596 PROCEEDINGS OF THE AMERICAN ACADEMY. 



During the standing in acid solution the oxidation of the residual 

 phosphorous acid progressed, but even at the end of the period it was 

 not quite complete. Precipitation of the silver bromide immediately 

 after acidification invariably resulted in darkened salt, but after forty- 

 eight hours an apparently pure product was obtained, if no appreciable 

 excess of silver was used. 



The bromine in each sample was determined in two ways, first by 

 finding the amount of silver necessary to combine with it, second, by 

 weighing the silver bromide formed. Both determinations could be 

 carried out with the same sample of tribromide. 



A quantity of silver equivalent to the tribromide within a very few 

 tenths of a milligram was weighed out and dissolved in nitric acid in a 

 flask fitted with a column of bulbs to prevent loss of material by spat- 

 tering. After the silver solution had been freed from oxides of nitro- 

 gen by dilution and heating, about 40 cc of concentrated nitric acid 

 was added and the whole was diluted to a volume of about one liter. 

 The silver solution was then quantitatively added to the bromide 

 solution, with constant agitation, and the stoppered precipitating flask 

 was allowed to stand for several days with occasional shaking. When 

 the precipitate had coagulated and settled so completely that the 

 supernatant liquid was apparently clear, portions of the solution were 

 tested in a nephelometer ^ for excess of bromide or silver. As a matter 

 of fact, bromide was always found in slight excess. The deficiency of 

 silver, which was never more than a very few tenths of a milligram, 

 was made up as nearly as could be estimated by means of a hundredth 

 normal solution of silver nitrate. After thorough shaking and stand- 

 ing until the solution was perfectly clear, tests for excess of bromide or 

 silver were again made. If necessary this process was repeated, until 

 eventually the amounts of bromide and silver were equivalent. Owing 

 to the slight solubility of silver bromide at this point the solution re- 

 mained essentially clear, even in the nephelometer, when either bromide 

 or silver solution was added. 



The solution was now allowed to stand for a week with occasional 

 shaking and then was tested again in the nephelometer. Usually a de- 

 ficiency of one or two tenths of a milligram of silver was found, owing 

 apparently to extraction of occluded soluble bromide from the silver 

 bromide. This deficiency of silver was supplied and the solution was 

 left for another week before being tested again, and this process was 

 repeated until in the course of a week no further change in the solu- 

 tion took place. In a few analyses the solutions were tested over a 



' Richards and Wells, Amer. Chem. Jour., 31, 235 (1904); 35, 510 (1906). 



