A Convenient Small Osmometer

A description is given of a simple, rugged instrument for determining the osmotic pressure developed by a solution of a high polymer. Other small osmometers described previously [1, 2, 3, 4, 5]/ have been, in some respects, too inconvenient to use. Although relatively easy to make, glass osmometers are fragile and may require frequent repairs. Small metal osmometers, in addition to being rugged, can be designed for easy and rapid assembly and simplicity of operation. The instrument described here is a modification of the apparatus described by Sands and Johnson [4]. The new design permits the instrument to be assembled, rinsed, and filled conveniently, while keeping the membrane wet with solvent at all times., The total volume of the solution cell is less than 2 ml so that a relatively small quantity of polymer suffices for a complete osmotic pressure curve. The Zimm and Myerson osmometer [5] has been used in the Bureau for some time. The outstanding advantages of this instrument are the ease of adjusting the solution meniscus and its relative speed in attaining equilibrium compared with other small osmometers. On the other hand, its assembly is tedious and time consuming. Particularly in rinsing the cell, it is difficult to empty the osmometer without risk of drying the membrane. After the instrument is finally filled with polymer solution, air bubbles must be removed from each osmometer. Because considerable clamping pressure on the membrane (which serves as its own gasket) is required to insure an adequate seal, there is danger that the glass edges may be chipped. This is especially likely if the plane of the membrane is not exactly perpendicular to the axis of the cylinder forming the cell of the Zimm osmometer.