Measurements of tonicity

• Measurements of Tonicity can be assessed by measuring osmolality or osmolarity, which indicate the concentration of solute particles in a solution.
• Two common methods for measuring osmolality, and subsequently assessing tonicity, are the haemolytic method and the cryoscopic method.

Haemolytic Method of Measurements of tonicity:

• A biological technique that assesses the tonicity of a solution by observing its effect on red blood cells (RBCs).

Procedure:

• RBCs are exposed to the test solution, and the degree of haemolysis (rupture of RBCs) is monitored.
• In an isotonic solution, RBCs maintain their normal shape with no net water movement.
• In a hypotonic solution, water enters the RBCs, causing them to swell and burst.
• In a hypertonic solution, water leaves the RBCs, causing them to shrink and crenate (shrink).

Conclusion:

• By comparing the haemolysis in the test solution with known solutions, you can determine the tonicity of the test solution.

Cryoscopic Method of Measurements of tonicity:

• Also known as freezing point depression osmometry, this method measures the decrease in freezing point of a solution relative to pure solvent.

Procedure:

• The freezing point of a solution is lower than that of a pure solvent due to the presence of solute particles.
• The amount of freezing point depression correlates with the osmolality of the solution.

Conclusion:

• By calculating the osmolality and comparing it with the osmolality of a reference solution (e.g., blood plasma), the tonicity can be determined.
• Isotonic solutions have similar osmolality to the reference, hypotonic solutions have lower osmolality, and hypertonic solutions have higher osmolality.

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