PHYSICO-CHEMICAL INTRODUCTION 



27 



of the freezing-point of the solvent. The extent to which the freezing- 

 point of a solution is lower than that of the solvent is proportional 

 to the concentration of the dissolved substance. 



Determination of the Lowering of the Freezing-Point. The appar- 

 atus generally employed for this purpose is known as Beckmann's 

 (Fig. 16). It consists of a tube. ,4 , placed in a 

 jacket, B, provided with a special thermom- 

 eter, D, and a platinum or nickel wire stirrer. 

 The jacket B fits into a metal plate which 

 covers a thick glass jar, C, also provided with 

 a stirrer. When the experiment is to be 

 made, this jar is filled with a freezing mixture, 

 which will give a temperature about 2-3 below 

 the F.P. of the solvent. A known weight 

 (10-20 c.c.) of the solvent is placed in A, and 

 its cork carrying the thermometer and stirrer 

 inserted. The temperature of A is first low- 

 ered by placing it in the freezing mixture; 

 but as the freezing-point is approached it is 

 fitted into the jacket B and stirred regularly, 

 so that a steady fall of temperature is 

 assured. The thermometer is carefully 

 watched; after a time the mercury ceases 

 to fall, then suddenly rises and remains 

 stationary for a moment before starting to fall 

 again. The point risen to gives the F.P.. of 

 the solvent for pure water, C. A known 

 weight (1-2 gms.) of the solute (the body to 

 be dissolved) is now introduced through the 

 side-tube, and after it has dissolved the F.P 

 is again determined in a similar manner. It 

 is well not to cool too rapidly or too much; 

 the thermometer should not rise more than 

 0-4 to 0-5 C. to its final position, otherwise 

 the operation must be repeated. Excessive 

 supercooling causes the separation of a con- 

 . siderable quantity of the solid solvent when 

 freezing occurs, and this makes an appreci- 

 able increase in the concentration of the 

 solution. The freezing-point method has 

 been extensively used in studying the 

 osmotic pressure of the blood in different 

 animals, and also the urine in patho- 

 logical conditions in man. It has been shown that 

 the body fluids of invertebrate marine animals is 



C- 



FIG. 10. BECKMANN'S AP- 

 PARATUS FOR DETERMIN- 

 ING THE DEPRESSION OF 

 FREEZING-POINT. 



the F.P. of 

 the same as 



that of the water in which they Iiv3; they are incapable of preserv- 

 ing any difference of osmotic pressure; if the osmotic pressure of 

 the water be varied, that of the body fluids varies also. T> " 1 - : ~ **"* 



But in the 



