Anomalies in Osmotic Pressure Measurements

Abstract

IN the course of a study of the molecular weights of coal-tar pitch fractions, we have observed unusual anomalies with two samples in benzene solution. A large number of fractions have been examined in benzene and in other solvents using modified Zimm–Meyerson osmometers fitted with specially prepared polyvinyl alcohol membranes¹ of the lowest possible permeability. We and other workers² have found samples of average molecular weight as low as 250 to be retained by the membranes, and nearly all our samples have followed van't Hoff's law almost exactly. As the full results of the investigation are being published elsewhere, attention is now directed only to the behaviour of two fractions produced by molecular distillation of that part of a coke-oven tar pitch which is insoluble in petroleum ether of boiling range 100–120° C. but soluble in benzene.