Temperature calibration dropping weight method

Table 3.1. Fusible Link Temperature Calibration Data. Dropping Weight Method (53)...

Vapor Pressure Osmometry. VPO is a very practical method for determining Mn values in a wide range of solvents and temperatures. Recently, results obtained with classical pendant-drop instruments showed a significant dependence of the calibration constant upon the molecular weight of the standards (8,9). On the other hand, with an apparatus equipped with thermistors allowing the volume of the drops to be kept constant, this anomaly is not observed (10,11). 

Another method (ASTM D-2878) provides a procedure to calculate these properties from test data on evaporation. In the method, the sample is dissolved in an appropriate solvent. A drop each of this solution and the solvent are suspended on separate thermistors in a closed chamber saturated with solvent vapor. The solvent condenses on the sample drop and causes a temperature difference between the two drops. The resultant change in temperature is measured and used to determine the molecular weight of the sample by reference to a previously prepared calibration curve. This procedure is based on an older method (ASTM D-972) in which the sample can be partly evaporated at a temperature of 250-500°C (482-932°F), and fluids not stable in this temperature range may require special treatment. 

For molecular weights less than about 20,000, another technique has been automated. In the so-caUed vapor pressure osmometer, there is no membrane [21]. A drop of solution and a drop of pure solvent are placed on adjacent thermistors. The difference in solvent activity brings about a distillation of solvent from the solvent bead to the solution. The temperature change that results from the differential evaporation and condensation can be calibrated in terms of the number-average molecular weight of the solute. The method is rapid, although multiple concentrations and extrapolation to infinite dilution are still required. 

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