158 RESEARCHES UPON ATOMIC WEIGHTS. 



Sample III. — The most dilute nitric acid which was used in the preparation 

 of the silver dichromate was about 0.16 normal, solutions of this concentration 

 being employed in the precipitation and crystallization of Sample III. This 

 sample was made from chromic acid and silver nitrate, and was six times crys- 

 tallized from 0.16 normal nitric acid. 



The chief difference in the purification of the three specimens, aside from the 

 concentration of acid used in their preparation, lies in the fact that Sample I was 

 prepared from recrystallized potassium dichromate and II and III from chromic 

 acid. All three samples were crystallized many times as silver dichromate. 



After the final drainage in the centrifugal apparatus, the crystals were dried 

 in an electric oven at 150° for several hours. Then they were powdered gently 

 in an agate mortar and kept in platinum vessels. 



DRYING OF SILVER DICHROMATE. 



In preparing the silver dichromate for analysis, the complete elimination of 

 moisture by fusion of the salt was impossible, owing to the ease with which silver 

 dichromate decomposes. Even at the comparatively low temperature of the 

 melting point of the dichromate, about 400°, oxygen is given off rapidly, while 

 at temperatures considerably below this point, 300°, and to a very slight extent 

 at 250°, there seemed to be evidence of decomposition, since salt heated to these 

 temperatures did not give an absolutely clear solution in dilute nitric acid. In 

 order to be on the safe side, the drying of the salt took place at 200° C. 



The heating of the dichromate was efifected much as described in the preced- 

 ing paper in the case of silver chromate. The salt, contained in a weighed 

 platinum boat, was heated in a current of pure dry air in a hard-glass tube for 

 4 hours at 200° C, the air being purified and dried by passing over hot copper 

 oxide, solid potassic hydroxide, concentrated sulphuric acid containing dichro- 

 mate, and resubhmed phosphorus pentoxide successively. The oven composed 

 of solid aluminum blocks (page 78) was used, by means of which the tempera- 

 ture could be maintained constant within a very few degrees. 



DETERMINATION OF SILVER IN SILVER DICHROMATE. 



After the boat had been allowed to cool in the tube, it was transferred to the 

 weighing-bottle by means of the bottling apparatus (page 8), and was re- 

 weighed. Then the dichromate was transferred to a flask and was dissolved in 

 hot 0.8 normal nitric acid, the boat and the weighing-bottle being carefully 

 cleansed with nitric acid and the rinsings being added to the main solution. The 

 solution, which was always perfectly clear, was quantitatively transferred to the 

 3-liter glass stoppered precipitating flask, and at a dilution of about i liter was 

 reduced by the addition of a very slight excess of sulphur dioxide. When the 

 solution was cold, a slight excess of hydrobromic acid was diluted to about 

 800 c.c. and then was slowly added to the silver solution with continual agitation. 

 The flask was stoppered and vigorously S'haken. After 24 hours' standing the 

 flask was again shaken, and then was allowed to stand 2 days or more, until the 

 supernatant solution was clear. 



