32 THE ANALYSIS OF POTASSIC BROMIDE. 



finally concentrated in the extreme mother liquor, and was found to be 

 due to a trace of platinum ; no iron was present. Even the first crystals 

 were perfectly white, the second and third mother liquors also showed 

 no trace of color, and only the first of the mother liquors gave a test 

 for platinum. 



The material thus prepared, after being three times recrystallized in 

 platinum with centrifugal draining, was called sample I, and served for 

 analyses 1 to 11. 



The second sample of potassic bromide was prepared from the second 

 sample of four times recrystallized potassic oxalate. The method was the 

 same as with the previous sample. An excess of bromine was added to 

 the oxalate in quartz ; when action ceased, the bromine was driven off by 

 heating on a steam-bath. However, on testing for oxalate, a perceptible 

 quantity was found still undecomposed. This was unexpected, in view of 

 the fact that the first preparation of bromide had given no trouble in this 

 respect. The difficulty was traced to the presence of a small percentage 

 of acid salt in the oxalate ; the presence of acid has an inhibitory effect on 

 the reaction of bromine on the oxalate ion. Obviously, the simplest 

 remedy would have been to add pure potassic hydroxide ; but this was not 

 added because at that moment there was none at hand. Moreover, 

 because the reaction was being performed in quartz, it was thought best 

 not to use a caustic alkali for fear that the dish might be slightly attacked. 

 The addition of a small amount of very pure recrystallized bromate to 

 this slightly acid solution served better, because its use was unattended 

 with danger. Its employment was entirely effective ; the ionized hydrogen 

 was removed, and after adding more bromine and heating, a solution of 

 bromide was obtained which contained no trace of oxalate. 



This specimen was recrystallized entirely in quartz, in order to prevent 

 a recurrence of the trouble experienced with sample I, as well as to see 

 if the variation of conditions affected the atomic weight. Two crystalli- 

 zations were made with centrifugal draining, and every precaution ; and 

 there was obtained finally a very pure, colorless product with which were 

 made two analyses, Nos. 12 and 13. That a uniformly pure material had 

 been made is shown by the close agreement of these two analyses with 

 the mean of all. 



As a further precaution against accidental error, it was thought best 

 to develop a method essentially different from that just discussed, and 

 to prepare therewith a third specimen of bromide. In brief, this method 

 consisted in the separate preparation of pure ammonic bromide and 

 potassic hydroxide, and the production of potassic bromide by the evapor- 

 ation of the mixed solutions of these substances. Thus can be obtained 

 a salt contaminated only with excess of volatile ammonic bromide. 



