Osmometer

An osmometer is a device or instrument for measuring the osmotic value or osmotic pressure of a solution.

Measurement methods

Direct and indirect methods of measurement

The osmotic value ( the osmolarity or osmolality ) of a solution depends on the van 't Hoff - law in addition to the temperature substantially on the number of dissolved particles from ( colligative property). Differences in osmotic value become apparent as the pressure difference between two solutions that are available through a semipermeable membrane with each other. This difference can be measured directly as osmotic pressure. Osmometer that will work according to this principle referred to as membrane osmometer, well-known examples are the Pfeffersche cell and Stoye osmometer. It usually pure water is used as a reference.

As a colligative property of the osmotic value affects the freezing and boiling point of a solution, so that by determining the freezing point depression or vapor pressure lowering an indirect measurement. In the indirect method, no reference solution is required for the measurement, but usually a calibration of the instrument using standard solutions is necessary to have a fixed and known osmolarity.

Static and dynamic measurement method

In Pepper and shear cell and Stoye osmometer measurement can be evaluated only after filling when there is no more pressure change occurs and the thermodynamic equilibrium is reached. Several hours may elapse until equilibrium is reached. This type of measuring is referred to as static. Rapid measurements are possible if the measurement chamber, an external pressure is applied to the just cut off the osmotic flow. In this case, the externally applied pressure corresponds exactly to the osmotic pressure; the measurement is performed dynamically within a few seconds or minutes.

Osmometry

With a known concentration can from the osmotic value of a solution, the molar mass of a substance to be determined (see osmometry ). For measuring the freezing point, this method is referred to as a cryoscopic method.

Historical Background

A first osmometer was described in 1828 by the French botanist Henri Dutrochet in his work Nouvelles Recherches sur l' endosmosis et l' exosmose. When he used an animal bladder membrane whose properties in no way corresponded to an ideal semipermeable membrane. After describing colloidal precipitate membranes by Moritz Traube succeeded Wilhelm Pfeffer these membranes in the pores of Tonzellen store ( Pfeffersche cell), which allowed measurements at relatively high pressures. The preparation of the pepper 's cell was connected with a large expenditure of time and placed high demands on the quality of Tonzellen used. Was further developed by Harmon Northrop Morse the membrane osmometer, who succeeded in particular, to simplify the manufacturing process and to improve the behavior at high pressures.