NEW PROCEDURE FOR MOLECULAR-WEIGHT DETERMINATION BY VAPOR-PHASE OSMOMETRY

Inconsistent results on molecular weights determined by different methods reported in the recent literature prompted this study. A detailed investigation of the behaviour of benzil-chloroform solutions and fourteen other systems resulted in a major revision of the experimental procedure used in the VPO method for molecular-weight determination. The revision was based on a clear recognition of a 'rectilinear region' and a 'curved region' in the relation between the osmometer readout (ΔV) and non-volatile solute concentration. Only the rectilinear region shows a direct relation between ΔV and the number of solute molecules in an infinitely dilute concentration range, a fact which has not been clearly identified before. A method of determining a concentration range for the rectilinear region was devised and used to develop a simpler experimental procedure. Molecular weights of eleven model compounds were checked using the new procedure. Molecular weights of two coal-derived liquid fractions were also determined and the presence of association in their mixture was examined. Shortcomings in the present theory and two other experimental procedures are pointed out. Molecular weights of model compounds including those in the experiments quoted ranged from 122 to 1238. © 1979.