62 The Estimation of Nitrogen [91, 92 



91. Apparatus required.— This is shown in figs. 28, 29, 

 and 30, and described in the text. 



Chemicals required. — Sulphuric acid free from nitrogen. 

 Strong caustic soda solution. This is prepared by dissolving 

 357 grams of 98 per cent, caustic soda in the least possible 

 quantity of water and making up to a litre. It is kept in the 

 arrangement shown at c, fig. 25. The bottle has a soda lime 

 tube above to prevent the access of C0 2 - By opening the 

 pinchcock, the solution is allowed to run into the tube, which 

 has two marks upon it. When it is full up to the topmost mark, 

 the cock is closed and the liquid is run out below into a beaker, 

 until the level in the tube is that of the bottom mark. This 

 measures out exactly 100 c.c. 



Anhydrous potassium sulphate is also required. 



92. Heating with Acid. — Weigh out about 1 gram of 

 linseed cake, and introduce it into an 8-oz. flask, taking care 

 that none adheres to the neck. Measure out 20 c.c. of strong 

 H 2 S0 4 free from nitrogen. Pour it on to the cake, and heat 

 gradually up to the boiling-point of the acid. 1 A very con- 

 venient stand for this is shown in fig. 28. The stand itself is 

 made of iron, and a piece of asbestos cardboard, a, pierced 

 with four holes, each 3 inches diameter, is used as a rest for 

 the flasks. A sheet-iron support, b, is fixed at the back of the 

 stand, so that the flasks whilst heating may be tilted as in the 

 figure ; this prevents loss by spirting. The heat is supplied 

 by the four Argand burners c, c, c, c. The temperature ob- 

 tained by this means is quite sufficient for the purpose, whilst 

 the strong illumination of the flask and its contents supplied by 

 the bright flame is a great advantage to the operator. 



When the liquid has ceased frothing (from twenty to forty 



1 It need scarcely be mentioned that the heating of a carbonaceous sub- 

 stance with sulphuric acid will give off large quantities of sulphur dioxide 

 This operation must therefore be carried out in a draught cupboard. 



