There are several chemical compounds all around us utilised to make a variety of goods that we use daily. The majority of the chemicals are either acidic, basic, or neutral in nature. As a result, testing the acidic or basic nature of a chemical before employing it is critical. Acids have a sour flavour, while bases have a bitter taste. However, because chemicals might be deadly, we cannot taste them to determine if they are acidic or basic.

As a result, we will need some indicators that can tell whether anything is acidic or basic without tasting it. In the laboratory, we occasionally see a paper strip that turns red or blue when a drop of a chemical solution is introduced. When turmeric stains on our garments are exposed to soaps or detergents in laboratories and at home, the yellow colour of the stains turns crimson. These all are examples of acid-base indicators. Let’s learn about these acid-base indicators precisely in this article.

Acids and Bases

One of the leading chemical properties of a substance is its acidity and basicity. Before learning about acid-base indicators, we need to understand what are the acids and bases.

Acid – The term acid is derived from the Latin word – ‘Acidus,’ which means ‘sour.’ Acids are any substance in an aqueous solution that tastes sour, changes blue litmus paper to red, and liberates protons. Examples of acidic substances are lemon, orange, grapes, tamarind, etc.

Base – The mild bitter-tasting compounds are bases. Their aqueous solution turns red litmus paper to blue and liberates hydroxide ions. Examples of basic substances are soap, household cleaners.

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What is an Acid-Base Indicator?

The major challenge is how to infer whether a substance is acidic or basic. Thus, for this purpose, we need indicators. Substances that help us detect whether a given solution is acidic, basic or neutral without tasting it are known as acid-base indicators. These indicators are dyes that change colour when brought in contact with the solution according to the pH. Hence, they may also be known as “\({\rm{pH}}\) Indicators.” Apart from colour change, some indicators also change their smell when exposed to an acidic or basic solution.

Characteristics of Acid-Base Indicators

While choosing an indicator, it should have some characteristics for appropriate use. Some of them are as follows:

1. It should be chemically pure and must not react with other substances in the solution that has to be tested.
2. An indicator should remain chemically inert and must not degrade in the whole reaction.
3. When the reaction reaches the equilibrium point or end-point during titration, the indicator must have noticeable colouration.

Acid-Base Indicators in Chemistry

Chemically, indicators are weak organic acids with complex structures. Let’s take a general representation of an indicator by the formula \({\rm{HIn}}\) and its ionisation in a solution is shown by the chemical equation given below:

Equilibrium constant is: Equation (i)

\({{\rm{K}}_{{\rm{In}}}} = \frac{{\left[ {{{\rm{H}}^ + }} \right]\left[ {{\rm{I}}{{\rm{n}}^ – }} \right]}}{{\left[ {{\rm{HIn}}} \right]}}\)

On rearranging the above equation: (ii)

\(\frac{{\left[ {{\rm{I}}{{\rm{n}}^ – }} \right]}}{{\left[ {{\rm{HIn}}} \right]}} = \frac{{\left[ {{{\rm{K}}_{{\rm{In}}}}} \right]}}{{\left[ {{{\rm{H}}^ + }} \right]}}\)

From the above equation, we can infer how the concentration of \([{{\rm{H}}^ + }]\) and indicator \([{\rm{I}}{{\rm{n}}^ – }]\) is sensitive towards the colour change.

We know that:

\({\rm{p}}{{\rm{K}}_{{\rm{In}}}} =  – \log ({{\rm{K}}_{{\rm{In}}}})\)

Taking the negative log of equation (i),

\(- \log {{\rm{K}}_{{\rm{In}}}} = \, – \log \left[ {{{\rm{H}}^ + }} \right] – \log \frac{{\left[ {{\rm{I}}{{\rm{n}}^{\rm{ – }}}} \right]}}{{{\rm{HIn}}}}\)

Therefore:

\({\rm{pH}} = {\rm{p}}{{\rm{K}}_{{\rm{In}}}} + \log \frac{{[{\rm{I}}{{\rm{n}}^ – }]}}{{[{\rm{HIn}}]}}\)

The above equation shows the relation between pH and the concentration of indicators. In this manner, the colour of the indicator changes with pH and helps us to determine whether a substance is acidic or basic.

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Types of Acid-Base Indicators

Acid-base indicators are of different types:

1. Natural Indicators
2. Artificial Indicators
3. Olfactory Indicators

1. Natural Indicators

The natural indicator is a naturally occurring substance that can determine whether the substance is acidic or basic. Some examples of natural indicators are given below:

1. Litmus – It is a purple dye that is extracted from a type of algal bloom called ‘lichens’. In an acidic medium, blue litmus paper turns red, and red litmus paper turns blue in a basic or alkaline medium.

2. Turmeric – Turmeric is a common spice present in our kitchen that remains yellow in acidic and neutral solution, but turns bright red when comes in contact with basic solution. In this manner, turmeric behaves as an acid-base indicator.

3. Red cabbage – Red cabbage contains the pigment ‘anthocyanin’, which changes colour when they come into contact with an acid or an alkali. In acidic conditions, it turns red, whereas, in alkaline conditions, it turns green to yellow. In neutral conditions, it remains purple.

4. China Rose – China rose is a natural indicator that turns an acidic solution to dark pink or magenta colour and basic solution to green.

2. Artificial Indicators

Acid-base indicators that are artificially synthesised by using chemicals are known as artificial indicators. Some examples of artificial indicators are given below:

1. Phenolphthalein 

It is an important acid-base indicator commonly used in chemical laboratories in titration. It is a chemical compound that belongs to the ‘phthalein’ family having the chemical formula \({{\rm{C}}_{20}}{{\rm{H}}_{14}}{{\rm{O}}_4}\) It is used as a colourless reagent having a \({\rm{pH}}\) range of \(8.5,\) which turns pink to deep red by increasing \({\rm{pH}}\) above \(9.0.\)

2. Methyl Orange – It is one of the most important and very common azo dye or pH indicators used in acid-base titration in chemical laboratories and also in various industries such as paper, printing, textile industry, etc. Methyl orange turns red in acidic medium and yellow in basic medium.

Bromocresol Blue – Bromocresol Blue (BCG) is a triphenylmethane dye used as a pH indicator. In an alkaline medium, it turns blue and yellow in an acidic medium. In a neutral solution, BCG turns green. Thus, it is mostly used in laboratory applications that require measuring substances that would have a relatively neutral \({\rm{pH}}\) (nearly \(7\)). It is commonly used for measuring carbonic acid in a solution.

Eriochrome Black – It is a bluish-black indicator used in complexometric titrations. The most common use of Eriochrome Black is to determine the hardness of the water. As it consists of calcium carbonate, when all calcium is dissolved in water during titration, the indicator shows an electric blue colour

3. Olfactory Indicators

The substances whose odour varies in the acidic and basic medium are known as olfactory indicators. Such indicators are very useful for visually impaired students to detect acid and base in chemical laboratories as they cannot see the colour change. Some examples of olfactory indicators are given below:

1. Vanilla Essence – In an acidic medium, vanilla essence retains its smell but loses it in basic medium. This helps to identify whether a substance is acidic or basic in nature.

2. Onion – In acidic medium onion retains its smell but loses it in basic medium—this helps to identify whether a substance is acidic or basic.

3. Clove oil – It smells as such in an acidic medium, but the smell of clove oil disappears in basic medium.

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Universal Indicators

Some indicators are used as a collective mixture of different types of indicators that give different colours at different pH values. Hence, it is used to measure the \({\rm{pH}}\) of a solution. The pH of a solution is measured by dipping a piece of Universal Indicator paper in the solution. The pH value is then found by comparing the colour obtained with a colour chart.

The pH Meter

The \({\rm{pH}}\) meter is a device used to measure the \({\rm{pH}}\) of a solution. It consists of a \({\rm{pH}}\) electrode that is connected to a meter. When the electrode is dipped into the solution, the meter shows the \({\rm{pH}}\) value either on a scale or digitally. It is one of the most reliable and accurate methods of measuring \({\rm{pH}}\) than the universal indicator paper.

Uses of Acid-Base Indicators

In Chemical Research

1. Indicators are used to determine whether a substance is acidic, basic or neutral.
2. Acid-base indicators help monitor the change in acidity during volumetric analysis in neutralisation reactions when an acid reacts with a base to form a neutral product.
3. It is used to signal the end-point of an acid-base titration.

In Everyday Life

1. Indicators are used in soil testing to determine whether the soil of a particular area is acidic or basic. This also helps to decide the type of vegetation to be grown and also neutralise the soil if needed.
2. Acid-base indicators are used to measure the acidity of water in a swimming pool.

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Summary

In a nutshell, acid-base indicators are chemical compounds that help determine whether a substance is acidic, basic, or neutral in nature. Acidity and basicity are regarded as pH indicators because they are related to \({\rm{pH}}\). Natural indicators, synthetic or artificial indicators, and olfactory indicators are all examples of indicators. Litmus, turmeric, red cabbage, China rose, and other natural acid-base indicators are examples. Artificial acid-base indicators, on the other hand, include Phenolphthalein, methyl orange, and others.

In acidic and basic media, they show a substantial colour change that lets us determine the acidity or basicity. Acids turn blue litmus red, while bases turn red litmus blue. As a result, acid-base indicators are essential for detecting acidity or alkalinity because we cannot taste all compounds to determine whether they are acidic or basic.

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 FAQs on Acidic-Basic Indicators

Q.1. What are the indicators used in acid-base titration?
Ans: Phenolphthalein and methyl orange are the indicators commonly used in the acid-base titration. They are synthetic indicators having complex structures.

Q.2. What is the most common acid-base indicator?
Ans: Litmus is the most common acid-base indicator used widely in all chemical laboratories. Litmus solution or litmus paper is very easily available. Acids turn blue litmus red, and bases turn red litmus blue. The colour of litmus remains unchanged in a neutral solution.

Q.3. What are the four types of indicators?
Ans: Four types of indicators are 
(i) Natural indicators such as turmeric, China rose, red cabbage, etc.
(ii) Artificial indicators such as Phenolphthalein, methyl orange, etc.
(iiii) Olfactory indicators such as vanilla essence, clove oil, onion, and
(iv) Adsorption Indicators, indicate an excess of a reactant in argentometric titrations. They are used in a precipitation reaction and become coloured when adsorption indicators are adsorbed.

Q.4. What is the chemical name of the indicator?
Ans: Acid-base indicators are weak acids. Phenolphthalein is the chemical name of an indicator used in an acid-base titration, also known as \({\rm{pH}}\) indicator.

Q.5. What are acid-base indicators examples?
Ans: Examples of natural acid-base indicators include litmus, turmeric, red cabbage, China rose, etc. Whereas examples of artificial acid-base indicators include Phenolphthalein, methyl orange, etc. and Vanilla essence, onion, and clove oil are examples of olfactory indicators.

We hope this article on the Acid-Base indicators is helpful to you. If you have any queries on this page or in general about the acid-base indicator, ping us through the comment box below and we will get back to you as soon as possible.