Lesson 5


                                             Osmometry


Read the text in your group and try to fill the gaps.


Osmometry is a technique for m_ _ _ _ _ _ _g  the concentration of particles in a s_ _ _ _ _ _ _n,
i.e., osmolar concentration.

When a s_ _ _ _ e is dissolved in a pure s_ _ _ _ _ t, the following properties of the solvent are
changed:

The f_ _ _ _ _ _g p_ _ _ t is depressed;

the b_ _ _ _ _ g p_ _ _ t is raised;

the o_ _ _ _ _ c p_ _ _ _ _ _ e is increased;

the v_ _ _ _ r p_ _ _ _ _ _ e is lowered.


The f_ _ _ _ _ _g p_ _ _ t of pure w_ _ _ r is precisely 0 ^oC at atmospheric pressure.

Ideally, 1 mol of a non-dissociating s_ _ _ _ e such as glucose, dissolved in 1 kg of water,
depresses the f_ _ _ _ _ _g p_ _ _ t by 1.86 ^oC.

The f_ _ _ _ _ _g p_ _ _ t depression also depends upon the degree of dissociation of the s_ _ _ _
_ e.

For e_ _ _ _ _e, if 1 mol of sodium chloride were to completely dissociate into two i_ _ _ c
species (Na^+ and Cl^-) in 1 kg of water, the freezing point would _ _ depressed by 3.92 ^oC.


The freezing point o_ _ _ _ _ _ _r is the most commonly used m_ _ _ _ d for measurement of
osmolality in the chemistry laboratory. When using this d_ _ _ _ e, a patient s_ _ _ _ e is s_ _ _
_ _ _ _ _ _ d below its freezing point; the sample in the measurement cell is still fluid. In the
measurement chamber, immersed in the sample, is a t_ _ _ _ _ _ _ _ e sensing thermistor and a s_ _
_ _ _ _ g w_ _ e. When the wire agitates the supercooled sample begins to f_ _ _ _ e. The process
of freezing releases h_ _t and the supercooled solution warms to its freezing temperature.

Supercooling

The tendency of a substance to remain in the l_ _ _ _ d state when cooled b_ _ _ w its freezing
point.


Crystallisation temperature

Aqueous s_ _ _ _ _ _ _ _s can be induced to freeze (i.e. crystallise) most reliably when
supercooled. Supercooled c_ _ _ _ _ l formation is induced by agitating the solution (freeze
pulse).