6 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Towards the end of the distillation the thermometer may be seen 

 to rise slowly and the last portion to distil 1 higher than the first por- 

 tion. Most pure liquids boil over a range of -5 or 1 or sometimes 

 more. 



Chloroform and aniline may be used as examples. 



Correction for Boiling-point. 



A correction should be applied when the mercury in the thermometer 

 passes outside the neck of the flask as this portion is ccoled below the tem- 

 perature of the vapour of the liquid. The following amount is added to the 



rved temperature T 



n(T /) x "000154 



where n is the number of scale divisions projecting, / the temperature of the 

 air recorded by another thermometer held at a point about the centre of the 

 projecting thread and screened from heat radiation from below by a sheet of 

 cardboard. 



A short thermometer registering only 50 and which can be completely 

 inserted in the vapour is used for more accurate determinations. Sets of 

 thermometers registering intervals of 50 between o and 360 can be pur- 

 chased. 



Determination of the Boiling-point of Small Quantities. 



When insufficient liquid is available for distillation its boiling-point can be 

 determined by placing it in a test tube and heating it through an opening in 

 a sheet of asbestos. The thermometer is held in the vapour. 



If only a few drops of liquid are available the boiling-point can be ascer- 

 tained by introducing it into a small test tube, attaching the test tube to a 

 thermometer by a rubber band and heating the two together in a beaker con- 

 taining sulphuric acid. In the liquid is placed a piece of capillary tubing, 

 a melting-point tube (p. 24) which is sealed near the end immersed in the 

 liquid. As the temperature of the bath rises bubbles escape from the capillary 

 and ascend through the liquid. At the boiling-point they form a continuous 



;m and the temperature of the bath is noted. Several determinations are 

 made with fresh pieces of capillary and the mean will give the boiling-point 

 of the liquid. 



