Aspects of vapor pressure osmometry

Abstract

SynopsisThree aspects of vapor pressure osmometry were examined: drop size, solute volatility, and the constancy of the calibration factor. It was shown that there is a drop size effect which is dependent on the solute concentration. Solutes having vapor pressures as low as 0.3 mm Hg under the operating conditions were found to give unsatisfactory results. The calibration factor was determined using eight compounds in the molecular weight range of 128 to 883 in chloroform, toluene, and methyl ethyl ketone a t several temperatures; a small but significant dependence of the calibration factor on molecular weight was demonstrated. The results fitted, with a high degree of accuracy, the relation log A, = a + b log (AV/C)c=o, where a and b are constants for a given solvent/temperature combination. However, measurements with standard polystyrene samples, using extrapolations of this relationship to higher molecular weights, revealed considerable discrepancies in results for the one compound under different solvent/temperature combinations.

Raoult's law‐based method for determination of coal tar average molecular weight

“…The coal tar from site 1 was highly viscous with a taffy‐like consistency, and the potential interferences with the VPO method described earlier may have resulted in this discrepancy. This low estimate from the VPO method has been observed for other relatively high‐molecular weight compounds, where VPO measurements have given a low average molecular weight compared to other methods [43]. This is an important consideration when using the VPO method, as use of the average molecular weight determined from the VPO method for site 1 in Equation 7 will underpredict the aqueous concentrations by an order of magnitude, and this can have significant impacts if the values are used in risk assessments or to develop remediation scenarios for contaminated soils.…”

“…The VPO method often is applied to coal tars and asphaltenes due to its simplicity. Potential interferences with this method include the presence of volatile solvents in the sample [45], of which coal tar contains benzene, toluene, ethyl‐benzene, and xylene, among others; variation of the calibration factor with the molecular weight of the sample [43,44]; existence of surface films [45]; differences in diffusion coefficients in the mixture and solvent [45]; and differences in drop sizes and shapes [43–46]. The choice of solvent [38,39,42,44,47,48] and operating temperature [38,39,48] also have been shown to impact the measured molecular weight.…”

“…Chem. 24, 2005 1889 sizes and shapes [43][44][45][46]. The choice of solvent [38,39,42,44,47,48] and operating temperature [38,39,48] also have been shown to impact the measured molecular weight.…”

“…Coal tar average molecular weight commonly is determined using vapor pressure osmometry (VPO) [7,40–42]. With the VPO method, the average molecular weight is calculated from the change in temperature that occurs as a solvent condenses onto a solute (e.g., coal tar) droplet [38,43]. This measurement is based on a colligative property of solutions and, therefore, leads to the number average molecular weight via Equation 1 [44].…”

Accuracy, Repeatability, and Limitations for Determination of Chemical Activities from Vapor Pressure Osmometry

“…For low concentration samples, the dilution due to the condensation is not apparent over the measurement interval, but for more concentrated samples, there is a noticeably decreasing signal. Although both dilution and volatile contaminants in the sample cell have been cited to cause decreasing signal in VPO, 7,8 the correlation with sample concentration suggests that dilution is the more probable explanation. Normally, the sample is injected and the signal rapidly increases and then levels off to a plateau.…”