THE ANALYSIS OF SILVER CHROMATE. 14$ 



avoided. The simplest fashion of drying a substance perfectly is to fuse it in a 

 current of dry gas. In the case of the silver chromate, however, this is not 

 practicable, for even at 300° incipient decomposition sets in. Upon attempt- 

 ing to dissolve in nitric acid samples dried in air at that temperature, a slight 

 insoluble residue was always obtained, while heating in a current of oxygen 

 gave no better results. Since the moisture can not be entirely expelled from 

 silver chromate by heating at a moderate temperature, it must be determined 

 by the analysis of separate portions of the substance which have been treated 

 in some definite fashion. 



Experiments showed that at temperatures below 225° the salt was not 

 appreciably changed, hence this temperature was chosen as a suitable one at 

 which to heat the salt preparatory to analysis. The silver chromate was there- 

 fore always heated in a current of pure dry air for 2 hours at 225°, in order to 

 obtain the separate portions in as nearly as possible the same condition. 



The drying apparatus was constructed entirely of glass, rubber connections 

 being especially avoided. A current of air was passed first over red-hot copper 

 oxide to destroy organic matter, then through successive Emmerling washing 

 towers. In the first were beads drenched with silver nitrate solution, in the 

 second with a strong solution of potassic hydroxide containing much potassic 

 manganate, and in the last three with concentrated sulphuric acid. The already 

 very dry air was then passed through a long tube containing resublimed phos- 

 phoric anhydride spread over a large surface of glass beads and ignited asbestos. 

 From the drying apparatus the air p^ss^d into the tube in which the boat con- 

 taining the silver chromate was placed. 



DETERMINATION OF, SILVER IN SILVER CHROMATE. 



During the drying of the silver chromate it was contained in a platinum boat 

 which had been weighed, in a weighing-bottle, by substitution for a similar 

 bottle which with its contents displaced the same amount of air as the bottle 

 with the boat. The boat was placed in a hard-glass tube connected by a care- 

 fully ground joint with the bottling apparatus.^ The tube was heated by means 

 of the solid aluminum oven described on page 78. After 2 hours' heating at 

 225° the boat was transferred to the weighing-bottle and was allowed to stand 

 in a desiccator near the balance for several hours before being weighed. 



Next, the weighed quantity of silver chromate was transferred to a 3-liter 

 glass stoppered Jena flask with a carefully ground stopper, and, after the boat 

 and bottle had been cleaned with hot dilute nitric acid and water, the rinsings 

 were poured into the flask and the silver chromate dissolved by the applica- 

 tion of gentle heat. If the salt had not been heated above 225°, the solution 

 was absolutely clear. Specimens heated above this temperature always showed 

 more or less turbidity, 



* See page 8. 



