424 AGRICULTURAL ANALYSIS 



within an hour or two, the volume of water obtained with a sam- 

 ple of Chile saltpeter can be compared directly with that given 

 oft" by the same weight of a pure potassium or sodium nitrate 

 without correction. 



Example. Two hundred and fifty milligrams of potassium 

 nitrate, containing 34.625 milligrams of nitrogen, displaced in a 

 given case 60 cubic centimeters of water ; therefore, one cubic 

 centimeter of water equals 0.578 milligram of niirogen. If 289 

 instead of 250 milligrams be taken, then the number of cubic cen- 

 timeters of water displaced divided by five will give the per cent, 

 of nitrogen. 



366. Method of Difference. In the analysis of Chile salt- 

 peter by the direct method a variation of 0.25 per cent, in the 

 content of nitrogen is allowed from the dealers' guaranty. This 

 would allow a total variation in the content of sodium nitrate 

 of 1.52 per cent. Dealers and shippers have always been accus- 

 tomed to estimate the quantity of sodium nitrate in a sample by 

 difference ; i. e., by estimating the constituents not sodium nitrate 

 and subtracting the sum of the results from 100. Chile saltpeter 

 usually contains sodium nitrate, water, insoluble ferruginous mat- 

 ters, sodium chlorid, sodium sulfate, magnesium chlorid, sodium 

 iodate, calcium sulfate and sometimes small quantities of potas- 

 sium nitrate. 



When the total sodium nitrate is to be estimated by difference, 

 the following procedure, suggested by Crispo, may be followed : 53 



Water. Dry 10 grams of the finely powdered sample to con- 

 stant weight at i5O-i6o. 



Chlorin. The residue, after drying, is dissolved and the vol- 

 ume made up to one-fourth liter with water and the chlorin de- 

 termined in one-fifth thereof and calculated as sodium chlorid. 



Insoluble. Twenty grams are treated with water until all solu- 

 ble matter has disappeared, filtered on a tared gooch, and the 

 crucible dried to constant weight. 



Sulfuric Acid. The sulfuric acid is precipitated by barium 

 chlorid in the slightly acid filtrate from the insoluble matter. 

 M L'Engrais, 1894, 9 : 877. 



