Acids, Bases and pH

4.1 Arrhenius and Brønsted–Lowry Definitions

Arrhenius (1884):

• Acid: Substance that produces H⁺ (protons) in water. E.g., HCl → H⁺ + Cl⁻.
• Base: Substance that produces OH⁻ in water. E.g., NaOH → Na⁺ + OH⁻.

Brønsted–Lowry (1923) — more general:

• Acid: Proton (H⁺) donor.
• Base: Proton (H⁺) acceptor.

4.2 pH Scale

Developed by Søren Sørensen (1909), the pH (potential of Hydrogen) scale measures hydrogen ion concentration [H⁺] in solution:

pH = −log[H⁺]

pH Range	Nature	Examples
0–2	Strongly Acidic	Hydrochloric acid (0), stomach acid (1.5–2)
3–6	Weakly Acidic	Lemon juice (2–3), coffee (5), urine (6)
7	Neutral	Pure water, blood serum
8–9	Weakly Basic	Seawater (8.1), baking soda solution (8.3)
10–14	Strongly Basic	Soap (10), bleach (11), NaOH (14)

Human body pH values (RPSC-relevant):

Fluid	pH	Significance
Blood	7.35–7.45	Acidosis (<7.35) or Alkalosis (>7.45) = medical emergency
Stomach acid	1.5–2.0	HCl aids protein digestion, kills bacteria
Saliva	6.2–7.4	Amylase starts starch digestion
Urine	4.6–8.0	Varies with diet; kidney regulation
Intestinal fluid	7.0–8.5	Pancreatic bicarbonate neutralises stomach acid

4.3 Indicators and Salts

Indicators are substances that change colour based on pH:

• Litmus (from lichen): Red in acid, blue in base.
• Phenolphthalein: Colourless in acid, pink in base (pH > 8.2).
• Methyl orange: Red in acid (pH < 3.1), yellow in base (pH > 4.4).
• Universal indicator: Shows gradual colour change across full pH range.

Neutralisation: Acid + Base → Salt + Water. HCl + NaOH → NaCl + H₂O.

Important Salts:

• Baking Soda (NaHCO₃): Used in cooking; releases CO₂ when heated.
• Washing Soda (Na₂CO₃·10H₂O): Water softener, cleaning agent.
• Bleaching Powder (Ca(OCl)Cl): Disinfectant, water purification. Made by passing Cl₂ over slaked lime Ca(OH)₂.
• Plaster of Paris (CaSO₄·½H₂O): Used in dentistry, casting; sets hard when water added.

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