kept there. These all chemicals may appear similar to us but they are not. Some of these may be acids, bases or salts. We can easily identify these chemicals with the help of the labels on the bottles. But what if you want to know whether a chemical is an acid or base without its label. Have you ever wondered how scientists identify the acids or bases in a lab or classify the product of a chemical reaction as an acid or a base? This is where indicators are really helpful to us. Let us discuss indicators in this article. Before we look into what are indicators and their types, we shall discuss what acids and bases are:

• Acids: In old times, anything that tasted sour was considered to be an acid. According to the modern definition of an acid, any compound that can donate a hydrogen ion to another molecule or compound is an acid. In other words, we can say that any molecule that can give away an H+ ion and still exist is known to be an acid. Acids generally have a sour taste and can be strong acids or weak acids.
• Bases: In old times, anything that was bitter in taste was considered to be a base. According to the modern definition of the base, any compound that can accept a hydrogen ion given away by acid is said to be based. In other words, we can say that any molecule that can accept an H+ ion from acid and still exist is known to be a base.

Water-soluble bases are termed alkalis. Acids and bases react with each other to give rise to salt and water. This reaction is termed a neutralization reaction. 

Acid+Base → Salt+Water

e.g.

HCl (Acid) + NaOH (Base) → NaCl (Salt) + H2​O (Water)

What are Chemical Indicators?

Indicators are weak acids or weak bases that show a change in colour as the concentration of Hydrogen ions in a solution changes or the pH of a solution changes. The indicators dissociate slightly in the water to form ions. 

Some examples of indicators are Litmus, turmeric, phenolphthalein, etc.

Any material that offers a visual evidence of the presence or absence of a threshold concentration of a chemical species, such as an acid or an alkali in a solution, generally by a colour change are called the chemical indicators. A chemical called methyl yellow, for example, gives an alkaline solution a yellow colour. When acid is gradually added, the solution remains yellow until all of the alkali has been neutralised, at which point it becomes red.

Types of Indicators

Indicators are mainly of two types:

1. Natural Indicators: The indicators that occur naturally in the environment are called natural indicators. e.g. Litmus, Turmeric, etc.
2. Artificial Indicators: Those indicators that are obtained through some chemical reaction but not naturally or are prepared artificially in the laboratory are artificial indicators. e.g. Phenolphthalein, Methyl orange, etc.
3. Olfactory Indicators: These are those indicators that do not show a color change but a change in smell when added to acid or a base. Thus olfactory indicators are those indicators whose smell changes depending upon the nature of medium, whether it is acidic or basic.

Natural Indicators 

Natural Indicator is a sort of indicator that may be found in nature and can be used to detect whether a material is acidic or basic. Red cabbage, turmeric, grape juice, turnip peel, curry powder, cherries, beetroots, onion, tomato, and other natural indications are examples.

Hydrangeas, for example, can tell you if the soil is acidic or basic. If the soil is acidic, the blooms turn blue, purple if the soil is neutral, and pink if the soil is basic. The colour intensity is determined by the quantity of acid or base in the soil. Deep blue flowers bloom in extremely acidic soil, whereas deep pink blooms bloom in strongly basic soil.

Following are some natural indicators that are commonly used:

• Litmus: Litmus is obtained from lichens and is a mixture of water-soluble dyes that are obtained from the lichens. Litmus is generally made into a soluble dye and then absorbed onto a paper that is then called litmus paper. The natural colour of litmus is purple. Litmus can be used to test if a compound is an acid or a base. A red litmus paper turns blue in a basic solution and does not show any colour change in acidic solutions. A blue litmus paper turns red in an acidic solution and shows no colour change in a basic solution. Neutral litmus turns red and blue in acidic and basic solutions respectively.

• Turmeric: Turmeric is a naturally occurring indicator and is commonly known as Haldi. It is bright yellow in colour. Turmeric paste can be used to test for acids or bases. In an acidic medium, turmeric is yellow in colour and does not show any colour change while in a basic solution its colour turns red.

• Red Cabbage Indicator: Red cabbage is commonly used as a vegetable but its extract can also be used as an indicator. Red cabbage is a natural indicator. Red cabbage contains a pigment anthocyanin that is responsible for the colour change of red cabbage in an acid or base. Red cabbage extract is used as an indicator. Red cabbage has a deep purple colour naturally. The red cabbage extract turns red in an acidic medium with a pH < 7 and bluish-green in an alkaline or basic medium with a pH > 7.

Artificial (or Synthetic) Indicators

Synthetic indicators, often known as artificial indicators, are acid-base indicators that are made from artificial chemicals.

Following are some artificial or synthetic indicators discussed below:

• Methyl Orange: Methyl Orange is also an indicator and shows colour change in acids and bases. It is orange in colour originally. Methyl orange can be only used in the case of mineral acids and strong bases. It cannot identify weak acids or weak bases. In an acidic medium, methyl orange turns red, while in a basic medium, it turns yellow. The structure of methyl orange is as shown below:

• Phenolphthalein: Phenolphthalein is a water-soluble dye and is dark purple in color. It is used in form of a solution to test for acid and base. In an acidic medium, it turn colorless while in a basic medium it turns light pink in color. It is mostly used in acid-base titrations. The color of phenolphthalein is shown below in acidic and basic medium:

What are the Universal Indicators?

The common indicators may or may not show colour change over a wide range of pH but only for a small range. This makes it difficult to identify the acids or bases. This problem is solved by universal indicators. Multiple indicators are mixed to form universal indicator which change their colour over a wide range of pH values.

Olfactory Indicators: 

An olfactory indicator is a chemical that changes its scent depending on whether it’s combined with an acidic or basic solution. In the laboratory, olfactory indicators can be used to determine whether a solution is a basic or an acid, a technique known as olfactory titration. Simply defined, olfactory markers are compounds in acid and basic solutions that have distinct odours. For example, vanilla extract, onion and clove oil, and so on have distinct aromas. Olfactory indicators are very helpful to identify acids or bases for visually impaired students.

Some of the olfactory indicators are as follows:

• Onion Extract: Onion Extract can be obtained by boiling the water and adding chopped onion into it. When the water cools down, the onion extract is ready. Onion extract shows the change in smell in acid and bases. When it is added to an acid, it retains its pungent smell but in bases, it becomes completely odourless.
• Vanilla Extract: Vanilla extract works just like onion extract. It retains its pleasant smell in an acidic solution but loses its smell in a basic medium.

Following are the list of the indicators and their colours in acids and bases respectively:

Importance of Indicators

The nature of a material is significant in biology, chemistry, civil engineering, water purification, agriculture, forestry, food science, environmental science, water treatment, oceanography, medicine, nutrition, and agronomy, among other fields.

Lichens are even used to make litmus. It’s a colour combination that dissolves in water. It is then absorbed into filter paper to create one of the first types of pH indicator, which is used to determine the acidity or basicity of things.

Sample Questions

Question 1: What do you mean by an indicator?

Answer:

An indicator is generally a weak acid or weak base that dissociates in a solution to form ions. Indicators are used to test whether a medium is acidic or basic in nature.

Question 2: An indicator turned red in HCl and blue in NaOH solution. Identify the indicator.

Answer:

We know that HCl is acidic in nature and NaOH is basic in nature. As the indicator turned red in acid and blue in base, it is surely litmus.

Question 3: What do you mean by olfactory indicators? Give examples.

Answer:

Those indicators that change their smell depending upon the acidic or basic nature of the medium are termed as olfactory indicators. Example: Onion extract, Vanilla Extract

Question 4: Red cabbage turned red in a solution while red litmus showed no change in colour. Identify the nature of the solution.

Answer:

As red cabbage turned red and red litmus did not change its color in the solution, the solution must be acidic in nature.

Question 5: HCl vapours were passed onto a blue litmus paper but it did not show any colour change. Explain.

Answer:

As we know that indicators show color change when the acid or base dissociates into hydrogen or hydroxide ions. Here HCl was converted to vapors and did not dissociate to ions, thus litmus paper did not show any color change.

Question 6: A stain of turmeric was washed with soap and later lemon was rubbed onto it. Identify the colour changes.

Answer:

When turmeric stain was rubbed with soap it turned reddish brown in color but it again changes to its original yellow color when lemon is rubbed onto it.

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