The Osmotic Balance

Abstract

IN the case of polydisperse high polymers, different methods of molecular weight determination give different kinds of average values¹. The two most important absolute methods are ultra-centrifuging and measurement of osmotic pressure. In the first case one gets, according to the method of calculation used, either weight averages or so-called z-averages, while osmotic pressure measurement always gives number-averages. The knowledge of several different average values for one substance helps us to judge the degree of polydispersity. When dealing with thread-like molecules of high molecular weight, the osmotic measurements are rendered difficult partly by the fact that even comparatively dilute solutions of this type possess high viscosity, partly owing to breakdown of the ideal gas laws at experimentally usable concentrations. One therefore has to resort to a suitable extrapolation procedure for obtaining correct zero-concentration pressure values. These circumstances make it necessary to perform the osmotic measurements at as low concentrations as are experimentally possible.