pH Scale and Indicators Explained
The pH scale measures the acidity or alkalinity of a solution using the concentration of hydrogen ions (H+). pH indicators are substances that change color
TL;DR: The pH scale measures the acidity or alkalinity of a solution using the concentration of hydrogen ions (H+). pH indicators are substances that change…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
pH Scale and Indicators: The pH scale measures the acidity or alkalinity of a solution using the concentration of hydrogen ions (H+). pH indicators are substances that change color
pH Scale and Indicators Explained Simply
The pH scale measures the acidity or alkalinity of a solution using the concentration of hydrogen ions (H+). pH indicators are substances that change color This is a key Chemistry concept for Class 10.
🎬 Watch the animated, step-by-step pH Scale and Indicators diagram →
What is pH?
pH is defined as the negative logarithm of hydrogen ion concentration: pH = -log[H+]. It measures the concentration of H+ ions in a solution. The pH scale ranges from 0 to 14. pH = 7 represents neutral solutions (equal H+ and OH- concentrations), pH < 7 represents acidic solutions (more H+ than OH-), pH > 7 represents alkaline/basic solutions (more OH- than H+). At 25°C, a change of 1 pH unit represents a 10-fold change in H+ concentration.
pH Values and Solution Types
Strong acids have pH 0-2 (e.g., HCl, H2SO4). Weak acids have pH 2-6 (e.g., acetic acid, citric acid). Neutral solutions have pH = 7 (e.g., pure water, certain salt solutions). Weak bases have pH 8-12 (e.g., ammonia solution, sodium acetate). Strong bases have pH 12-14 (e.g., NaOH, KOH). The relationship: pH + pOH = 14, so [H+][OH-] = 10^-14 at 25°C.
pH Indicators and Their Properties
pH indicators are weak organic acids or bases that have different colors in their ionized and unionized forms. They change color within specific pH ranges (transition ranges). Methyl orange (red in acidic, yellow in basic, transition pH 3.1-4.4) is used for strong acid-strong base titrations. Methyl red (red in acidic, yellow in basic, transition pH 4.4-6.2) is used for weak acid-strong base titrations. Phenolphthalein (colorless in acidic, pink in basic, transition pH 8.2-10.0) is used for strong acid-weak base titrations.
Universal Indicator and pH Paper
Universal indicator contains a mixture of several indicators and produces different colors across the pH range 1-14. pH paper is filter paper impregnated with universal indicator. When pH paper is dipped in a solution, it develops a color that is compared to a standard color chart to determine pH. pH paper gives approximate pH values (usually to nearest 0.5-1 unit). Digital pH meters give precise pH measurements but are expensive and require calibration.
Buffers and pH Control
A buffer is a solution that resists changes in pH when small amounts of acid or base are added. Buffers consist of a weak acid and its conjugate base, or a weak base and its conjugate acid. Buffer capacity is the ability to resist pH change. Blood in the human body is buffered by bicarbonate buffer system (H2CO3/HCO3-) to maintain pH around 7.4. Buffers are important in laboratories, medicine, and industry.
Applications of pH in Daily Life
Soil pH affects nutrient availability to plants. Acidic soil (pH < 6.5) requires lime addition. Digestive system requires acidic environment (pH ~2). Water quality for drinking must have pH 6.5-8.5. Shampoos are slightly acidic (pH ~5.5) to protect hair. Swimming pools are maintained at pH 7.2-7.6 for comfort and disinfection efficiency. Agriculture in India uses pH testing to optimize crop productivity.
Worked Examples
Example 1. Calculate pH of a solution with H+ concentration of 1 x 10^-5 mol/L.
Solution: Given: [H+] = 1 x 10^-5 mol/L
Using pH = -log[H+]
pH = -log(1 x 10^-5)
pH = -(-5)
pH = 5
The solution is weakly acidic.
Example 2. A solution has pH = 9. Calculate the H+ concentration and pOH.
Solution: Given: pH = 9
Step 1: Calculate [H+]
pH = -log[H+]
9 = -log[H+]
[H+] = 10^-9 mol/L
Step 2: Calculate pOH
pH + pOH = 14
9 + pOH = 14
pOH = 5
The solution is weakly alkaline with [H+] = 10^-9 mol/L.
Example 3. Identify which indicator is suitable for titration between weak acid and strong base: methyl orange (pH 3.1-4.4), methyl red (pH 4.4-6.2), or phenolphthalein (pH 8.2-10.0)?
Solution: For weak acid and strong base titration:
Weak acid (pH ~5-7) reacts with strong base (pH ~12-14)
Equivalence point occurs in basic range (pH ~8-10)
Phénolphthalein with transition pH 8.2-10.0 covers the equivalence point.
Methyl orange (3.1-4.4) is too low.
Methyl red (4.4-6.2) is too low.
Answer: Phenolphthalein is the suitable indicator for weak acid-strong base titrations.
Where You See This in Real Life
- Indian farmers test soil pH to determine acidity or alkalinity and adjust with lime or sulfur to optimize crop growth.
- Rainfall in industrial areas of India can be acidic (pH < 5.6) due to atmospheric pollution, affecting crops and water bodies.
- Stomach acid has pH around 1.5-3.5, which is necessary for digestion of food. Antacids like calcium carbonate neutralize excess acid when pH becomes too low.
- The water in the Indian Ocean has pH around 8.1-8.3 (slightly basic), while freshwater lakes and rivers typically have pH 6-8 depending on rocks and soil.
Common Mistakes Students Make
- Forgetting that pH is logarithmic, not linear. A pH change from 5 to 4 means 10 times more H+ concentration, not twice.
- Confusing pH and pOH. Remember pH + pOH = 14 only at 25°C in aqueous solutions.
- Using wrong indicator for a titration. The indicator's transition range must overlap with the equivalence point pH.
- Not considering temperature effects. pH and the ion product of water depend on temperature. At higher temperatures, pure water pH is less than 7.
Frequently Asked Questions
Why is milk slightly acidic if no acid is added to it?
Milk contains lactic acid naturally, produced by bacteria during storage. Fresh milk is slightly acidic (pH ~6.6) due to dissolved CO2 and other weak acids, not from added acids.
Can pH be negative or greater than 14?
Yes, pH can be negative for very strong acid solutions with [H+] > 1 mol/L. Similarly, pH can exceed 14 for very strong base solutions. However, the simple pH scale (0-14) applies to dilute solutions at 25°C.
Why do we add indicators to acid-base titrations instead of using pH paper?
pH indicators show a sharp, distinct color change at the equivalence point, making it easy to detect when titration is complete. pH paper gives approximate values and is not precise enough for quantitative analysis.