A Comparative Method for Measuring Aqueous Vapor and Dissociation Pressures with Some of its Applications

Abstract

The method here described is a comparative one because the aqueous pressure over an aqueous sulfuric acid solution, whose concentration can later be found, becomes equal at the same temperature to the aqueous pressure of the system under investigation. Since the variation of aqueous pressure with temperature of the two systems thus under comparison is, fortunately, very similar, refined temperature control is not a necessity. Measurements may therefore be made with ease up to temperatures well over 100°C., at which close control of temperature over lengthy periods has proved difficult to most workers. It is in part due to this difficulty that records of measurements of dissociation pressures of salt hydrate systems are scanty as well as notoriously discordant for temperatures much above room temperature, and we believe that this situation can be remedied by use of the method here described. It is essential to realize, however, that in measurements of dissociation pressures, there enters at the lower temperatures another little understood source of error which must be avoided, especially in dealing with systems of lower hydrates which of necessity exliibit very large surface areas when prepared by efflorescence from higher hydrates. The errors in question have been attributed to adsorbed water (23) and are here further discussed in the light of experimental results which we believe are little vitiated by the better recognized sources of error.We shall show below the application of our procedure to the measurement of the aqueous pressures of saturated solutions and to the dissociation pressures of salt hydrate systems. COMMENTS UPON CERTAIN OTHER METHODSThe first three methods mentioned below, because absolute rather than relative or comparative, require accurate temperature control, which is the more difficult the higher the temperature.1 From a thesis presented by Edward M. Collins to the Faculty of Princeton University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, January, 1933.

Vapor pressures and isopiestic molalities of concentrated CaCl2(aq), CaBr2(aq), and NaCl(aq) to T=523 K

“…The parameters in equations (16) to (20) for CaCl 2 (aq) and CaBr 2 (aq) are given in table 3 figure 3 is the locus of the saturated solutions, essentially in agreement with the CODATA review [10]. At temperatures above 500 K, however, the uncertainty of the solubility increases significantly.…”

“…It was somewhat surprising that the several sets of data reported between room temperature and the lower limit of both the present data and the data of Holmes et al (about 380 K), at high molalities exceeding 4.5 mol AE kg À1 , were in substantial disagreement with each other. This was the main reason why, besides the interpolation between the present experimental results and accurate data at T = 298.15 K and below, we decided to rely upon the older data for saturated solutions [15,16,59]. In this region only the two results of Bechtold and Newton [60], recalculated by Anathaswamy and Atkinson [5] for 7 mol AE kg À1 CaCl 2 at T = (308.15 and 318) K, were found to be consistent with all the key results.…”

“…The maximum differences between the datasets shown in figure 6 exceed 0.1 at both temperatures, but at T = 323.15 K (the top set of curves) there is very good agreement between our model and the model of Ananthaswamy and Atkinson at molalities up to 7.5 mol AE kg À1 . There is little doubt that the behavior of the model of Ananthaswamy and Atkinson at higher molalities, characterized by a rather sudden turn down, is due to the absence in their database of the saturated-solution data [15,16] and the inclusion of the T = 323.15 K results of Jakli and van Hook who reported the high-molality plateau of / at only 2.72 at this temperature. On the other hand, the high-molality values of the osmotic coefficient recommended by CODA-TA represent an extrapolation of the results of Holmes et al [7,11] to the saturated-solution data, with a relatively high uncertainty (±0.14) noted by the authors on the basis of their substantial disagreement with Jakli and van Hook [68].…”

“…The experimental data used by Pitzer and Oakes [13] above 373 K included vapor pressures over saturated solutions measured directly 125 years ago by Roozeboom [15] at temperatures up to 473 K and isopiestically by Collins and Menzies [16] (over 66 years ago) at temperatures up to 398 K. The only newer data consisted of several vapor pressures at T = 523.15 K reported by Ketsko et al [17] and Valyashko et al [18]. The reason for this scarcity of good data is that the simple isopiestic method with external weighing is increasingly more inconvenient to use as temperature increases significantly above ambient, while the accuracy required for derivation of meaningful osmotic coefficients from directly measured vapor pressures at low and moderate concentrations is difficult to achieve [19,21].…”

“…Only the most careful experiments using well-designed equipment and including pressure gauges capable of high precision in a wide range of pressures can be expected to yield direct vapor pressure results of the quality that is necessary to significantly contribute to the results currently available and their extrapolations. Since both Pitzer and Oakes [13] and Sterner et al [14] model parameters are based on the same sources [15][16][17] containing a small number of high temperature and high molality experimental data that are all subject to question, these treatments could not be considered accurate enough for support of CaCl 2 (aq) as a primary isopiestic reference standard.…”