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Definition and Measurement

• Osmotic pressure is the pressure needed to prevent solvent movement across a semipermeable membrane, from a lower solute concentration to a higher concentration.
• It is a colligative property, depending on solute particle concentration, not identity.
• Osmometers measure osmotic pressure.
• These usually measure the pressure needed to stop water flowing into a higher-solute-concentration solution through a semipermeable membrane.

Factors Affecting Osmotic Pressure

• Solute concentration: Higher concentration results in higher osmotic pressure (direct proportionality).
• Temperature: Higher temperature increases kinetic energy of particles, leading to higher osmotic pressure (direct proportionality).
• Number of solute particles: More dissolved particles increase osmotic pressure, maintaining constant solvent concentration.

Equations and Calculations

• The osmotic pressure equation, derived from the ideal gas law, is: π = iCRT
  • π = osmotic pressure (Pascals or atmospheres)
  • i = van't Hoff factor (particles per formula unit; e.g., NaCl = 2)
  • C = molar concentration of solute (mol/L)
  • R = ideal gas constant (8.314 J/mol·K)
  • T = absolute temperature (Kelvin)
• The van't Hoff factor corrects for solute dissociation.

Biological Significance

• Cell membrane function: Osmotic pressure maintains cell homeostasis. Different solutions (isotonic, hypertonic, hypotonic) have different pressures, affecting water movement into and out of cells.
• Osmosis in the body: Water movement across membranes, regulated by osmotic pressure, is crucial for nutrient transport, waste removal, and blood pressure maintenance. For example, kidney function, intestinal absorption, and cellular hydration rely on this process.
• Treatment of dehydration: Understanding osmotic pressure assists in creating solutions that balance water in the body for medical treatment.

Applications

• Food preservation: High osmotic pressure draws water from microorganisms and spoilage agents, preserving food. Methods like salting and sugaring use this.
• Water purification: Reverse osmosis uses pressure to force water through a semipermeable membrane, removing dissolved solutes, crucial for water desalination.
• Pharmaceutical formulations: Solutions with specific osmotic pressures are made for intravenous use and other medical applications, regulating solute concentrations in the body without harm.

Isotonic, Hypertonic, and Hypotonic Solutions

• Isotonic solutions: Have the same osmotic pressure as a cell's internal environment, maintaining balance and no net water movement.
• Hypertonic solutions: Have higher osmotic pressure than the cell, drawing water out, causing cell shrinkage (crenation).
• Hypotonic solutions: Have lower osmotic pressure than the cell, drawing water in, potentially causing cell swelling or bursting (lysis).

Differences between Osmotic pressure and Osmosis

• Osmosis: Net water movement across a semipermeable membrane from high water concentration to low concentration, equalizing concentrations.
• Osmotic pressure: The pressure needed to prevent osmosis. It's the driving force behind osmosis.