The solutions of the pure salts and of the sample to be analyzed are made of such a strength as to contain sixty-six grams of sodium nitrate, or eighty grams of potassium nitrate, in a liter. Five cubic centimeters of such a solution will yield a little less than 100 cubic centimeters of nitric oxid under usual conditions. Let the volume of gas obtained with the pure salt be _v_; and that with the sample be _v′_. The calculation is then made from the equation: _v′_ _x_ ---- = ----. _v_ 100 _Example_: Let ninety-five cubic centimeters be the volume of gas from five cubic centimeters of the pure salt (sodium nitrate), and 91.5 cubic centimeters be the volume of gas from five cubic centimeters of the sample; then 91.5 _x_ ---- = ----, whence _x_ = 96.31. 95 100 Hence the sample analyzed contains 96.31 per cent of sodium nitrate. Since the pure sodium nitrate contains 16.47 per cent of nitrogen the sample under examination would contain 16.47 × 96.31 ------------- = 15.86 per cent. 100 It is evident that this comparative method is quite easy of application when the sample under examination has no other nitrate in it except that combined with the one base. =216. Volumetric Method of Gantter.=—The process proposed by Gantter for determining the nitrogen, volumetrically, in Chile saltpeter and other nitrates is based on the following principles:[180] (1) If a nitrate be heated in contact with sulfuric and phosphorous acids, nitrous acid will be formed. (2) If nitrous acid be boiled with ammonium chlorid, nitrogen will be quantitatively evolved from both compounds. These processes are illustrated by the following formulas: (a) N₂O₅ + P₂O₃ = N₂O₃ + P₂O₅. (b) N₂O₃ + 2NH₄Cl = 2N₂ + 3H₂O + 2HCl. [Illustration: FIGURE 16. GANTTER’S NITROGEN APPARATUS.]