pH and Its Measurement
The term pH stands for “potential of Hydrogen” and is a scale used to specify the acidity or basicity of an aqueous solution. Introduced by Danish chemist Søren P. L. Sørensen in 1909, the pH scale ranges from 0 to 14: pH and Its Measurement
- pH < 7: Acidic solutions
- pH = 7: Neutral solution (e.g., pure water)
- pH > 7: Basic (alkaline) solutions
pH Formula
The pH is calculated using the formula:
pH = -log10[H⁺]
Where [H⁺] is the concentration of hydrogen ions in moles per liter (mol/L).
Methods of Measuring pH
1. Litmus Paper
Litmus paper quickly tests acidity. It turns red in acids and blue in bases.
2. Universal Indicator
A universal indicator is a mixture of dyes that changes color over a wide pH range:
| pH Range | Color | Nature |
|---|---|---|
| 0–3 | Red | Strong Acid |
| 4–6 | Orange-Yellow | Weak Acid |
| 7 | Green | Neutral |
| 8–11 | Blue | Weak Base |
| 12–14 | Violet | Strong Base |
3. pH Meter
A pH meter is an electronic device giving an accurate numeric pH using a glass electrode and a reference electrode.
Importance of pH in Everyday Life
- Digestive system: Stomach acid (pH ≈ 1.5–3.5) helps digest food.
- Soil health: pH affects nutrient availability for plants.
- Oral care: Tooth decay starts when oral pH drops below 5.5.
- Water quality: Safe drinking water pH ranges between 6.5–8.5.
Fun Fact
Human blood has a narrow pH range of 7.35–7.45. Even small deviations can be fatal!
Fill in the Blanks
1. A solution with pH 2 is __________. (Answer: Strong Acid)
2. A pH meter uses a __________ and reference electrode to measure pH. (Answer: Glass electrode)
Quiz: pH and Its Measurement
Q1: What does a pH of 7 indicate?
A) Acidic
B) Alkaline
C) Neutral
D) Corrosive
Answer: C) Neutral
Explanation: A pH of 7 indicates a perfectly neutral solution, like pure water.
Q2: Which instrument is used for precise pH measurement?
A) Litmus paper
B) Universal indicator
C) pH meter
D) Thermometer
Answer: C) pH meter
Explanation: A pH meter gives an accurate numerical pH value using electrodes.
Q3: What is the color of universal indicator in a strongly basic solution?
A) Red
B) Green
C) Violet
D) Yellow
Answer: C) Violet
Explanation: Strongly basic solutions (pH 12–14) turn violet with universal indicators.
