Colligative properties of biological liquids

Colligative properties are properties of solutions that depend on the number of molecules in a given volume of solvent and not on the properties (e.g. size or mass) of the molecules. Colligative properties include: lowering of vapor pressure; elevation of boiling point; depression of freezing point and osmotic pressure. Measurements of these properties for a dilute aqueous solution of a non-ionized solute such as urea or glucose can lead to accurate determinations of relative molecular masses. Alternatively, measurements for ionized solutes can lead to an estimation of the percentage of ionization taking place. Colligative properties are mostly studied for dilute solutions. Vapor pressure

The relationship between the lowering of vapor pressure and concentration is given by Raoult's law, which states that: The vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution.

The individual vapor pressure p for each component is

p = p0 x , where p is the partial pressure of the component in

mixture, p0 is the vapor pressure of the pure component, x is the mole fraction of the component in solution. Boiling point elevation

Boiling point is achieved in the establishment of equilibrium between liquid and gas phase. At the boiling point, the number of gas molecules condensing to liquid equals the number of liquid molecules evaporating to gas. Adding any solute effectively dilutes the concentration of the liquid molecules, slowing the liquid to gas portion of this equilibrium. To compensate for this, boiling point is achieved at higher temperature.

ΔTb = molality * Kb * i,

(Kb = ebullioscopic constant, which is 0.51°C kg/mol for the boiling point of water; i = Van 't Hoff factor) Van't Hoff factor is the actual number of particles in solution after dissociation : i=1+α(n-1) Freezing point depression

Freezing point, or the equilibrium between a liquid and solid phase is generally lowered in the presence of a solute compared to a pure solvent.

ΔTf = molality * Kf * i,

(Kf = cryoscopic constant, which is 1.86°C kg/mol for the freezing point of water,; i = Van 't Hoff factor) Osmosis

Osmosis is the movement of water across a partially permeable membrane from an area of low solute concentration to an area of high solute concentration.

A semipermeable membrane is a thin layer of material that contains various sized holes, or pores. Smaller solutes and fluid pass through the membrane, but the membrane blocks the passage of larger substances (for example, red blood cells, large proteins) Osmotic pressure

Osmotic pressure is the pressure applied by a solution to prevent the inward flow of water across a semipermeable membrane. The phenomenon of osmotic pressure arises from the tendency of a pure solvent to move through a semi-permeable membrane. This process is of vital importance in biology as the cell's membrane is selective towards many of the solutes found in living organisms. Osmoregulation is the homeostasis mechanism of an organism to reach balance in osmotic pressure.

Morse equation: πV = nRTi, where: π = osmotic pressure; V is the volume; T is absolute temperature; n is the number of moles of solute; R = 8.3145 J K-1 mol-1, the molar gas constant; i = Van 't Hoff factor. Osmoregulation in mammalian cells

Ion Concentration in Concentration in cytosol (millimolar) blood (millimolar) Potassium 139 4 Sodium 12 145 Cloride 4 116 Bicarbonate 12 29 Amino acids in 138 9 proteins Magnesium 0.8 1.5 Calcium <0.0002 1.8

The loss of sodium and chloride ions compensates for the osmotic effect of the higher concentration of organic molecules inside the cell. Balance in osmotic pressure

Hypertonic solution causes cells to shrink. Hypotonic solution causes cells to swell. Isotonic solution produces no change in cell volume. Dialysis

In medicine, dialysis is primarily used to provide an artificial replacement for lost kidney function in people with renal failure.

Substances in water tend to move from an area of high concentration to an area of low concentration. Hemodialysis remove wastes and excess water from the blood.

A hemodialysis machine