PRINCIPLES OF THE METHOD 425 



The acidity is produced by a few drops of nitric acid. The rest 

 of the process is conducted in the usual way. 



Magnesia. This is precipitated by ammonium sodium phos- 

 phate, filtered, ignited, and weighed as pyrophosphate. The mag- 

 nesia is then calculated as chlorid. Magnesia is rarely found 

 in excess of one-fourth per cent. When this amount is not ex- 

 ceeded the estimation of it may be neglected without any great 

 error. As has already been said, the chlorin is all calculated as 

 sodium chlorid. If a part of it be combined with one-fourth 

 per cent, of magnesia, it would represent 0.59 per cent, of mag- 

 nesium chlorid instead of 0.73 per cent, sodium chlorid. In 

 omitting the estimation of the magnesia, therefore, the importer 

 is only damaged to the extent of 0.14 per cent, of sodium nitrate. 



Sodium lodate. This body, present only in small quantities, 

 may also be neglected. In case the content of this body should 

 reach one-fourth per cent, the estimation of chlorin by titration, 

 using potassium chromate as indicator, is impracticable. Such an 

 instance, however, is rarely known. 



Approximate Results. When the determinations outlined above 

 have been carefully made, it is claimed that the result obtained 

 by subtraction from 100 will not vary more than from two-tenths 

 to three-tenths per cent, from the true content of sodium nitrate. 

 The method, however, cannot be considered strictly scientific and 

 is much more tedious and chronophagous than the direct deter- 

 mination. In the direct determination, however, the analyst must 

 assure himself that potassium is present in only appreciable quan- 

 tities, otherwise the per cent, of sodium nitrate will be too low. 



The presence of potassium nitrate is a detriment in this re- 

 spect only; viz., that it contains a less percentage of nitrogen 

 than the corresponding sodium salt. As a fertilizer, the value 

 of Chile saltpeter may be increased by its content of potassium. 



ESTIMATION OF NITRIC ACID BY OXIDATION OF A 

 COLORED SOLUTION 



367. Principles of the Method. Solutions of organic coloring 

 matter in certain conditions are decolorized by nitric acid. The 

 process is one of oxidation and the disappearance of the natural 



