62 THE QUANTITATIVE SYNTHESIS OF ARGENTIC NITRATE, ETC. 



in an atmosphere free from ammonia would have been used in a blank 

 experiment for comparison ; but with such a small trace of impurity this 

 was not worth while. 



In summing up the results of the test for ammonia, the amount should 

 be calculated for the quantity of argentic nitrate produced from 100.000 

 grams of silver. Thus it is found that 157.480 grams of fused argentic 

 nitrate could not have contained over 0.0007 gram of ammonic nitrate. 



The next impurity to be studied, nitrous acid, was sought by means of 

 sulphanilic acid and naphthylamine hydrochloride. These were proved 

 with a standard nitrite solution of which 1 liter contained 0.1 gram of 

 nitrogen. To insure parallel conditions in testing by comparison, pure sil- 

 ver nitrate was prepared with some care. It was thrice recrystallized 

 from nitric acid with centrifugal treatment, fused in porcelain, then recrys- 

 tallized three times more from the purest water in platinum, with centrif- 

 ugal draining as before. The specimens of fused argentic nitrate result- 

 ing from quantitative determinations were dissolved in known amounts 

 of water, and comparison solutions of equal concentration prepared. The 

 crystallized silver nitrate gave no trace of pink color with the reagents. 

 A fresh solution to which 0.1 cc. of nitrite solution (0.01 mg.) had been 

 added gave a pronounced color, while determinations 2, 3, and 11 all 

 gave a much fainter, barely visible color ; hence it was permissible to con- 

 clude that the loss of oxygen from the nitrate on fusion could not have 

 exceeded 0.000005 gram, a wholly negligible quantity. 



In seeking for the next impurity, free nitric acid, Stas tested aqueous 

 solutions of his fused silver nitrate with "tournesol" and found them alka- 

 line rather than acid. This alkali may have come from his glass vessels. 

 We used methyl orange, a more sensitive indicator, and noted that the 

 purest crystallized neutral specimens and the fused salt alike gave equally 

 pronounced acid reactions in solution. The color was not modified by the 

 addition of half a milligram of nitric acid ; therefore the equality in color 

 does not disprove the presence of free acid in our fused product. Dilute 

 sodic hydroxide freed from carbonate by baric hydroxide threw down a 

 precipitate without changing the pink color. Hence the indicator was dis- 

 carded and the investigation continued with the nephelometer. 



The test for acid with the help of this instrument was conducted by add- 

 ing a very dilute standard caustic alkaline solution to the solution to be 

 tested and by observing if this addition caused a permanent cloud. 



Preliminary trials were necessary in the first place in order to determine 

 the conditions best suited for accuracy. It was found that a concentration 

 of nitrate no stronger than 4 grams in 0.025 liter was best adapted to this 

 work, as argentic hydroxide is distinctly soluble in double this concentra- 

 tion of argentic nitrate. On the other hand, argentic hydroxide precipi- 



