OSMOLARITY

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OSMOLARITY

Definition of osmolarity:

The concentration of osmotically active particles expressed in terms of osmoles of solute per litre of solution.

Osmolarity is determined by osmometer. An osmometer is a device for measuring the osmotic strength of a solution, colloid, or compound.

What is osmole?

In chemistry, the osmole (osmol) is a non-SI unit of measurement that defines the number of moles of a chemical compound that contribute to a solution's osmotic pressure. The term comes from the phenomenon of osmosis, and is typically used for osmotically-active solutions. For example, a solution of 1 mol/L NaCl corresponds to an osmolarity of 2 osmol/L. The NaCl salt particle dissociates fully in water to become two separate particles: an Na+ ion and a Cl- ion. Therefore, each mole of NaCl becomes two osmoles in solution. Similarly, a solution of 1 mol/L CaCl2, gives a solution of 3 osmol/L (Ca2+ and 2 Cl-).

A strict definition of an osmole is the amount of osmotically active particles that when dissolved in 22.4 L of solvent at 0 degrees celsius exerts an osmotic pressure of 1 atmosphere. This is equivalent to the observation that 1 mole of a perfect gas occupying a volume of 22.4 L exerts a pressure of 1 atmosphere.

1 osmole = 1 mole of osmotically active particles.

mosmol: A unit of osmotic pressure equal to one thousandth of osmole or osmotic pressure of 0.001 molar solution of a substace that does not dissociate.

 

Formula for Calculating Osmolarity

If one does not have an osmometer, the osmolarity of medium can be increased by a calculated amount using the following method:

1. Use 0.0292 g of NaCl per liter to raise the osmolality 1 mOsmole.

2. Therefore, if you use a 5 M NaCl stock solution (theoretically 10,000 mOsmole), 1 ml/liter will raise the osmolality 10 mOsmole.

 

Example: Medium A has an osmolarity of 300 mOsmole according to the manufacturer's information.

One wishes to study the effect of increasing osmolarity in the range of 300¨C400 mOsmole.

Experimental Conditions: medium A at 300, 325, 350, 375, and 400 mOsmole.

1. Control 300 mOsmole = Medium A with no additions

2. 325 mOsmole = 10 ml medium A + 25 μl sterile 5 M NaCl solution

3. 350 mOsmole = 10 ml medium A + 50 μl sterile 5 M NaCl solution

4. 375 mOsmole = 10 ml medium A + 75 μl sterile 5 M NaCl solution

5. 400 mOsmole = 10 ml medium A + 100 μl sterile 5 M NaCl solution

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