Molecular weight determination of macromolecules by vapor pressure osmometry

A new vapor pressure osmometry (VPO) apparatus having a quite good sensitivity was constructed on the basis of a Hitachi 115 molecular weight measurement apparatus. In order to match a pair of thermistors, a conventional Wheatstone bridge circuit was modified by introducing a matching resistor, which enabled us to detect the temperature difference of ca. 6. 10 −6 °C. By employing this apparatus the measurement of the temperature difference Δ T s between a solution drop and a solvent drop at steady state was carried out as a function of concentration with solutions of the three normal alkanes (C 18 , C 28 and C 36 ) dissolved in the normal alkanes (C 6 , C 8 and C 10 ), and of eight atactic polystyrenes (the number‐average molecular weights ranging from M̄ n = 2030 to 4,1.10 4 ) in benzene over a wide temperature range. The ratio of the calibration parameter K s to the corresponding constant K e (≡( RT V 0 )/Δ H , R = gas constant, T 0 = measuring temperature in K, V 0 = molar volume of solvent, Δ H = partial molar heat of condensation), K s / K e , which represents the efficiency of measurement was found to be about 0,5, almost independent of the molecular weight of the solute, ranging from 200 to 20.10 4 . The maximum number‐average molecular weight measurable by this apparatus with a precision of ±10% was estimated to be ca. 1·10 5 . The time necessary to reach a steady state was about 9 min regardless of the molecular weight.