A new cryoscopic micro-method for the determination of molecular weights

Abstract

INTItODUCTIONThe known cryoscopic methods for the determination of molecular weights arc based on the determination of initial freezing or final melting points. These methods may give reliable results if solute and solvent form no solid solutions, the heat of dilution is ncgligiblc and solid and liquid are physically stable near the melting point. But thcrc is little or no mformation given as to whether these conditions are fulfilled or not.The method described in this paper is based on the determination of a tcmperatureheat content curve (melting curve). This curve permits the calculation of molecular weights even when mixed crystals occur and the heat of dilution is not negligible. Further, it yields information as to whether the various equilibrium conditions are sufficiently approached. IThe method permits extrapolation to infinite dilution and is less open to expcrimental errors than the known methods.The determination of a temperature-heat content curve presents no serious difficulty. With the aid of a simple apparatus' the curve can be obtained within two hours. Only 200-300 mg of solvent and r-3 mg of solute are required.Uriefly, the method is as follows. A heating cuxvc permits the determination of the equilibrium temperature of solid and liquicl as a function of the fraction of heat of melting consumed. This fraction is nearly equal to the fraction which is molten. (This approximation is not actually introduced in our theoretical considerations and serves only for the preliminary picture.)The substance of which the molecular weight is to be determined isdistributed between the solid and the liquid phase of the solvent. (When no mixed crystals occur the solute is entirely dissolved in the liquid phase of the solvent.)As the melting proceeds a series of concentrations of the solute and the pertaining depressions is obtained. This series permits extrapolation to zero concentration. As will be shown m the theoretical part it is more exact to plot amounts of heat consumed as a function of the depression. THEORl%TICAL PARTThe molecules of a polar substance when dissolved in a solvent may show a strong tendency to mutual association. This mutual association may be largely prevented when solvents showing association with the solute arc applied. Association with the excess solvent hardly effects the molecular ratio of solvent and solute; moreover its And Chnr.Acta, 22 (xg60) 8-16

Purification of Monoglycerides From Palm Stearin by Liquid–Liquid Extraction With Aqueous Ethanol

“…Because the model that was used to correlate the experimental liquid–liquid equilibrium data is expressed in terms of mole fractions (see ahead in this paper), it was necessary to determine the average molecular weight for the mono‐, di‐, and triglyceride pseudocomponents. Molecular weights can be experimentally determined by several methods, such as ebullioscopy (Kennedy, Fries, Sullivan, & Willingham, ; Mair, ), cryoscopy (Mikulak & Runsquist, ; Smit, Ruyter, & Van Wijk, ), and vapor pressure osmometry (Brzezinski, Glowala, & Kornas‐Calka, ; Elias, ). However, as a first approximation, we found useful an estimation procedure based on the fatty acid profile of each pseudocomponent, as determined by gas chromatography.…”