Note on the molecular weight dependence of the calibration constant in vapour pressure osmometry

Abstract

VAPOUR pressure osmometry is widely used for the determination of number average molecular weights, /~,I,, of oligomers and polymers in the lower range of values. Calculation of NIn requires calibration of the apparatus using pure compounds of known molecular weight. Normally, pure compounds of mol. wt in the range 200-600 are used as calibrating substances, and mol. wt up to 20,000 are calculated assuming constancy of the calibration constant over the whole range of molecular weights. C1~ Analysis of the literature data (2,3) and our own experience concerning methods based on Raoult's law, namely ebulliometry and cryoscopy as well as vapour pressure osmometry, reveal that all these methods give too low results for substances of supposedly higher molecular weights. These phenomena might be partly explained by the effect of low molecular weight admixtures or impurities33~ To elucidate the question of constancy of the calibration constant, K, over the whole molecular weight range applicable to vapour pressure osmometry, we have performed a series of measurements for several calibrating substances of known molecular weight. The following substances were used: