Determination of freezing point depression constant using Beckmann’s procedure

Determination of freezing point depression constant using Beckmann’s procedure

1. A given mass of the solvent (m₁) is placed in glass tube A into which a thermometer is immersed. This tube is fitted with a wire stirrer, s₁, and a side arm, m, through which the solute can be introduced.

Apparatus

2. the tube A is surrounded by a wider tube, B, which provides an air jacket to ensure uniform cooling
3. The freezing agent is contained in vessel C consisting of a mixture of ice and salt and is kept stirred by stirrer S₂.
4. The freezing point of the pure solvent T₁, is determined by stirring both freezing agent and the solvent and recording the constant temperature at which a liquid turns into a solid.
5. After determining the freezing point of a solvent the solid is warmed to melt.
6. A given mass of solute m₂ is introduced into the solvent through side arm, m, and stirred until completely dissolved.
7. Repeat procedure 4 and note the temperature (T₂) at which pure solvent alone separate out from solution as solid.

Treatment of results

Mass of solvent                                              = M₁

Mass of solute                                                 = m₂

∆t                                                                       = (T₁-T₂)

Molecular mass of solute                              = Mr

NB. this method is ideal for solvents with small depression constants.

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