In a strict sense, osmometry is measurement of osmotic pressure.
But in many labs (clinical, environmental etc) the osmometer reading is interpreted as
a measure of total concentration of solute particles. Interconnections between Osmotic
Pressure, Solute Concentration and Colligative Properties. Osmotic
pressure is one of four COLLIGATIVE PROPERTIES of solutions:
- Osmotic pressure
- Depression of vapour pressure
- Elevation of boiling point
- Depression of freezing point
Colligative = property determined ONLY by concentration of solute particles in the
solution.
Many other properties (density, absorbance, viscosity etc) depend on solute
concentration but also depend on nature of solute particles. They are NOT
colligative.
Osmolality
Osmolality is a concentration scale to express total concentration of solute
particles - directly related to any of the 4 colligative properties.
It is total particle concentration that matters, so one must consider:
- Does the solute dissociate?
A non-electrolyte eg glucose = only 1 particle per dissolved molecule. So,
for a glucose solution, osmolarity = mole.L-1 concentration.
Electrolytes dissociate in solution ---> more than one particle. eg NaCl = 2
particles per dissolved molecule(Na+ & Cl-), so osmolarity =
2 x (mole.L-1 concentration).
- Liquids expand when heated
Need concentration scale that doesn't depend on volumes ---> temperature
independent. Why? Colligative properties are measured at different temperatures
(freezing pt depression at <0°C, osmotic pressure at 37°), and different from temp.
at which solutions prepared.
Molality (m) - concentration scale not based on volumes, so temperature
independent.
| Molality = Moles of solute per Kg of solvent |
Osmolality is derived from molality by factoring in the dissociation of
electrolytic solutes. ie m x no. of particles/molecule.
Solutions display thermodynamic non-ideality
Dissolved particles interact with one another, so that their effective concentration or thermodynamic
activity is less (usually) than the particle concentration.
This affects all thermodynamically related properties eg equilibrium positions of
reactions, and the set of 4 colligative properties (which arise out of a solute's effect
on Free Energy of the solvent).
For colligative properties, non-ideality is corrected by the OSMOTIC COEFFICIENT (f).
Overall, Osmolality (q) = osmotic coefficient x
particles/molecule x molality
Measurement of Osmolality
Why measure osmolality?
Because it is the determinant of osmotic pressure, which determines movement of water in
and out of cells. Hence...
Clinically, indicator of electrolyte imbalances, kidney function etc.
Environmentally, relevant to species adaptation to aquatic conditions, eg
salinity.
How to measure osmolality:
In theory, measure any colligative property:
- Boiling Pt elevation not favoured for biological samples (structural degradation)
- Osmotic pressure slow to equilibrate, need large volume, membrane behaviour not
always reproducible.
- V.P. depression most accurate, but slow and requires very precise temperature
control.
- Freezing Pt depression (cryoscopic method), convenient, rapid, small volumes.
Favoured method.
Cryoscopic Osmometry
Supercool sample, then freeze by vibration or ice crystal. F.Pt =
plateau in freezing curve.
DT, the difference in freezing point between sample solution
and pure water, is directly related to the solution osmolality by:
where R = Gas constant (8.3 Joule.K-1.mole-1), Tf
= Freezing point of water (273.2K), Lf = Latent heat of melting for water (3.33
x 105 Joule.Kg-1)
The factor RTf2/Lf is
constant for aqueous samples (evaluates to 1.86 K.Kg.mole-1) and is built into calibration
of instrument ---> readout in osmolality.
As well as measuring osmolality as a physiological parameter, cryoscopic osmometry can
be used for MOLECULAR MASS DETERMINATION, to follow POLYMER FORMATION OR BREAKDOWN,
monitor TITRATIONS where the NO. OF PARTICLES in the solution CHANGES.
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