EUROPEAN PHARMACOPOEIA 7.3 2.2.35. Osmolality
01/2012:20235
2.2.35. OSMOLALITY
Osmolality is a practical means of giving an overall measure of
the contribution of the various solutes present in a solution to
theosmoticpressureofthesolution.
An acceptable approximation for the osmolality ξmof a given
aqueous solution is given by:
If the solute is not ionised,
υ
=1;otherwise
υ
is the total
number of ions already present or formed by solvolysis from
1 molecule of solute.
m=molality of the solution, that is the number of moles
of solute per kilogram of solvent;
=molal osmotic coefficient which takes account of
the interactions between ions of opposite charge in
the solution. It is dependent on the value of m.As
the complexity of solutions increases, becomes
difficult to measure.
The unit of osmolality is osmole per kilogram (osmol/kg),
but the submultiple milliosmole per kilogram (mosmol/kg) is
usually used.
Unless otherwise prescribed, osmolality is determined by
measurement of the depression of freezing point. The following
relationship exists between the osmolality and the depression of
freezing point ∆T:
Apparatus. The apparatus (osmometer) consists of :
— a means of cooling the container used for the measurement ;
— a system for measuring temperature consisting of a resistor
sensitive to temperature (thermistor), with an appropriate
current or potential-difference measurement device that
may be graduated in temperature depression or directly in
osmolality ;
— a means of mixing the sample is usually included.
Method. Prepare reference solut ions as described in
Table 2. 2.35.-1, as required. Determine the zero of the apparatu s
using water R. Calibrate the apparatus using the reference
solutions : introduce a suitable volume of sample into the
measurement cell, as indicated by the equipment supplier,
and start the cooling system. Usually, the mixing device is
programmed to operate at a temperature below that expected
throug h cryoscopic depression to preve nt supercooling. A
suitable device indicates attainment of equilibrium. Before each
measurement, rinse the measurement cell with the solution to
be examined.
Table 2.2.35.-1. – Reference solutions for osmometer
calibration
Mass in grams of
sodium chloride R
per kilogram of
water R
Real
osmolality
(mosmol/kg)
Ideal
osmolality
(mosmol/kg)
Molal osmotic
coefficient
Cryoscopic
depression
(°C)
3.087 100 105.67 0.9 463 0.186
6.260 200 214.2 0 0.9337 0.372
9.463 300 323.83 0.9264 0.558
12.684 400 434.07 0.9215 0.744
15.916 500 544.66 0.9180 0.930
19.147 600 655.24 0.9157 1.116
22.380 700 765.86 0.9140 1.302
Carryoutthesame operationswiththetestsample. Read
directly the osmolality or calculate it from the measured
depression of freezing point. The test is not valid unless the
value found is within 2 values of the calibration scale.
General Notices (1) apply to all monographs and other texts 3777
