Learn about the types of universal indicators, their uses, advantages, and how they work.
The pH is a significant factor that determines whether a solution is acidic or alkaline. It determines the concentration of hydrogen ions (H+) in the aqueous solution on a negative logarithmic scale from 0 to 14.
There are many activities in research, pharmaceuticals, science classes, and agriculture sectors, etc that involve pH testing, environmental science, titration in chemistry, water quality testing, and biological processes are among them. Depending on the application they use the pH paper, litmus paper (red and blue), pH indicators, and potentiometer or pH meter.
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What is a universal indicator in chemistry?
A universal indicator is a pH indicator composed of a solution of a different compound that shows multiple color changes over a broad range of pH values to indicate the solution’s acidity or alkalinity. To determine the pH value of the acid or base solution a sample is added to the universal indicator, which generates a new color that is used to match the color to the colors on a pH chart.
Definition: “A universal indicator is a combination of multiple indicators that produce distinct colors at various pH levels on the scale”
The indicator will show colors such as orange, red, yellow, orangish-yellow in an acidic medium. It will change color to purple or blue-violet in basic medium and its color will change to green in neutral medium. However, several universal pH indicators are commercially available, most of which are variations of a formula patented in 1933 by Yamada.Example: Solution or pH paper
History of indicators:
Sir Robert Boyle first mentioned the use of a natural dye as an acid-base indicator in his essays collection "Experimental History of Colors" in 1664. By using markers for the experimental classification of acids and bases, Boyle made significant contributions to the early theory of acids and bases.
Theory of indicators:
The two primary theories that lead to the working principle of acid-base indicators are the Ostwald theory and the quinonoid theory.
Ostwald's theory:
According to Ostwald's theory, due to the ionization of the acid-base indicator, the color changes. The color of the unionized form differs from that of the ionized form.
Quinonoid theory:
According to Quinonoid theory, an acid-base indicator exists in two tautomeric forms with different structures that are in equilibrium. One form is called the benzenoid form and the other is called the quinonoid form.
What is a universal indicator made of?
Generally, the universal indicator is usually composed of water (H2O), phenolphthalein (C20H14O4), 1-Propanol (C3H8O), methyl red (C15H15N3O2), sodium hydroxide (NaOH), thymol blue (C27H30O5S), sodium bisulfite (NaHSO3), and bromothymol blue (C27H28Br2O5S), etc.
Phenolphthalein, methyl red, thymol blue, and bromothymol blue are the major components of a universal indicator in the form of a solution. This mixture is significant because depending on the acidity or basicity of the solution being tested, each component loses or gains electrons.
Types of universal indicators:
A universal indicator can be a solution or a piece of paper that changes color in a solution and tells you how acidic or basic it is.
Solution:
Thymol blue, bromothymol blue phenolphthalein, and methyl red as a solution are the major components of universal indicators. This combination is crucial since each component loses or acquires protons which depend on the acidity or alkalinity of the solution being analyzed. In a colorless solution, this type of universal indicator is useful. This will improve the precision of the endpoint or indication.
There are several indicators available in the market where you can buy them, although a mixture of thymol blue, methyl red, bromothymol blue, and phenolphthalein is the most commonly used.
Paper form:
It's a piece of colored paper that turns blue when the solution is basic and turns red when the solution is acidic, e.g. litmus paper. To see the color change compared to the reference scale of 01–14, a drop of the sample solution can be placed on the pH paper strip or it may be immersed in the solution. When it comes into touch with the sample solution to be examined, it changes color depending on the pH of the sample solution.
Generally, both types of universal indicators are most commonly used in different classes or laboratory practical’s because they are a simple, fast, and reliable way of measuring acidity or alkalinity.
How does a universal indicator work?
The pH scale may be used to determine whether a solution is acidic or basic. The change of color of the pH indicator is due to the H+ ion being dissociated from the indicator itself. Remember that pH indicators include both natural dyes and weak acids. The weak acid indicator dissociates, causing the solution to change color.
Universal indicator color range or chart:
pH Range
Category
Colour
< 3
Strongly acidic
Red
3 to 6
Weakly acidic
Orange to Yellow
7
Natural
Green
8 to 11
Weak alkali
Blue
> 11
Strong alkali
Indigo to Violet
How to perform universal pH test:
- Liquid pH universal indicators are used by dropping a few drops of the solution into the substance being tested. Generally, it uses a 00.20 ml indicator for every 10.00 ml of sample to be tested.
- For pH universal indicator papers (using a dropper) add a drop of the sample solution that can be placed on the pH paper strip or it may be immersed in the solution.
- The acidity level of a substance is determined by the color that is produced as a result of the reaction.
Uses of universal indicators:
- The basic use of universal indicators is to determine the strength of an acid or basic solution.
- Universal indicators are more used compared to other indicators because of their wide range of results and applications.
- It is commonly used in research, scientific laboratories, to teach in classes (practical) of science colleges, and pharmaceutical industry, etc.
- It is used to determine the acidity or alkalinity of a wide range of substances, including food, sludge, soil, sewage, and water or beverage, etc.
- It may be applied in different types of titration such as acid-base, redox, precipitation, and complexometric to determine whether a reaction (endpoint) is complete.
Advantages of universal indicator:
- The advantage of a universal indicator is that it can indicate a wide range of colors on the pH scale of 01 to 14.
- It works similarly to a pH meter in that there is no waiting for equilibrium and measurements are made quickly.
- It is a simple, fast, and economical method that gives high accuracy results.
FAQ (Frequently Asked Questions):
What color would hydrochloric acid (pH 1) turn universal indicator?
Since it has a very strong pH, HCL causes the universal indicator to turn red.
What is the pH value of dilute HCL on the universal indicator?
Dilute hydrochloric acid has a pH value of 0-02 (acidic) which produces a red color when measured.
Difference between the indicator and universal indicator
Simple acid-base indicators can only be used to determine if a sample is a base or an acid, while the universal indicator is used to evaluate a sample's strength by indicating its pH value.
What are natural indicators?
Natural indicators are indications that can be acquired from plants. Examples: Turmeric, beetroot, red cabbage, cherries, and grape juice.
What is the color of universal indicator in neutral solution?
Green is the color of a universal indicator in a neutral solution.
References:
https://en.wikipedia.org/wiki/Universal_indicator
http://www.idc-online.com/technical_references/pdfs/Theory_of_indicators.pdf
Jorgensen,J.H., Pfaller , M.A., Carroll, K.C., Funke, G., Landry, M.L., Richter, S.S and Warnock., D.W. (2015) Manual., of Clinical Microbiology, 11th Edition. Vol. 1.
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