Chrominfo: What is indicator in chemistry and its types

Learn the three types of indicators, natural, artificial, and olfactory which are used to determine whether a substance is acidic or basic. It is a weak acid or weak base that dissociates in a solution to form ions.

The concentration of hydrogen in a solution is expressed using the pH (potential of hydrogen) value. The negative logarithm of hydrogen ion concentration is related to the pH scale, which ranges from 0 to 14. An acid is a substance that gives hydrogen ions, so when a solution has more hydrogen ions than hydroxide ions, it is acidic. A base is a substance that accepts hydrogen ions, so when a solution has more hydroxide ions than hydrogen ions, it is alkaline.

Table of Contents:

What is indicator?
History of indicators
Theory of indicators
How pH indicators work?
Characteristics of indicators
Different types of indicators
Uses of indicators

What is an indicator in chemistry?

Indicators are compounds that change color when introduced to acidic or alkaline solutions. Colored indicators are commonly used to determine pH and can be added to the reaction mixture to determine the endpoint or equivalence point of the different types of titration.

In the laboratory (for practicals) of science classes, indicators such as litmus, phenolphthalein, and methyl orange are most commonly used. As well as to determine the pH of the compound a pH paper, universal indicator, litmus paper (Blue or Red), and pH meter are generally used.

Definition of the indicator:
“Indicator is a substance that shows a color change when exposed to acids and bases.”

An indicator has a defined pH range in which it changes from an acid to a base form. For example, litmus paper is blue in a basic solution and red in an acidic solution. The range is from 0 to 14, with 07 being the neutral value. Acidic is indicated by a pH less than 07, while the base is indicated by a pH greater than 7. Indicators cannot work outside their pH range, as an indicator does not change color over a wide range of pH values.

Indicators are weak acids or bases (usually derived from plant pigments) that have unique colors in their ionized and non-ionized states and are related to the pH of the solution being analyzed.

For example, using phenolphthalein as an indicator for a titration with a strong base as the titrant and acid as the titrated ingredient would result in a color change in the conical flask from colorless to pink towards the equivalence point.

History of indicators:

In his collection of essays "The Experimental History of Dyes", Sir Robert Boyle first described the use of a natural dye as an acid-base indicator in 1664. Boyle made major contributions to the early theory of acids and bases by using markers for the experimental classification of acids and bases.

Theory of acid-base indicators:

The theory of the acid-base indicator is explained by two theories: Ostwald's theory and the quinonoid theory.

Ostwald's theory:

Ostwald's theory, which is based on the Arrhenius theory, was proposed by Ostwald in 1891. According to this theory, the acid-base indicator ionization causes the color change. The color of the unionized form varies from that of the ionized form.

Quinonoid theory:

According to the quinonoid theory, the color change of the acid-base indicator results from a structural change. An indicator is thought to exist in equilibrium as two tautomeric forms, the benzenoid and quinonoid forms. One form occurs in an acidic solution and the other form in an alkaline solution. The two forms have two different colors, and as the pH of the solution changes, the color of the solution containing the indicator changes.

How do pH indicators work?

Indicators are both weak acids and bases, by reacting with H+ and OH-, pH indicators detect the presence of H+ and OH-. The deprotonation produces a color change when an indicator is a weak acid that is colored and its conjugate base is a different color.

The color we observe is determined by the ratio of the concentration of the indicator, HInd, and its conjugate base, Ind-. According to the Henderson–Hasselbalch equation, this ratio depends on the pKa and pH of the indicator.

Characteristics of indicators in chemistry:

It should change color when the pH reaches a point close to the endpoint of the reaction.
It should be sensitive to the equivalence point of the titration.
The color change of the sample solution should be very sharp and clear.
The produced color should be brilliant and stable.
Two colors of an indicator must be contrasted so that they can be identified.
The indicator should be cheap and non-toxic in nature.

Different types of indicators:

Different types of indicators are used in different types of titrations like acid-base, redox, precipitation, complexometric, etc. Each of them has its pKa, pH range, and produces a particular color at different pH values.

There are three types of indicators in chemistry: natural indicators, artificial indicators, and olfactory indicators, although artificial and natural indicators are the two types of chemical indicators that are most commonly used.

Natural indicators:

Natural indicators are types of indicator that occurs naturally and can be used to identify whether a substance is acidic or alkaline. Some examples of natural indicators are litmus, turmeric, china-rose, curry powder, red cabbage, grape juice, cherries, tomato, and onion, etc.

To detect hydrogen ions (H+) and hydroxyl ions (OH-) in a sample solution, these natural indicators are used. Some examples of household pH indicators are red cabbage, rose-petals and turmeric.

Artificial indicators:

Artificial indicators are types of indicators that are prepared synthetically in the laboratory or obtained through a chemical reaction that does not occur naturally.

Some examples of artificial indicators are phenolphthalein, methyl orange, malachite green, and methyl red, etc. These synthetic indicators are used for titration as well as the identification of acids and bases.

Olfactory indicators:

Olfactory indicators are types of indicators that do not indicate a color change however a change in odor when added to acidic or a basic sample. Olfactory indicators, unlike other indicators, change their smell rather than their color to indicate whether a sample solution is acidic or basic.The odor varies depending on the nature of the medium.

Some examples of olfactory indicators are onion, garlic, vanilla extract, clove oil, etc.

Indicators pH range and color change:

Here are some common indicators are listed in table form along with their pH range, pKa value, and color change under acid and basic environments.

What is indicator in chemistry and its types

Universal indicator:

A universal indicator is a pH indicator composed of different compounds that changes color over a wide range of pH values to indicate the acidity or alkalinity of the sample solution. The new color that is formed is then compared with the pH chart.

Usually, it is formulated by, phenolphthalein, water, methyl red, 1-Propanol, sodium bisulfite, sodium hydroxide (NaOH), bromothymol blue, and thymol blue, etc. However, there are various universal pH indicators on the market, most of which are modifications of the formula patented by Yamada in 1933.

Litmus paper:

Litmus paper is a kind of indicator that is used to determine if a compound is acidic or basic. There are two types of paper strips available, blue and red, which are made by mixing water-soluble dyes that are absorbed into the filter paper strips. It is impregnated with lichens, giving it the capacity to change color when exposed to acids and bases.

Red litmus paper of a weak diprotic acid and changes color red to blue when exposed to a basic solution or gaseous sample. Blue litmus paper already consists of a blue conjugate base and it changes color blue to red when expose to the acidic solution or gaseous sample.

Phenolphthalein:

Phenolphthalein (C20H14O4) is a phthalein-family organic compound that is commonly used as an acid-base indicator. The ionic form of phenolphthalein is In-, while the molecular form is HIn. The molecule loses a hydrogen ion and becomes a negative ion in the basic solution. Phenolphthalein is the preferred indicator when titrating weak acids with strong bases.

It is a fine crystalline powder that is yellowish-white to pale orange and appears colorless in the liquid form up to pH 8.5, after which it changes from pink to dark red. It has a pKa value of 9.3, is slightly soluble in water, and is prepared by dissolving in alcohol as an indicator for acid-base titration experiments.

Methyl orange:

Methyl orange (C14H14N3NaO3S) is an organic dye commonly used as a pH indicator in titrations, since of its clear and different color variations at different pH values. In an acidic medium, methyl orange turns red, while in a basic condition, it turns yellow.

Methyl orange is preferred for titrations involving weak bases and strong acids, such as ammonia solution (NH4OH) and hydrochloric acid (HCl) because its pKa (3.4) lies in the acidic region of the pH scale.

Bromothymol blue:

Bromothymol blue is an ionic dye also known as bromothymol sulfone phthalein and BTB. Bromothymol blue (C27H28Br2O5S) is a pH indicator that can be used to detect weak acids and bases. It has a bluish-green color in neutral solution and turns blue at pH 7.6 and yellow at pH 6.0. BTB is a good choice for titrations that have an equivalence point near the neutral.

Phenol red:

Phenol red (C19H14O5S) is a water-soluble dye also known as phenolsulfonphthalein that changes color from yellow to red over pH 6.6 to 8.0, then turns a bright pink color above pH 8.1. Phenol red is a pH indicator dye that can be used in a variety of medical and cell biology studies.

Methyl red:

Methyl red (C15H15N3O2) is a dye used as an acid and base indicator in the laboratory and also used in textile industries. It is an azo dye, comes in a dark red colored crystalline powder; indicator is also available in liquid form.

At pH 4.4 and below, methyl red turns red, and when pH 6.2 is achieved, it turns yellow. The color orange is found between these color endpoints, in the pH range of 4.4 to 6.2.

Turmeric:

Turmeric is a yellow-colored natural indicator, also as well as household indicator. Turmeric solution or paper becomes reddish-brown by adding base and adding acid does not change its color.

Onion:

Onion indication is a kind of olfactory indicator in which the smell of onion paste or juice is lost when it is mixed with the base. It does not change its odor when exposed to acid. Red onion turns pale red in acidic solution and green in alkaline solution.

Uses of indicators in chemistry:

The most typical use of indicators is to determine the endpoints of a titration.
The role of indicators in chemistry is to determine whether a reaction has completed or undergone a chemical change.
It is used in laboratory experiments such as titration; it is one of the most primary experiments for secondary school of chemistry, analytical chemistry students, and it necessitates the use of a burette, pipette, and conical flask, etc.
Titratable acidity or alkalinity of the sample is determined using indicators.
Chemical indicators are used to detect the presence of certain compounds in water, such as pollutants.
It is also used in industrial chemical processes, such as pharmaceutical, food, agriculture, and environmental science, etc.