1. Introduction
Measuring pH helps us understand how acidic or basic a substance is. In everyday life and in laboratories, we use different tools to check pH, such as litmus paper, pH paper, universal indicators, and digital pH meters.
Each method tells us something about the nature of the solution, from simple colour changes to exact numerical values.
2. Using Litmus Paper
Litmus paper is the simplest way to check whether a substance is acidic or basic. It does not give an exact pH number but tells us the type of solution.
2.1. Blue Litmus Paper
Blue litmus paper turns red when dipped in an acid.
2.2. Red Litmus Paper
Red litmus paper turns blue when dipped in a base.
2.3. Limitation
Litmus can only tell whether a substance is acidic or basic — it cannot show the exact pH value.
3. Using pH Paper
pH paper is more advanced than litmus. It changes colour depending on the exact pH of the solution.
The paper is dipped into the solution, and the resulting colour is compared with a colour chart to find the pH value.
3.1. How pH Paper Works
pH paper contains special dyes that show a different colour for each pH value from 1 to 14.
- Red/orange → Strong acids
- Yellow/green → Neutral or weak acids
- Blue/purple → Bases
3.2. Advantages
- Gives a more accurate pH than litmus.
- Easy to carry and use.
- Safe for quick testing of many substances.
4. Using Universal Indicator
A universal indicator is a mixture of dyes that gives a wide range of colours for different pH values. It can be used as a liquid or on pH paper.
4.1. Colour Range
The universal indicator gives colours that match specific pH values:
- pH 1–3 → Red (strongly acidic)
- pH 4–6 → Orange/yellow (weak acids)
- pH 7 → Green (neutral)
- pH 8–10 → Blue (weak bases)
- pH 11–14 → Purple (strong bases)
4.2. Where It Is Used
Universal indicators are used in classrooms, labs, and environmental testing where a full pH range is needed.
5. Using a pH Meter
A pH meter is a digital instrument that measures pH very accurately. It is commonly used in laboratories, industries, and water testing.
5.1. How a pH Meter Works
The device has a glass electrode that detects hydrogen ion concentration in the solution. The meter then displays the pH on a digital screen.
The reading is highly accurate, usually up to two decimal places.
5.2. Advantages
- Gives exact pH value (e.g., 7.23)
- More accurate than paper methods
- Useful for scientific experiments and industrial processes
5.3. Precautions
- Electrode must be cleaned before and after use.
- The meter should be calibrated regularly.
- Cannot be used in very hot or very dirty solutions.
6. Summary
There are several ways to measure pH:
- Litmus paper → Tells only acid or base
- pH paper → Gives an approximate pH value
- Universal indicator → Shows wide colour range
- pH meter → Gives exact numerical pH
Each method is useful depending on how accurate the results need to be.