Osmometer
An osmometer is a device or measuring device for determining the osmotic value or osmotic pressure of a solution .
Measurement method
Direct and indirect measurement methods
The osmotic value (the osmolarity or osmolality ) of a solution depends, according to van-'t-Hoff's law, in addition to the temperature , essentially on the number of particles dissolved in it ( colligative property ). Differences in the osmotic value become noticeable as the pressure difference between two solutions that are connected to one another via a semipermeable membrane . This difference can be measured directly as the osmotic pressure . Osmometers that work according to this principle are called membrane osmometers , well-known examples are the Pfeffer's cell and the Stoye osmometer . Pure water is usually used as a reference.
As a colligative property, the osmotic value influences the freezing and boiling points of a solution, so that an indirect measurement is possible by determining the lowering of the freezing point or the lowering of the vapor pressure . In the case of the indirect method, no reference solution is required for the measurement, but in most cases the measuring device needs to be calibrated using standard solutions that have a fixed and known osmolarity.
Static and dynamic measurement methods
With the Pfefferscher cell and Stoye osmometer, the measurement can only be evaluated after filling when there is no longer any pressure change and the thermodynamic equilibrium has been established. Several hours can pass before equilibrium is reached. This type of measurement is called static . Faster measurements are possible if an external pressure is applied to the measuring chamber, which just interrupts the osmotic flow. In this case the externally applied pressure corresponds exactly to the osmotic pressure; the measurement takes place dynamically within a few seconds or minutes.
Osmometry
If the concentration is known, the molar mass of a substance can be determined from the osmotic value of a solution (see osmometry ). When measuring the depression of the freezing point, this procedure is known as cryoscopy .
Historical background
A first osmometer was described in 1828 by the French botanist Henri Dutrochet in his work Nouvelles Recherches sur l'Endosmose et l'Exosmose . He used an animal bladder as the membrane , the properties of which by no means corresponded to those of an ideal semipermeable membrane. After the description of colloidal precipitation membranes by Moritz Traube , Wilhelm Pfeffer succeeded in storing these membranes in the pores of clay cells ( Pfeffer's cell ), which made measurements possible at relatively high pressures. The production of Pfeffer's cells was very time-consuming and made high demands on the quality of the clay cells used. The membrane osmometer was further developed by Harmon Northrop Morse , who succeeded in particular in simplifying the manufacturing process and improving the behavior at high pressures.
literature
- Béla G. Lipták (Ed.): Analytical Instrumentation. Chilton Book Company, Radnor PA 1994, ISBN 0-8019-8397-5 .
Individual evidence
- ^ Henri Dutrochet: Nouvelles recherches sur l'endosmose et l'exosmose: suivies de l'application expérimentale de ces actions physiques à la solution duproblemême de l'irritabilité végétale et à la détermination de la cause de l'ascension des tiges et de la descente des racines . J.-B. Baillière, Paris 1828 ( full text in the Google book search).
- ^ Wilhelm Pfeffer. Osmotic studies . Wilh. Engelmann, Leipzig 1921. (2nd, unchanged edition of the first print from 1877).
- ^ Harmon Northrop Morse: The Osmotic Pressure of Aqueous Solutions: Report on Investigations Made in the Chemical Laboratory of the Johns Hopkins University During the Years 1899-1913 . Carnegie institution of Washington, 1914.