13. ANTACIDS

Introduction to Antacids

Antacids are alkaline drugs used to neutralize excess gastric acid in the stomach, providing relief in conditions such as hyperacidity, peptic ulcers, and heartburn. In hyperchlorhydria, excessive hydrochloric acid is produced, and antacids help reduce acidity by neutralization.

The efficacy of an antacid is measured by its acid-neutralizing capacity. An ideal antacid should neutralize excess acid without leading to achlorhydria or causing undesirable side effects.

Ideal Properties of an Antacid

  • Insoluble in water
  • Fine particle form for better surface area
  • Non-absorbable and non-toxic
  • Should not cause systemic alkalosis
  • Should act gradually and have prolonged effect
  • Should not cause constipation or laxative effects
  • Stable, readily available, and should not produce gas

Classification of Antacids

1. Systemic Antacids

These are absorbable and water-soluble. They can disturb the body’s acid-base balance and cause systemic alkalosis. Example: Sodium bicarbonate.

2. Non-Systemic Antacids

These are insoluble and non-absorbable, acting locally in the stomach. Examples: Aluminium hydroxide, Magnesium hydroxide, Calcium carbonate.

Common Metal-Based Antacids

  • Aluminium Compounds: Aluminium hydroxide, Aluminium phosphate, Aluminium carbonate (cause constipation).
  • Calcium Compounds: Calcium carbonate, Tribasic calcium phosphate (cause constipation).
  • Magnesium Compounds: Magnesium hydroxide, Magnesium trisilicate, Magnesium carbonate (cause diarrhea).

Combination Antacid Preparations

To minimize side effects, antacids are often combined — for example, aluminium and magnesium hydroxide gel or simethicone-based antacids. – Aluminium and calcium compounds cause constipation, while magnesium causes diarrhea. – Combining them balances the overall effect. – Simethicone is added as an antifoaming agent to suppress gastric gas formation caused by CO₂ during neutralization.

Neutralizing Capacity of Antacids

  • Measured in milli-equivalents (mEq) of hydrochloric acid neutralized per dose.
  • Each antacid product must neutralize at least 5 mEq HCl per dose unit.
  • Normal fasting stomach pH ≈ 3.5.
  • Half-potent antacid = 2.5 mEq/dose; double-potent = 10 mEq/dose.

Official Compounds of Antacids

1. Sodium Bicarbonate (NaHCO₃)

  • Synonym: Baking soda
  • Purity: 99–100.5%
  • Preparation: Formed by carbonation of sodium hydroxide and sodium carbonate with CO₂.
  • Properties: White, odorless crystalline powder; saline taste; soluble in water.
  • Assay: Acid-base titration with 0.5 N H₂SO₄ using methyl orange indicator.
  • Uses: Systemic antacid, buffer component, local treatment for burns, insect bites, ingredient in effervescent mixtures.

2. Potassium Citrate (K₃C₆H₅O₇·H₂O)

  • Synonyms: Tripotassium citrate, Kalii citras
  • Purity: 99–101%
  • Preparation: By neutralizing citric acid with potassium bicarbonate or carbonate, followed by evaporation.
  • Properties: White crystalline powder; saline taste; soluble in water and glycerin.
  • Assay: Acid-base back titration with 0.5 N NaOH after carbonization and reaction with sulfuric acid.
  • Uses: Antacid, osmotic diuretic, expectorant, mild laxative, systemic alkalizer.

3. Aluminium Hydroxide Gel [Al(OH)₃]

  • Purity: 3.5–4.4% Al₂O₃
  • Preparation: Mixing hot solutions of potash alum and sodium carbonate to precipitate aluminium hydroxide.
  • Properties: White viscous suspension, tasteless.
  • Assay: Complexometric titration using EDTA and lead nitrate with xylenol orange indicator.
  • Uses: Slow-acting antacid for hyperchlorhydria and intestinal toxemia; adsorbent for toxins and gases.

4. Dried Aluminium Hydroxide Gel

  • Purity: ≥47% Al₂O₃
  • Properties: Colorless, amorphous powder; insoluble in water; soluble in acids.
  • Assay: Complexometric titration using EDTA and lead nitrate.
  • Uses: Antacid (0.5–1 g/day).

5. Magnesium Hydroxide (Mg(OH)₂)

  • Purity: 95–100.5%
  • Preparation: From seawater treated with calcium hydroxide or during milk of magnesia preparation.
  • Properties: White amorphous powder; slightly alkaline; insoluble in water.
  • Assay: Acid-base back titration with 1 N H₂SO₄ and NaOH.
  • Uses: Antacid and mild laxative.

6. Magnesium Trisilicate (2MgO·3SiO₂·xH₂O)

  • Purity: 29–32% MgO; 65–68.5% SiO₂
  • Preparation: By reacting sodium silicate with magnesium sulfate.
  • Properties: White fine, odorless, tasteless powder; insoluble in water.
  • Assay: MgO determined by complexometry, SiO₂ by gravimetry.
  • Uses: Antacid, absorbent, and emulsifying agent; relieves hyperchlorhydria and duodenal ulcers.

7. Heavy Magnesium Carbonate [3MgCO₃·Mg(OH)₂·5H₂O]

  • Purity: 40–45% MgO
  • Preparation: By double decomposition of magnesium sulfate and sodium carbonate.
  • Properties: White, light powder; insoluble in water.
  • Assay: Complexometric titration using EDTA and murexide indicator.
  • Uses: Antacid (500 mg–2 g); laxative (2–4 g).

8. Light Magnesium Carbonate [3MgCO₃·Mg(OH)₂·3H₂O]

  • Purity: 40–45% MgO
  • Properties: White, odorless, tasteless powder; insoluble in water.
  • Assay: Complexometric titration using EDTA and murexide indicator.
  • Uses: Antacid (500 mg–2 g); laxative (2–4 g).

9. Calcium Carbonate (CaCO₃)

  • Purity: 98–100.5%
  • Preparation: By reacting calcium chloride with sodium carbonate.
  • Properties: White, tasteless crystalline powder; insoluble in water.
  • Assay: Direct complexometry using EDTA and naphthol green indicator.
  • Uses: Antacid and dentifrice; may cause constipation, often combined with magnesium salts.

10. Bismuth Carbonate [Bi₂O₂(CO₃)₂·H₂O]

  • Purity: 90–92% Bi₂O₃
  • Preparation: Precipitated by reacting bismuth salts with sodium carbonate.
  • Properties: White to pale yellow powder; insoluble in water; soluble in dilute HCl.
  • Assay: Gravimetric determination of bismuth oxide.
  • Uses: Astringent, protective, antidiarrheal; used in ulcerative colitis and dysentery.

11. Magaldrate

Magaldrate is a combination antacid containing aluminium hydroxide and magnesium hydroxide. It helps balance the constipating effect of aluminium with the laxative effect of magnesium.

  • Composition: 28–39% MgO and 17–25% Al₂O₃
  • Properties: White, odorless, crystalline powder; insoluble in water; soluble in acids.
  • Assay: 3 g sample dissolved in 100 ml 1 N HCl; excess acid back titrated with 1 N NaOH to pH 3.0 potentiometrically.
  • Uses: Antacid used in treatment of duodenal and gastric ulcers.

Detailed Notes:

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