Acids, Bases & Buffer Solution

Acids, Bases & Buffer Solution

Acid

Definition

An acid is a substance which:

1. is sour in taste

2. turns blue litmus paper into red

3. contains replaceable hydrogen

4. gives hydrogen ions (H⁺) in aqueous solution (Arrhenius theory)

5. can donate a proton (Bronsted & Lowry concept)

6. can accept an electron (Lewis theory)

Uses of Acids

1. As Food

• Citric acid – lemons or oranges (Citrus fruits)

• Lactic acid - Sour milk

• Butyric acid - Rancid butter

• Tartaric acid - Grapes

• Acetic acid - Vinegar

• Maleic acid - Apples

• Stearic acid - Fats

• Oxalic acid – Tomato, wood sorrel

• Carbonic acid – Soda water, aerated drinks

2. Other Uses

1. Hydrochloric acid (HCl) is used in digestion.

2. Nitric acid (HNO₃) is used in the purification of gold & silver.

3. Conc. H₂SO₄ and HNO₃ are used to wash iron for its galvanisation.

4. Oxalic acid is used to remove rust spots.

5. Boric acid is a constituent of eyewash.

6. Formic acid is present in red ants.

7. Uric acid is present in the urine of mammals

Indicator Properties of an Acid

Strength of Acids

• Strong acid (completely ionised in water) – HCl, HNO₃, H₂SO₄

• Weak acid (partially ionised in water) – CH₃COOH, H₂C₂O₄, HCOOH

Classification of Acids

• Hydra acids – NH₃, H₂S, HCl, HBr, HF

• Oxy acids – HNO₃, H₂SO₄, HClO, HIO

Basicity of an Acid

The number of removable hydrogen ions from an acid is called the basicity of that acid.

• Monobasic acid (one removable H⁺ ion) - HCl, HNO₃

• Dibasic acid (two removable H⁺ ions) – H₂CO₃, H₂SO₄, H₃PO₃

• Tribasic acid (three removable H⁺ ions) – H₃PO₄

Acidic Strength

1. HF < HCl < HBr < HI

2. CH₃COOH < H₂SO₄ < HNO₃ < HCl

Uses of HCl

1. HCl present in gastric juices is responsible for digestion.

2. Used as a bathroom cleaner

3. As a pickling agent before galvanisation

4. In the tanning of leather

5. In the dyeing and textile industry

6. In the manufacture of gelatin from bones

Uses of HNO₃

1. In the manufacture of fertilisers like ammonium nitrate

2. In the manufacture of explosives like TNT, TNB, and Picric acid

3. Nitroglycerine (Dynamite)

4. Found in rainwater (first shower)

5. It forms nitrates in the soil.

6. In the manufacture of rayon

7. In the manufacture of dyes & drugs

Uses of Sulphuric Acid (H₂SO₄)

1. In a lead storage battery

2. In the manufacture of HCl

3. In the manufacture of Alum

4. In the manufacture of fertilisers, drugs, detergents & explosives

5. In a car battery

Uses of Boric Acid

• As an antiseptic

Uses of Phosphoric Acid

1. It's calcium salt that makes our bones.

2. It forms phosphatic fertilisers.

3. PO₄³⁻ is involved in providing energy for chemical reactions in our body

Uses of Other Acids

• Ascorbic acid: Source of Vitamin C

• Citric acid: Flavouring agent & food preservative

• Acetic acid: Flavouring agent & food preservative

• Tartaric acid: 1. Souring agent for pickles 2. Component of baking powder (sodium bicarbonate + tartaric acid)

Bases

Definition

A base is a substance which:

1. is bitter in taste

2. turns red litmus paper into blue

3. gives hydroxyl ions (OH⁻) in aqueous solution

4. can accept a proton (Bronsted & Lowry concept)

5. can donate electrons (Lewis theory)

Indicator Properties of Bases

Other Facts about Bases

• Oxides & hydroxides of metals are bases.

• Water-soluble bases are called alkalies, e.g., NaOH and KOH.

• All alkalis are bases, but not all bases are alkalis because all bases are soluble in water.

Strength of Bases

• Weak bases – NH₄OH, Fe(OH)₃

• Strong bases - NaOH, KOH

Acidity of a Base

The number of removable hydroxyl (OH⁻) ions from a base is called the acidity of a base.

• Acidity of NaOH = 1

• Acidity of KOH = 1

• Acidity of Ca(OH)₂ = 2

The pH Scale

• pH of a solution is the negative logarithm of the concentration of hydrogen ions in moles per litre:
  pH = -log[H⁺]

• pH < 7 → acidic

• pH > 7 → basic

• pH = 7 → neutral

pH Values of Some Common Liquids

Importance of pH in Everyday Life

1. Our body works within a narrow pH range of 7; plants and animals also survive in a narrow range.

2. Digestive system: Hydrochloric acid in the stomach helps in digestion. Excess acid causes pain and irritation. Antacids like milk of magnesia (Mg(OH)₂) are used to adjust pH.

3. Tooth decay prevention: Foods like chocolates and sweets are degraded by bacteria. When pH falls to 5.5, tooth decay starts. Saliva is slightly alkaline, and toothpaste neutralises excess acid.

Buffer Solution

Definition

A solution whose pH is not altered to a great extent by the addition of small quantities of either an acid (H⁺ ions) or a base (OH⁻ ions) is called a buffer solution.

How to Obtain a Buffer Solution

1. Mixing of a weak acid and its salt with a strong base:

• Example: CH₃COOH + CH₃COONa

2. Mixing of a weak base and its salt:

• Example: NH₄OH + NH₄Cl

The buffering action of blood is due to the presence of H₂CO₃ and HCO₃⁻.

Salt

When an acid reacts with a base, salt and water are formed:

Uses of Some Important Salts

1. Sodium Chloride (NaCl)

• As a flavouring agent in food

• In saline water for a patient with dehydration (0.9% NaCl)

• In the manufacture of HCl

2. Sodium Iodate

• Iodised salt to prevent Goitre disease

3. Sodium Carbonate

• As washing soda

• In the manufacturing of glass

4. Sodium Benzoate

• As a food preservative for pickles

5. Potassium Nitrate (KNO₃)

• As a fertilizer giving both K & N

• In gunpowder (C + S + KNO₃)

• In matchsticks

6. Calcium Chloride (CaCl₂)

• Dehydrating agent used for removing moisture from gases

7. Calcium Carbonate (Limestone)

• In the construction of buildings

• In the cement industry

• In the extraction of metals

8. Calcium Sulphate (CaSO₄·2H₂O, Plaster of Paris)

• For moulds & statues

• In the cement industry, in the form of Gypsum

9. Calcium Phosphate

• As a fertiliser (Superphosphate of lime)

10. Bleaching Powder

• As a disinfectant

• As a bleaching agent (removing colours)

11. Other Uses of Sulphates

• In the purification of water

• In the dyeing industry

• As an antiseptic aftershave

The Acidic and Basic Nature of Some Household Substances

Important Points About Acids & Bases

1. Acids are always stored in glass containers, not in metal containers, because they can react with metal.

2. The pH of human blood is 7.4, which is slightly basic.

3. The pH of milk is 6.5, which is slightly acidic.

4. Hydrochloric acid (HCl) destroys bacteria in the stomach.

5. If the pH of a solution is 7, it is neutral.

6. Milk of magnesia (Mg(OH)₂) is an antacid.

7. A sharp pain caused by the sting of an ant is due to formic acid.

8. Gastric juice has a pH of about 1.2, which is acidic.

9. When the pH of rainwater is < 5.6, it is called acid rain. Acid rain lowers the pH of rivers.

10. Sulphuric acid (H₂SO₄) is also known as oil of vitriol.

Final Thoughts

A buffer solution is a special solution that resists changes in pH when small amounts of acid or base are added, and it can be prepared by mixing a weak acid with its salt (e.g., CH₃COOH + CH₃COONa) or a weak base with its salt (e.g., NH₄OH + NH₄Cl). Salts are formed when acids react with bases, producing water, and have various everyday uses.

For example, sodium chloride flavours food and treats dehydration, sodium carbonate is used in washing and glassmaking, and bleaching powder acts as a disinfectant.

Household substances can be acidic, like lemon juice, or basic, like soap. Maintaining proper pH is essential for health, digestion, and preventing tooth decay, while acids like HCl destroy stomach bacteria, and milk of magnesia neutralises excess acid.