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4.35 OSMOLALITY

4.35 OSMOLALITY

      Osmolality is a practical means of giving an overall measure of the contribution of the various solutes present in a solution to the osmotic pressure of the solution.

      An acceptable approximation for the osmolality (ξm) of a given aqueous solution is given by:

      ξm = vmΦ

where m = molality of the solution, that is the number of moles of solute per kilogram of solvent,

          Φ = 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, Φ becomes difficult to measure. 

      If the solute is not ionized, v is 1; otherwise, v is the total number of ions already present or formed by solvolysis from one molecule of solute.

      The unit of osmolality is osmole per kilogram (Osmol kg–1), but the submultiple milliosmole per kilogram (mOsmol kg–1) 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 ΔT:

Apparatus

      The apparatus (osmometer) consists of:
      (a) a means of cooling the container used for the measurement,
      (b) a system for measuring temperature consisting of a resistor sensitive to a temperature (thermistor), with an appropriate current or potential difference measurement device that may be graduated in temperature depression or directly in osmolality,
      (c) a means of mixing the sample is usually included.
Method
      Prepare reference solutions as described in Table 1, as required. Determine the zero of the apparatus using distilled water. Calibrate the apparatus using the reference solutions: introduce 50 to 250 μl of the solutions into the measurement cell and start the cooling system. Usually, the mixing device is programmed to operate at a temperature below that expected through cryoscopic depression to prevent supercooling. A suitable device indicates attainment of equilibrium. Before each measurement, rinse the measurement cell with the solution being examined.
      Carry out the same operations with the test sample. 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 two values of the calibration scale.
Table 1 Reference Solution for Osmometer Calibration

 

APPENDICES • 4.35 OSMOLALITY
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หมายเหตุ / Note : TP II 2011 PAGE 465-466