Molecular weight determination colligative property measurement

Molecular weight Determination by Colligative Property Measurements... 

Hydroxyl number and molecular weight are normally determined by end-group analysis, by titration with acetic, phthalic, or pyromellitic anhydride (264). For lower molecular weights (higher hydroxyl numbers), 19F- and 13C-nmr methods have been developed (265). Molecular weight determinations based on colligative properties, eg, vapor-phase osmometry, or on molecular size, eg, size exclusion chromatography, are less useful because they do not measure the hydroxyl content. 

Aiken G. R. and Gillam A. H. (1989) Determination of molecular weights of humic substances by colligative property measurements. In Humic Substances II In Search of Structure, (eds. M. H. B. Hayes, P. MacCarthy, R. L. Malcolm, and R. S. Swift) Wiley, Chichester, pp. 515-544. 

The number average molecular weight determines the colligative properties (i.e., those which depend only on the number of dissolved molecules) of polymer solutions. Measurements of freezing point depression (cryoscopy) or... 

The vapour pressure osmometer method is more acceptable of all the methods involving measurement of colligative properties because of the sensitivity of the detector. For ideal solvent-solvents with a low heat of vaporisation, the differential thermistors of the VPO can detect differences in temperature of the order of 0.001°C this sensitivity determines the Molecular weight of the samples upto 20,000. 

Of the preponderance of small ions, the colligative properties of polyelectrolytes in ionising solvents measure counterion activities rather than Molecular weight. In the presence of added salt, however, correct Molecular weights of polyelectrolytes can be measured by membrane osmometry, since the small ions can move across the membrane. The second virial coefficient differs from that previously defined, since it is determined by both ionic and non-ionic polymer-solvent interactions. 

Clearly then, if a colligative property of a polymer solution is measured this provides an estimate of of the solute. The choice of the solution property has determined the average molecular weight that the measurement yields. 

The next example shows applications of Equations 6.5-2 through 6.5-5 to the determination of a vapor pressure and phase-transition temperatures for a known solution concentration, and to the calculation of a solution composition and solute molecular weight from a measured colligative property. 

The molecular weight obtained by measurement of a colligative property is a number-average molecular weight. Two serious problems need to be addressed in determining number-average molecular weights of humic substances ... 

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