COMPOSITION OF ORGANIC COMPOUNDS 



43 



2. Raoult-Beckmann Method. Substances dissolved in a liquid 

 lower its freezing-point. It was shown by Raoult that the freezing- 

 point was lowered the same number of degrees when weights of 

 different substances proportional to their molecular weights were dis- 

 solved in the same volume of liquid. Each liquid was found to have 

 a definite freezing-point. By employing this value as a constant, the 

 molecular weight of an unknown substance can be found. It is given 

 by the formula 



M = 



IPO x C x w 



where C is the constant, w the weight of the substance, W the weight 

 of the solvent, and d the depression of the freezing-point. 



The constants are: water 19 benzene 49 

 acetic acid 39 phenol 76 



Conversely by determining the lowering of the freezing-point, the 

 amount of salt in a solution can be ascertained, e.g. in serum, urine. 



The apparatus (Fig. 22) devised by Beckmann con- 

 sists of the freezing-point tube C with side opening D. 

 It is closed by a cork through which a Beckmann ther- 

 mometer T and a stirrer S (through a glass tube) pass. 

 The Beckmann thermometer is a large thermometer 

 graduated usually in -^^ parts of a degree and having 

 a range of only 5-6 degrees. 1 The tube C is placed in 

 a wider tube B which serves as a jacket and prevents too 

 rapid cooling. This is fixed in position in a freezing 

 mixture of salt and ice in the vessel A by a cork which 

 fits the opening in the brass lid L. The brass lid has 

 also openings for the passage of a stirrer E and a 

 thermometer. In carrying out a determination a known 

 weight of solvent is placed in C and its freezing-point is 

 taken. The tube is then removed and the solid allowed 

 to melt. A known weight of substance is then intro- 

 duced through D, dissolved in the liquid and the freez- 

 ing-point again determined. 



Several determinations of the freezing-point of the 

 solvent and the solvent containing the substance should 

 be taken. Whilst the freezing-point is being taken the FIG 

 liquid becomes super-cooled. To prevent very great gj c y j 

 super-cooling it is vigorously stirred with the stirrer. 

 At the freezing-point the temperature rises and the highest point reached 

 taken as the freezing-point. 



Similarly, a rise in the boiling-point of a solvent, when substances 

 are dissolved in it, will give the molecular weight of the substance. 



Micro-Molecular Weight Determinations. 



Micro-molecular weight determinations may be made by Barger's 

 method. 2 



1 It is so constructed that mercury can be removed from the thread or introduced into the 

 thread from a small bulb at the top. It can thus be used for any liquid. 

 a Trans. Chem, Soc., 1904, 85, 286. 



2. (From 



Find- 

 : 



