A comparison of different membranes for osmotic determination of the molecular weight of nitrocellulose

Osmotic determinations of the degree of polymerization of cellulose nitrate have been performed using different types of cellulosic membranes. For unfractionated samples the results were dependent upon the type of membrane, whereas for fractionated samples good agreement was obtained for different membranes. The leakage of nitrocellulose through the membrane has been determined. In separate experiments the solution used for measurements in osmometers with coarse membranes were transferred to osmometers with tighter membranes. A considerable increase in osmotic pressure was observed in the latter case; this shows that the lower osmotic pressure obtained in osmometers with nonselective membranes cannot be ascribed solely to a lowering in the molar concentration because of leakage. The results are in agreement with the theory postulated by Staverman. Plots of osmotic pressure against time as well as direct determinations of the leakage give insufficient information about the error obtained when using nonselective membranes.