Isotonicity (Part 1) - Colligative Properties and Tonicity

Colligative Properties of Solutions

• A solution is a mixture of two or more components that is homogeneous down to the molecular level.
• Solvent: the component that determines the phase of the solution and constitutes the largest proportion of the system.
• Solute: dispersed as molecules or ions throughout the solvent; i.e. dissolved in the solvent.

Colligative Properties

• Depend only on the number of particles in the solution, not on their nature.
• Include:
  • Lowering of vapor pressure
  • Depression of freezing point
  • Elevation of boiling point
  • Osmotic pressure

Vapor Pressure

• The vapor pressure (P0) of a liquid is the pressure at which water vapor is in thermodynamic equilibrium with its condensed state.
• The higher the temperature, the higher the vapor pressure increases.

Vapor Pressure Lowering

• When a non-volatile solute is dissolved in a liquid solvent, the vapor pressure of the solvent is lowered.
• This occurs because surface molecules of the solvent are replaced by solute molecules, which do not contribute to the vapor pressure.

Boiling Point Elevation

• Boiling point: the temperature at which the vapor pressure of the liquid becomes equal to an external pressure of 760 mmHg.
• When a non-volatile solute is added to a solvent, the boiling point of the solution is higher than that of the pure solvent.
• An increase in the concentration of the added solute is accompanied by a further decrease in the vapor pressure of the solution and a further elevation in the boiling point.

Freezing Point Depression

• Freezing point: the temperature at which the solid and liquid phases are in equilibrium under a pressure of 1 atm.
• When a non-volatile solute is added to a solvent, the freezing point of the solution is lower than that of the pure solvent.
• Example: salt is added to ice, causing it to melt at a lower temperature.

Osmotic Pressure

• The minimum pressure required to prevent the inward flow of a solution's pure solvent through a semi-permeable membrane.
• Is the pressure that must be applied to the solution to prevent the passage of the solvent through a perfect semi-permeable membrane.
• Solutions of non-electrolytes of the same molal concentration have identical osmotic pressure, but this is not true for electrolytes.

Tonicity

• A measure of the osmotic pressure of two solutions separated by a semi-permeable membrane.
• Is influenced only by solutes that cannot cross the membrane, as only these exert an osmotic pressure.
• Solutes able to freely cross the membrane do not affect tonicity because they will always be in equal concentrations on both sides of the membrane.
• Classified into:
  • Hypertonic solutions (higher salt concentration and higher osmotic pressure than the cells)
  • Isotonic solutions (same salt concentration and same osmotic pressure as the cells)
  • Hypotonic solutions (lower salt concentration and lower osmotic pressure than the cells)

Measurement of Tonicity

• The hemolytic method
• Class I methods
• Class II methods
• Using Van't Hoff equation

Van't Hoff Equation

• π = R T C for non-electrolytes
• π = i R T C for electrolytes
• Where R is the gas constant, T is the absolute temperature, C is the concentration in moles per liter, and i is the number of ions into which the molecule dissociates.

This quiz covers the concept of colligative properties in solutions, with a focus on tonicity. Topics include the definitions of solvent and solute, as well as their roles in determining the phase and composition of a solution.