108] Nitrogen in Nitrates 73 



the tube dissolves one twentieth of its volume of gas. The 

 total volume of gas evolved is therefore the volume read off on 

 the collecting vessel + J a the volume of water standing above 

 the mercury. 



108. Calculation. — Having found the volume of gas, 

 including the correction for solubility, at a certain known 

 temperature and pressure, the first thing to do is to find what 

 that volume would be at the normal temperature and pressure 

 of o° C. and 760 mm. This is done by the formula 



V = V i x 2 73 */ 



(273 + /) x 760 



where V 2 =volume at o° C. and 760 mm. ; 

 V 1 =volume noted; 

 p= pressure noted in millimetres ; 

 t— temperature noted in degrees Centigrade. 



For the explanation of this formula the student is referred to any 

 elementary book on physics. 



Having reduced our volume to normal temperature and 

 pressure, we make use of the following data : 1 c.c. of NO at 

 o° C. and 760 mm. represents 1*343 milligram of NO. This is 

 equivalent to '627 milligram of nitrogen or 3*805 of NaN0 3 . 1 

 We have, therefore, only to multiply the reduced volume in 

 c.c. by -000627 to find the weight of N present in the KN0 3 , 

 and from this we may calculate the percentage. When a large 

 number of estimations have to be made this calculation be- 

 comes somewhat tedious. The table on pages 74-77 will 

 simplify this very much. This table contains a set of factors 

 which are used as follows : 



Having read off the temperature and the pressure, refer to 



1 Sodium nitrate is mentioned here, as it is the substance in which 

 agricultural analysts most commonly have to carry out this estimation. 

 Potassium nitrate is suggested for the student, as it is more easily prepared 

 in the pure state. 



