Osmometry by Dr. Anurag Yadav

Osmometer Principle, Types and mechanism of working .
application of osmometer.

1. Presenter : Dr Anurag Yadav Post-Graduate O smometry

2. Osmometer : is a device for measuring the osmotic strength of a solution, colloid, or compound. Osmometry : is a technique for measuring the conc of solute particle that contribute to the osmotic pressure in a solution.

3. Terminology Osmotic Pressure: is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane. Also defined as the measure of the tendency of a solution to take in water by Osmosis. Osmotic pressure governs the movement of solvent across membrane that separate two solutions.

4. Colligative properties are used for the measurement of the osmolality of solution (when solute is added);

5. Osmolarity: is defined as the number of osmoles of solute per  liter   (L) of solution  ( osmol /L or Osm /L). Osmolality: is a measure of the  osmoles  ( Osm ) of solute per  kilogram  of  solvent  ( osmol /kg or Osm /kg). Osmolality is thermodynamically more exact expression because solution concentration expresses on weight basis are temp independent ; whereas those based on volume vary with temperature. Osmolality is what the clinical laboratory measures.

6. Determination of Plasma and Urine Osmolality Major osmotic substance in normal plasma are sodium, chloride, glucose and urea. mOsmol /kg = Osmolal gap is difference between the measured osmolality and calculated osmolality. Theoretically all four Colligative properties could be used as basis to measure osmolality.

7. Types

8. 1. Boiling Point Elevation Boiling Point elevation is not useful for clinical samples because proteins will coagulate, causing gross changes in sample composition.

9. 2. Freezing Point Depression Osmometer Principle : the freezing point of a solution is related to the osmotic concentration of that solution. If concentration of the solute is increased, it will lower its freezing point. Normal Pure solvent (water) freezing point is - 0.53 C Most commonly used method for measuring the osmolality of serum or urine in clinical laboratory.

10. Components: Cooling fluid. Stirring rod. Thermistor. Galvanometer. Potentiometer.

11. Process :

12. 3. Vapor Pressure Osmometer Measurement related to the decrease in dew point of temp of pure solvent (water) caused by decrease in vapor pressure of solvent by the solute. Drawback: measurement of any of volatile solute in serum not better. (volatile gases if present will increase the vapor pressure of solvent) Hence not recommended for clinical laboratory.

13. 4. Membranous Osmometer Is a device used to indirectly measure the number average molecular weight of a polymer sample. One chamber contains the pure solvent and other a solution in which the solute polymer with unknown MW. The osmotic pressure of the solvent across the semipermeable is measured.

14. solvent is permitted through the membrane, a change in concentration causes the solvent to diffuse to the solute side of the chamber through the membrane separating it. As this occurs, the pressure of the solvent decreases until the pressure difference across the membrane just counteracts the chemical potential difference caused by the solute. Hence, this change in pressure is measured.

15. Different membrane Osmometry devices:

16. Application Normal values Urine Osmolality: 24hr specimen- 300-900 mOsm/kg of H 2 O Random sample- 50-1200 mOsm/kg of H 2 O After 12hr fluid restriction: >850 mOsm/kg of H 2 O Serum Osmolality: Adult – 280-303 mOsm/kg of H 2 O Newborn – upto 266 mOsm/kg of H 2 O Urine-serum ratio: 1:1 to 3:1

17. Osmolal gap : Abnormal levels >10 mOsm/kg of water Methanol Ethanol Isopropyl alcohol Mannitol Sever illness with shock, Lactic acidosis& renal failure. Interfering factor- Intravenous sodium administration. Intravenous Dextrose and Normal saline administration. Effect of anticoagulant: Heparin +0 mOsm/kg EDTA +15-30 Fluoride oxalate +150-300 Idoacetatic acid +5-10

18. Application Urine osmolality Serum osmolality Stool osmolality Clearance studies Monitoring electrolyte imbalance ADH & fluid therapy

19. Other Application Blood alcohol Various other fluid (CSF/sweat/gastric juices or any other) Dialyzer Fetal maturity Chromatographic elutes Quality control

20. Reference: Tietz – Clinical Chemistry And Molecular Diagnostic Keith Wilson and John Walker – Principle And Technique In Biochemistry And Molecular Biology. Kaplan : Clinical Chemistry Fischbach : A Manual of Laboratory and Diagnostic Tests Internet sources. THANK YOU