ตำรายาของประเทศไทย
Thai Pharmacopoeia
สำนักยาและวัตถุเสพติด กรมวิทยาศาสตร์การแพทย์ กระทรวงสาธารณสุข
Bureau of Drug and Narcotic, Department of Medical Sciences, Ministry of Public Health4.35 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