Measurement of elevation of boiling point
In dilute solution, the elevation of boiling point is very small which necessitates the use of very sensitive thermometers. The Beckmann thermometer, which is most widely used for the purpose, will be described below.
Beckmann thermometer: Beckmann thermometer is a differential thermometer. It was designed to record a very small change in temperature, and never measures the actual temperature of the solvent or of the solution. Several designs of this apparatus are now commercially available. One of the common forms is shown in Figure.
Figure: Beckmann thermometer
The thermometer essentially consists of n large bulb B at the bottom and a reservoir at mercury R at the top. The thermometer is calibrated from 0 to 6 K and the wale S is subdivided into 0.01 K The amount of mercury in the two reservoirs can be adjusted by careful manipulation depending on whether ebullioscopy or cryoscopy measurements are carried out.
Beckmann’s Method: The Beckmann apparatus generally used for measuring the elevation of temperature it even in the figure. A known volume of a given mass of the solvent is first taken into the boiling tube to fill about two-thirds of its capacity.
The boiling tube is connected to a condenser on one side, and there is a small side tube, E, for introducing the solute. The boiling tube is generally surrounded by an insulated jacket. The Beckmann thermometer is introduced into the boiling tube through a rubber stopper in such a way that the lower bulb completely immerses into the solvent.
Figure: Complete Beckmann apparatus
The heating of the boding tube is carried out by a Bunsen burner or by electric means. At first, the thermometer reading corresponding to the boiling of the pure solvent is recorded, and then the experiment is repeated after addition of a known mass of a solute. The difference of Beckmann reading in these two experiments gives a measure of elevation of boiling point. Since the mass of the solute, solvent, and K for the solvent are known, the relative molar mass of the solute can easily be determined.