Osmolality measurements using a osmometer, are regularly carried out in the pharmaceutical industry and research labs to establish the isotonicity of intravenous solutions, injections, nasal and eye drops. Preparations which are hypertonic or hypotonic can cause severe irritation when injected into the bloodstream or tissue or when applied to mucous membranes like cornea and wounds.
Basic principles of Osmometry
Osmosis is defined as the passage of solvent into a solution through a semi permeable membrane. This process eventually leads to equilibrium and the tendency of the solvent to move from one side of the membrane to the other can be measured by the closely related colligative property called osmotic pressure. Osmotic pressure is classically expressed in terms of atmospheres, but in clinical practice, it is expressed in terms of osmols (Osm) or miliosmols (mOsm).
Osmolarity is used more frequently than osmolality in the labelling of parenteral solutions in, pharmaceutical manufacturing, hospitals and pharmacies. But osmolarity cannot be measured and must be calculated experimentally by determining the osmolality of a solution.
For near ideal solutions, osmolarity is different from osmolality only by 1 or 2% , but as you go towards more concentrated solutions of polyvalent electrolytes together with buffers, preservatives and other ions these differences become larger.
An osmole is a unit of measurement that defines the concentration of a compound that contributes to osmotic pressure of a solution. It is commonly expressed in concentration unit of moles/liter (osmomolarity) or in moles/kg (osmomolality). Measurements of osmotic pressure are relevant to biological fluids such as blood and urine.
For accuracy in the preparation and labelling of parenteral solutions osmolality should be measured carefully with the vapour pressure or freezing point based osmometer rather than being calculated based on the concentrations of ingredients added.
Normal ranges of Osmolality
The osmolality of blood, measured using osmometer, ranges from 250 to 350 mOsm/kg and the normal osmolarity of body fluids given in medical books ranges from 275 to 295 mOsm/kg, but normal values usually fall in an even narrower range of 286±4 mOsm/kg. Parenteral preparations are developed to closely match the values to reduce the irritation caused by them
Osmolality measurement using osmometer is a pharmacopoeia requirement for numerous products. The United States Pharmacopoeia (USP) gives the option of using either vapour pressure based osmometer or a cryoscopic (depression increasing point based) osmometer for the measurement of osmolarity whereas the Indian Pharmacopoeia (IP) describes the cryoscopic osmometer for the measurement of osmolarity.