Beckmann thermometer is used to measure small temperature changes in the freezing point of pure solvent and solution. Beckmann thermometer is not used in determining the absolute value of freezing temperature of the solvent or that of the solution. It is therefore called a differential thermometer. Temperature differences of even 0.01K can easily be measured.
Beckmann thermometer (Fig.11.4) consists of a large thermometer bulb at the bottom of a free capillary tube (ii) which is connected to a reservoir of mercury (i) placed at the top. As the capillary has fine bore, a small change of temperature causes a considerable change in the height of mercury column (level) in the capillary. The whole scale of a Beckmann thermometer covers only about 6K. Initially the level of mercury in the capillary should be on the scale. This is achieved by transferring mercury from the lower bulb to the reservoir and viceversa. When the Beckmann thermometer is used at high temperatures, some of the mercury from the thermometer bulb is transferred into the upper reservoir. At lower temperature mercury from the reservoir falls down in to the thermometer bulb.
Measurement of freezing point depression by Beckmann method
A simple Beckmann apparatus is shown in Fig.11.5. It consists of a freezing tube (a) with a side arm (c) through which a known amount of a solute can be introduced. A stopper carrying a Beckmann thermometer (b) and a stirrer (d) is fitted in to the freezing tube. To prevent rapid cooling of the contents of the freezing tube, A, a guard tube (e) surrounds the tube so that there is an air space between a and e. This assembly, as a whole, is placed in a wide vessel V which contains a freezing mixture (f) maintaining a low temperature around 5C below the freezing point of the pure solvent.
A known weight of the pure solvent is placed in the tube (a). It is cooled with gentle and continuous stirring. As a result of super cooling, the temperature of the solvent will fall by about 0.5 deg C below its freezing point. Vigorous stirring is then set in when solid starts separating and the temperature rises to the exact freezing point. This temperature remains constant, for some time, until all the liquid solvent gets solidified and is noted as To.
The tube (a) is taken out, warmed to melt the solid and a known weight of the solute is added through the side arm (c). When the solute is dissolved in to the solvent forming a solution, the tube (a) is put back in to the original position and the freezing point of the solution (T) is redetermined in the same manner as before. The difference between the two readings gives the freezing point depression (∆Tf).
Depression in freezing point ∆Tf = To-T. From this value, the molecular mass of the non-volatile solute can be determined using the expression and known Kf value.
M2 = Kf . W2 / ∆Tf .W1
Solvent F. Pt. K Kf (K.kg.mole-1)
Acetic acid 289.60 3.90
Bromoform 281.30 14.30
Benzene 278.53 5.10
Cyclohexane 279.55 20.20
Camphor 451.40 37.70
Naphthalene 353.25 7.00
Nitrobenzene 278.70 6.90
Phenol 314.10 7.27
Water 273.00 1.86