pH and Buffers – Basic Concepts

pH and Buffers – Complete CSIR-NET Master Notes

Acids, bases, pH, and buffers are fundamental concepts in biochemistry and molecular biology. Every enzyme reaction, protein folding process, and metabolic pathway depends on pH.

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1️⃣ What are Acids and Bases?

Arrhenius Concept

• Acid: Produces H⁺ in aqueous solution
• Base: Produces OH⁻ in aqueous solution

Example:

HCl → H⁺ + Cl⁻

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Brønsted–Lowry Concept (Most Important for CSIR)

• Acid: Proton donor
• Base: Proton acceptor

Example:

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

NH₃ = Base H₂O = Acid ---

Lewis Concept

• Acid: Electron pair acceptor
• Base: Electron pair donor

Very important in enzyme catalysis. ---

2️⃣ Ionic Product of Water (Kw)

Water self-ionizes:

H₂O ⇌ H⁺ + OH⁻

At 25°C:

Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴

In pure water: [H⁺] = 10⁻⁷ M [OH⁻] = 10⁻⁷ M So neutral pH = 7 ---

Kw increases with temperature → Neutral pH is not always 7.

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3️⃣ What is pH?

Definition:

pH = – log [H⁺]

It is a logarithmic scale. Every 1 unit change = 10 fold change in acidity. Example:

• pH 3 → 100 times more acidic than pH 5

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pH of Acid and Base

Solution	[H⁺]	pH
Strong Acid (0.01M HCl)	10⁻²	2
Pure Water	10⁻⁷	7
Strong Base (0.01M NaOH)	10⁻¹²	12

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4️⃣ Ka and pKa

For weak acid:

HA ⇌ H⁺ + A⁻

Acid dissociation constant:

Ka = [H⁺][A⁻] / [HA]

Stronger acid → Higher Ka ---

pKa

pKa = – log Ka

Lower pKa → Stronger acid ---

Very Important Concept

When:

pH = pKa

Then:

[HA] = [A⁻]

This is maximum buffering capacity. ---

5️⃣ Henderson–Hasselbalch Equation

pH = pKa + log ([A⁻]/[HA])

Used to calculate:

• Buffer pH
• Ionization state of amino acids
• Drug absorption

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6️⃣ Buffers

Definition: A buffer is a solution that resists change in pH upon addition of small amounts of acid or base. ---

Types of Buffers

• Weak acid + its conjugate base (Acetate buffer)
• Weak base + its conjugate acid

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How Buffer Works?

If acid added: A⁻ + H⁺ → HA If base added: HA + OH⁻ → A⁻ + H₂O Thus pH remains stable. ---

Buffer Range

Effective when:

pH = pKa ± 1

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Buffer Capacity

Depends on:

• Total concentration
• Ratio of acid/base
• Maximum when pH = pKa

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7️⃣ Biological Buffers

1. Phosphate Buffer

Important inside cells.

2. Bicarbonate Buffer

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Maintains blood pH ≈ 7.4 ---

CSIR loves numerical problems from Henderson–Hasselbalch equation.

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8️⃣ CSIR-NET Important Points

• pH is logarithmic
• Kw changes with temperature
• pKa determines ionization of amino acids
• Buffers work best at pH = pKa
• Enzyme activity is pH dependent

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9️⃣ Practice MCQs

Q1. When pH = pKa, the ratio of [A⁻]/[HA] is:

A) 0.1 B) 1 C) 10 D) 100

Q2. If pH changes from 6 to 4, acidity increases by:

A) 2 fold B) 10 fold C) 100 fold D) 1000 fold

Q3. Buffer capacity is maximum when:

A) pH < pKa B) pH > pKa C) pH = pKa D) pH = 7

Answers: Q1 → B Q2 → C Q3 → C

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🔥 Final Revision in 30 Seconds

• pH = –log[H⁺]
• Kw = 10⁻¹⁴ at 25°C
• pKa = –log Ka
• Buffer works best at pH = pKa
• Biological systems strictly regulate pH