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DrX Whiz Niraj
Antacids: Ideal Properties & Combinations
Comprehensive exam notes on Gastrointestinal Agents: Antacids. Understand Ideal Properties, Combinations, and the Assay of Sodium Bicarbonate (*).
Antacids are weak alkaline (basic) inorganic substances that are used to neutralize excess hydrochloric acid (HCl) in the stomach. They are used to relieve symptoms of hyperacidity, heartburn, acid indigestion, and peptic ulcers.
No single antacid meets all ideal requirements. However, a perfect antacid should ideally possess the following characteristics:
- 1. It should be insoluble in water so that it does not absorb systematically into the blood.
- 2. It should not cause systemic alkalosis (making blood basic).
- 3. It should act rapidly but provide a prolonged duration of action.
- 4. It should not produce a large volume of CO₂ gas in the stomach (which causes belching/burping).
- 5. It should not cause constipation or diarrhea (should have no laxative or constipating effects).
- 6. It should not cause Acid Rebound (a sudden surge in acid production after the antacid wears off).
- 7. It should be palatable (taste good) and inexpensive.
Since no single antacid is perfect, they are often combined to balance out their side effects. The most classic combination is between Aluminum compounds and Magnesium compounds.
Why combine Al and Mg?
Magnesium salts (like Magnesium Hydroxide) act very fast but cause severe Diarrhea (Laxative effect).
Aluminum salts (like Aluminum Hydroxide) are slow-acting but cause severe Constipation.
When combined together (e.g., Digene, Gelusil), the fast action of Mg and slow action of Al provide prolonged relief, and the diarrheal effect of Mg is perfectly cancelled out by the constipating effect of Al, resulting in normal bowel function!
1. Sodium Bicarbonate (NaHCO₃)*
Also known as Baking Soda. It is a Systemic Antacid (highly soluble in water and absorbs into the blood).
- Disadvantage: Causes systemic alkalosis. Releases massive amounts of CO₂ in the stomach leading to belching.
- Assay: Acid-base titration against standard 0.5N Sulphuric Acid (H₂SO₄) using Methyl Orange indicator.
2. Aluminum Hydroxide Gel
Formula: Al(OH)₃. It is a Non-Systemic Antacid (insoluble, so it stays in the gut).
- Disadvantage: Causes severe constipation. It also binds to dietary phosphate in the gut, leading to phosphate deficiency (hypophosphatemia).
3. Magnesium Hydroxide Mixture
Formula: Mg(OH)₂. Also famously known as Milk of Magnesia.
- Nature: Non-systemic antacid.
- Disadvantage: Has a strong osmotic laxative effect, causing diarrhea.
Assay of Sodium Bicarbonate (*)
Viva Alert: Acid Rebound
Acid Rebound is a phenomenon where the stomach produces an abnormally high surge of acid after the antacid effect wears off. This is most commonly seen with Calcium Carbonate and Sodium Bicarbonate. For this reason, they are not preferred for long-term ulcer treatment.