Calibration of vapor pressure osmometers for molecular weight measurement

Abstract

Experimental molecular weights in the range 200 to 100,000 Daltons have been determined by vapor pressure osmometry using three different solvents. For all values in excess of 10,000, molecular weights were also determined by membrane osmometry. The general agreement found for molecular weights determined under a variety of conditions, leads to the conclusion that values as high as 100,000 Daltons can be determined by at least one type of vapor pressure osmometer when it is calibrated with a material of 200 Daltons. Thermistor self‐heating and diffusion in the liquid phase are shown to be unimportant for this particular instrument.

Molecular Weight Determination

“…For the Wescan instrument and another research vapor-pressure osmometer (69), both based on the same design (70), the calibration constant was independent of the material. These instruments have determined polystyrene Mn values of 400,000 (71), 100,000 (72), and 100,000 (69); polyolefins have been characterized at 140 • C (73) and styrene/methylmethacrylate copolymers (32). For higher molecular weight solutes, the virial coefficients become important.…”

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

“…7−13 The severity of these effects is dependent on the instrument design, 5,11,14,15 and careful design of the measurement cell and electronics 5,6,11 can promote accurate molecular weight determination. 11,16 Despite the reported issues, VPO can yield accurate chemical activity values and osmotic coefficients. 2,3,9,17,18 In particular, the work of Widera et al demonstrates that VPO can be more precise than direct vapor pressure measurements.…”

“…21 Noticeable signal fall off has also been observed, 22 in line with speculation that designs with mesh caps may be more prone to sample dilution compared to the hanging-drop design. 11 Those reasons spur this attempt to empirically assess the accuracy of chemical activity and osmolality measurements made using the UIC Jupiter 833 osmometer.…”

“…14 For molecular weight measurements, on the other hand, the vapor pressure reduction due to the solute build up in the reservoir is unlikely to significantly change the accuracy. 11 Additionally, the changing viscosity, heat transfer, and diffusion rates in the liquid drop could begin to significantly affect the accuracy of the measurement. While a calibration measurement can empirically correct for some of these changes, the problem would be more pronounced as the properties of the solutions of interest and the calibration solutions diverge.…”

“…The accuracy of a vapor pressure osmometer can be impacted by a number of conditions including sample dilution, drop size effects, thermistor self-heating, bridge voltage dependency, and solute build up in the reservoir. − The severity of these effects is dependent on the instrument design, ,,, and careful design of the measurement cell and electronics ,, can promote accurate molecular weight determination. , Despite the reported issues, VPO can yield accurate chemical activity values and osmotic coefficients. ,,,, In particular, the work of Widera et al demonstrates that VPO can be more precise than direct vapor pressure measurements.…”