Ebulliometry Boiling Point Elevation

In applying this method, the boiling point of a solution of known concentration is compared to that of the solvent at the same pressure. For ideally dilute solutions, the elevation of the boiling point, T -Tb, is related to the normal boiling temperature of the solvent Tb, its molar latent heat of evaporation Lg, and molecular weight Ml, and also to the molecular weight of the solute M2, and relative weights of solvent and solute Wi and W2, respectively, by 

Some other values of kg calculated in this way are water 0.51, acetic acid 3.0, benzene 2.5, acetone 1.7, methyl alcohol 0.8, chloroform 3.8, carbon tetrachloride 5.0. 

To determine a molecular weight, one measures isTb for a dilute solution of solute in solvent and calculates m2 from Eq. (4.31). The molecular weight M2 of the solute then equals the value 

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Colligative properties are dependent on the number of particles present and are thus related to M . M values are independent of molecular size and are highly sensitive to small molecules present in the mixture. Values of are determined by Raoult s techniques, which are dependent on colligative properties such as ebulliometry (boiling point elevation), cryometry (freezing point depression), osmometry, and end-group analysis. 

Raoult s law works for small polymers as well as small molecules. Determination of M is based for both ebulliometry (boiling point elevation) and cryometry (freezing point lowering) on the Clausius-Clapeyron equation ... 

Methods for the determination of Molecular weight based on colligative property are vapour-pressure lowering, boiling point elevation (ebulliometry), freezing-point depression (cryoscopy), and the Osmotic pressure (osmometry). 

The difference (G — mi) is measured by the difference [(F -l- r)— Pi]/V, in the osmotic pressure experiment. Other colligative properties are similarly measured in terms of the difference between a property of the pure solvent and that of the solvent in solution, at a particular concentration and common temperature. Specifically, boiling point elevation (ebulliometry) measurements result in... 

Ebulliometry Number average molecular weight Boiling point elevation <10 ... 

Ebulliometry. Ebulliometry (34,42-47) is another technique for determining the depression of the solvent activity by the solute. In this case the elevation of the boiling point is determined. The boiling-point elevation ATb is measured with sensitive thermocouples or matched thermistors in a Wheatstone bridge. The molecular weight Mn is calculated from... 

Ebulliometry n Method of measuring molecular weight of polymers under 20,000 based on vapor pressure lowering and boiling point elevation (Pethrick RA and Dawkins JV (1999) Modem techniques for polymer characterization. Wiley, New York). 

Measurements of the elevation of solvent boiling point ebulliometry) and of the depression of solvent freezing point (cryoscopy) caused by the presence of a polymeric solute enable M to be evaluated in a similar way to VPO. J4owadays, however, such measurements are rarely used to evaluate M for polymers and so will be given only brief consideration here. 

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