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The apparatus used in the determination of molecular weights from the depression of freezing point is shown in.
 | Fig: Beckmanns freezing-point apparatus |
A very sensitive thermometer, called a Beckmann thermometer, D, is used, which has a large bulb and only six degrees on the whole scale, the latter being graduated in hundredths of a degree. There is a reservoir at the top of the capillary tube, into which mercury can be shaken if higher temperatures are used (e.g., phenol, m. pt. 40°), or from which mercury can be drawn into the tube and bulb if lower temperatures (e.g., water, 0°) are to be used. The actual readings on the scale are of no consequence, only their difference, D, is required. About 20 gm. of the solvent are weighed into the tube A, and a stirrer of bent wire introduced. The thermometer is fitted into the tube through a cork, so that the bulb is covered with the liquid.
The tube A is fitted through a cork into a large test-tube, B, which serves as an air-jacket, and prevents too rapid fall in temperature. The tube B is supported in a freezing mixture (e.g., ice and salt) contained in the large jar, C. The stirrers in the solvent tube and outer jar are worked up and down, and the thermometer observed. The mercury falls steadily to a certain point, when the solvent is slightly supercooled. Freezing then commences, the temperature at once runs up to the freezing point, and afterwards remains stationary. It is then read off with a lens, the thermometer being gently tapped to prevent any adhesion of the mercury to the glass. Suppose the reading is 3.216°.
The tube A is then taken out and allowed to warm until the solvent liquefies. A weighed quantity of the substance under investigation is introduced through the side tube, and dissolved by working the stirrer. The tube is replaced in the air-jacket, and the latter again put into the freezing mixture. The process is carried out further exactly as with the pure solvent, and the freezing point of the solution read off. Suppose this to be 2.839°; then D, the depression of freezing point, is 3.216 - 2.839 = 0.377°.
A mixture of ice and salt is used in the outer jar if the solvent is water; ice and water are used for benzene, acetic acid, and formic acid; phenol is melted in warm water, and the inner tube and air-jacket supported in a clamp without outer jar. Acetic and formic acids, and phenol readily absorb moisture, which lowers their freezing points. Care must be taken to prevent this occurring during the experiment.
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