The steady‐state temperature difference in the Hill‐Baldes technique

Abstract

The molal steady-state temperature differences 6 for a number of solvents have been determined by the Hill-Baldes technique, using a precision themistof bridge. The values of d obtained in the present investigation as well as those available in the literature have been explained on the basis of the theory of evaporation of droplets after introducing a thermal efficiency factor, which is characteristic of the experimental method employed. The application of this technique for the determination of molecular weights in solution is also discussed.Baldes' technique of measuring the steady-state temperature difference, set up between a drop of solvent and a drop of a solution suspended in a chamber saturated with the vapour of the solvent, was first applied by Tay2or and Ha22 1) for the determination of molecular weights of substances in organic solvents. They observed that organic solvents, as compared with water, had a higher proportionality constant between the steady-state temperature difference and the effective molal concentration of solute. Neither these authors nor the subsequent investigators *) 3 ) could account quantitatively for the proportionality constants 8 in terms of the properties of the solvents. Recently4) an empirical relationship was proposed between the observed values of 6 and those evaluated from thermodynamics and the psychometric theory. In the present investigation 8 has been determined for various solvents by means of a thermistor bridge, which was previously used 3) 5 ) 6 ) for the determination of osmotic coefficients and molecular weights in solution. Many refinements have now been effected in this 1) G. B. Taylor and M. B. Hall, Anal. Chem. 23, 947 (1951).

A Thermometric Method for Thermodynamics Studies and Molecular Weight Determinations in Solutions

“…Thermometric determinations of molecular weights under quasi-isopiestic conditions have been carried out by several workers with systems of non-volatile solutes in various volatile solvents. [3][4][5][6][7][8][9] In the various methods used, a steady-state temperature above the ambient temperature is obtained in a partially isolated solution phase ex-posed to the solvent vapor in air. The temperature rise of the solution caused by the vapor condensing on its surface is the basis of the determination.…”

Time dependence and drop size effects in the determination of number-average colecular weight by vapor pressure osmometry

“…Since that time, the basic principles of this method have been explored in some detail with special emphasis on the heat dissipation and mass transfer which take place at the thermistor heat sensing beads (3,4). Several different types of apparatus have been described (3,5,6) employing either thermocouples or thermistors as temperature sensors, but recently a convenient commercial instrument has become available, a vapor pressure osmometer, series 300, from Mechrolab Division of F&M Scientific. In spite of the popularity of this instrument, very little published information has appeared concerning the more practical aspects of its use and it is hoped that the results published here will invoke further critical assessment of the variables which affect molecular weight determinations using this method.…”