Molecular weights from boiling-point elevation

Traditional methods for measuring molecular weight date from the dawn of modem physical chemistry in the 19th century. These relied on so-called colligative property measurements (see box opposite), such as boiling point elevation and freezing... 

A knowledge of the boiling-point elevation and the appropriate constant from Table 1-4 allows one to calculate the molality of the solution, and this figure coupled with the weight of material added gives the molecular weight. The values thus determined are usually accurate to about 10 per cent. 

For high-molecular-weight substances like orange juice and other organic colloids we can neglect boiling-point elevation and heat of dilution. Then the enthalpies of the feed and the liquor are essentially the properties of pure water under those conditions. The enthalpy of liquid water and water vapor can be read from the Steam Tables found in Appendices A.2-9 and A.2-10 (pages 857-861). ... 

Because the vapor is frequently water, which has a low molecular weight and a high specific volume, compressors are usually quite large and costly. Compressors require high purity of the vapor to avoid buildup on the blades of solids that result from evaporation of liquid as the vapor is superheated by compression. Liquids having high boiling point elevations are... 

The measurement of an osmotic pressure can also be carried out more accurately than can the measurement of a boiling-point elevation or a freezing-point depression. One difficulty in measuring very small osmotic pressures is the long time required for the system to reach equilibrium. This difficulty is sometimes overcome by imposing a pressure on the solution side of the membrane and observing how the rate of flow of liquid varies over time. The osmotic pressure can be calculated from this variation. Molecular weights of up to 3 000000 have been measured by the use of such techniques. 

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... 

The elevation of boiling point resulting from the addition to the solvent of a known weight of solute, enables the molality of the dissolved substance to be calculated, and hence the molecular weight of the solute if its weight concentration is known. This is the principle of the ebullioscopic method of determining molecular weights. 

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