DIRECT RAMAN DETERMINATION OF THE PARTIAL MOLAL HEATS OF DISSOLUTION OF H2O AND HCL MONOMERS FROM INTENSITY MEASUREMENTS OF THE VAPOR ABOVETHE CONCENTRATED ACID BETWEEN APPROXIMATE-TO-20 AND 110-DEGREES-C

A Raman method was developed for determining the partial molal heats o f dissolution of gaseous species vaporized from liquid mixtures, and t he method was applied to concentrated aqueous HCl solutions. Raman int ensities of the symmetric stretching modes from H2O and HCl at 3652 an d 2886 cm(-1), respectively, were measured from the vapor over concent rated HCl, 18.8-38.0 wt % HCl, between approximate to 20 and 110 degre es C. The In I versus 1/T plots (I is the Raman intensity) yielded the average (approximate to 65 degrees C) heats of dissolution (negative heats of solution), which equal the partial molal heats, because the l iquid concentration remains essentially constant. The (Raman) partial molal heats of dissolution for gaseous HCl are given by 16700 - 439n(2 ), in cal/mol (n(2) = HCl molality). The corresponding function calcul ated from Thomsen's 18 degrees C calorimetric data (ref 2), 17360 - 43 2n(2), differs slightly because different temperatures are involved. T he(Raman) partial molal heats of dissolution of gaseous H2O also agree with the aqueous values (Thomsen's data, Gibbs-Duhem analysis) if the heat of vaporization is added. The new Raman method may be used for a ny solution, provided that the vapor pressures are large enough for Ra man intensity measurements, or it may be useful for inaccessible or hi ghly reactive mixtures by virtue of Raman intensity analysis of their vapors.