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Emetics: Mechanism & Compounds
Comprehensive exam notes on Emetics. Understand the mechanism of vomiting (CTZ vs Local), and the detailed preparation & assay of Copper Sulphate (*).
Definition: Emetics are drugs or chemical agents that induce vomiting (emesis). They are primarily used in emergency situations to forcefully expel swallowed poisons or unabsorbed toxic substances from the stomach before they can enter the bloodstream.
Mechanism of Action:
Emetics induce vomiting via two main pathways:
- 1. Central Action: Drugs absorb into the blood and directly stimulate the Chemoreceptor Trigger Zone (CTZ) located in the medulla oblongata of the brain. (e.g., Apomorphine).
- 2. Local / Reflex Action: Drugs heavily irritate the gastric mucosa (stomach lining). This irritation sends nerve signals to the vomiting center in the brain, triggering a reflex to vomit. (e.g., Copper Sulphate, Mustard water, concentrated salt water).
Clinical Alert: When NOT to use an Emetic
Emetics must NEVER be given in the following cases of poisoning:
- Corrosive Poisons (Strong Acids/Alkalis): Vomiting will burn the esophagus and throat a second time while coming out.
- Petroleum Products (Kerosene, Petrol): High risk of aspiration (fumes entering the lungs), causing fatal chemical pneumonia.
- Unconscious or Convulsing Patients: Risk of choking on their own vomit.
Compounds marked with an asterisk (*) in the PCI syllabus require you to thoroughly study their Preparation and Assay.
Properties
- Formula: CuSO₄·5H₂O
- Synonyms: Blue Vitriol, Roman Vitriol, Cupric Sulphate.
- Appearance: Deep blue, large triclinic crystals. Highly soluble in water.
Preparation
It is prepared on a commercial scale by roasting copper scrap with sulfur to form Copper(II) Oxide (CuO), which is then dissolved in hot dilute Sulfuric acid.
Assay of Copper Sulphate (Iodometry)
The assay of Copper Sulphate is a classic example of an Iodometric (Indirect) Titration.
Principle of Assay:
Cupric ions (Cu²⁺) act as an oxidizing agent. When reacted with excess Potassium Iodide (KI) in an acidic medium, Cu²⁺ is reduced to Cu⁺ (forming a white precipitate of Cuprous Iodide, Cu₂I₂), and an equivalent amount of Iodine (I₂) is liberated.
The liberated Iodine is then titrated against standard Sodium Thiosulphate (Na₂S₂O₃) solution.
Indicator: Starch solution (added near the end point when the color fades to pale yellow). End point is the disappearance of the blue-black color.
Note: Often, a little Ammonium Thiocyanate (NH₄SCN) is added near the end point. This is to release any iodine that gets physically trapped inside the Cu₂I₂ precipitate, ensuring a sharp and accurate end point.
Uses of Copper Sulphate:
- Emetic: Used in doses of 300 mg (dissolved in water) to induce vomiting in poisoning cases.
- Astringent & Fungicide: Applied topically for skin/fungal infections.
- Reagent: It is a key ingredient in Benedict's and Fehling's solutions used to test for reducing sugars (like glucose in urine).
Properties & Formula
- Formula: NaKC₄H₄O₆·4H₂O
- Synonym: Rochelle Salt, Seignette salt.
- Appearance: Colorless, transparent crystals or white crystalline powder. It possesses a saline, cooling taste.
Uses
- Mild Emetic / Purgative: Although primarily a Saline Cathartic (laxative) that draws water into the intestine, in large specific doses, it can induce a mild emetic reflex.
- Chemical Reagent: It is an essential component of Fehling's Solution B. It acts as a complexing agent to keep cupric ions in solution at high alkaline pH.