Litmus paper is a widely used acid-base indicator that helps determine the pH of a substance. It is commonly seen in blue and red forms, but many people wonder about its natural colour before being treated with chemicals. The natural colour of litmus paper is purple, which results from the pigments derived from lichens.
In this topic, we will explore the origin of litmus paper, its natural colour, and how it changes in different pH environments.
1. What is Litmus Paper?
Definition and Purpose
Litmus paper is a pH indicator made by infusing paper with litmus dye, a natural pigment obtained from lichen species, mainly Roccella tinctoria and Lecanora tartarea.
How Does It Work?
Litmus paper changes colour depending on the acidity or alkalinity of a solution:
- Acidic solutions turn blue litmus paper red.
- Alkaline (basic) solutions turn red litmus paper blue.
- Neutral solutions (pH 7) do not change the colour of either.
2. Natural Colour of Litmus Paper
The Original Colour Before Treatment
Before being processed with acids or bases, litmus paper is naturally purple. This is due to the presence of a mixture of organic dyes extracted from lichens. The purple colour occurs because the litmus solution is naturally at a neutral pH (around 7).
Why is Purple Considered Neutral?
The pigments in litmus paper are sensitive to hydrogen ion concentration (H⁺) and hydroxide ion concentration (OH⁻). In a neutral environment (pH 7), the dyes appear purple, meaning they are balanced between acidic and basic states.
3. Colour Changes of Litmus Paper
Litmus Paper in Acidic Solutions
When purple or blue litmus paper is placed in an acidic solution (pH < 7), it turns red. This happens because:
- Acids increase hydrogen ions (H⁺) in the solution.
- The litmus dye reacts with these ions, causing a colour shift from purple to red.
Litmus Paper in Basic (Alkaline) Solutions
When purple or red litmus paper is placed in a basic solution (pH > 7), it turns blue. This occurs because:
- Bases increase hydroxide ions (OH⁻) in the solution.
- The litmus dye interacts with these ions, shifting the colour from purple to blue.
4. What Affects the Colour of Litmus Paper?
1. Concentration of the Solution
Stronger acids and bases cause a more intense colour change. For example:
- Diluted acids may produce a light red colour.
- Concentrated acids turn the paper deep red.
- Strong bases result in a dark blue colour.
2. Temperature and Environmental Conditions
Extreme temperatures or exposure to moisture can alter the performance of litmus paper.
- High humidity can lead to premature colour changes.
- Prolonged exposure to air may weaken the dye’s reactivity.
3. Presence of Other Chemicals
If the solution contains oxidizers, reducing agents, or contaminants, they might interfere with the dye, causing unexpected colours.
5. Applications of Litmus Paper
1. Laboratory Testing
- Used in schools, colleges, and research labs to test pH.
- Quick and cost-effective method to classify substances.
2. Industrial Uses
- Used in chemical manufacturing to test acidity and alkalinity.
- Helps in quality control for various products.
3. Household Uses
- Testing cleaning products (e.g., soaps and detergents).
- Checking the pH of drinking water.
- Measuring soil acidity for gardening.
6. Difference Between Litmus Paper and pH Paper
While litmus paper indicates whether a substance is acidic or basic, it does not provide an exact pH value.
| Feature | Litmus Paper | pH Paper |
|---|---|---|
| Natural Colour | Purple | Various |
| Acidic Reaction | Turns red | Specific pH range |
| Basic Reaction | Turns blue | Specific pH range |
| Neutral Reaction | Stays purple | Specific shade |
The natural colour of litmus paper is purple, indicating a neutral pH. It is derived from lichen-based dyes and is a simple, effective pH indicator. When exposed to acids, it turns red, and when exposed to bases, it turns blue.
Litmus paper is an essential tool in chemistry, industry, and everyday life, providing a quick way to check the pH of different substances. Understanding how it works can help in scientific experiments, environmental testing, and household applications.