538 Dr. Gibbs on the Determination of Nitrogen by Weight. 



also weighed with its cork. The increase in weight of the chlo- 

 ride-of- calcium tube gives the amount of moisture in the copper 

 and the water in the salt analyzed. The loss of weight in the 

 combustion-tube gives the nitrogen in the salt after correction 

 for the oxygen in the tube, for the moisture in the copper, and 

 for the water in the salt. The correction for the oxygen in the 

 combustion-tube absorbed by the copper is easily found, with a 

 sufficiently close approximation, by subtracting the volume of 

 the copper from that of the tube, finding the weight of the resi- 

 dual air, taking one-fifth of this as oxygen, and considering 

 the whole of this oxygen as absorbed by the copper. A piece 

 of asbestos may be placed between the copper and the cork with 

 advantage, but this renders an additional correction necessary. 

 Two analyses were executed by this method. In the first a 

 sample of pure saltpetre gave 13*86 per cent, nitrogen; the 

 formula KO, NO 5 requires 13*86 per cent. In the second a 

 specimen of the commercial salt gave 13*7 per cent, nitrogen, 

 while the same salt analyzed by Simpson's method, in which the 

 volume of the nitrogen is determined, also gave 13*7 per cent. 

 The whole analysis, with the weighings, may easily be executed 

 in an hour and a half by a single person. It is easy to see that 

 this method applies to all inorganic nitrates and nitrites, whether 

 hydrous or anhydrous, but that it cannot be employed in the 

 case of organic or ammoniacal salts. In the analysis of inor- 

 ganic nitrates or nitrites by Simpson's method, it is not neces- 

 sary to use oxide of mercury to prevent the formation of deut- 

 oxide of nitrogen. In all such cases it will be found sufficient to 

 mix the salt with pure metallic copper. In this manner the di- 

 mensions of the combustion-tube may be greatly diminished. I 

 have also found it advantageous to pump out the air from the 

 combustion-tube by a small hand air-pump before disengaging 

 carbonic acid from the carbonate of manganese. By alternately 

 pumping and filling the tube with carbonic acid, the air may be 

 completely expelled before the combustion commences. It is 

 also better to draw the tube out before a Bunsen's gas-blast, as 

 it is difficult to make a cork and india-rubber connector perfectly 

 tight. With a little practice the drawing out is easily effected, 

 even with the hardest combustion-tubes. Where many nitrogen- 

 determinations are made, it will be found convenient to employ 

 printed forms for logarithmic calculation, the logarithmic con- 

 stants of reduction being printed upon the form itself in their 

 proper places. 



