28 THE ANALYSIS OF POTASSIC BROMIDE. 



In common with others of the same type, the investigation easily resolves 

 itself into several sections and will be discussed tinder the following 

 heads : The preparation of materials ; the drying and weighing of potas- 

 sic bromide ; the determination of the ratio of argentic to potassic bro- 

 mide; the determination of the ratio of silver to potassic bromide; and 

 the discussion of the results. Of the laboratory tasks the preparation 

 of the materials was by far the most laborious and puzzling, while the 

 analytical work was comparatively simple. 



THE PREPARATION OF MATERIALS. 

 THE SOURCE OF THE POTASSIUM. 



The task of preparing pure potassic bromide, simple as it may appear, 

 is by no means an easy one. In view of our experience, it seems probable 

 that neither Marignac nor Stas ever prepared samples pure enough to 

 correspond with the other precautions which they took. As in the case 

 of sodic bromide, 1 the salt itself when once made can not be effectively 

 purified. In this respect it differs widely from the chloride. We found 

 after due trial that the potassium and bromine must be purified sepa- 

 rately in such forms as to introduce no foreign matter. Accordingly, 

 the two substances, already adequately purified, were caused afterwards 

 to yield potassic bromide ; and this salt, when recrystallized, gave the final 

 substance for analysis. The purification of the potassium materials will 

 be discussed first. 



It was necessary to provide potassium material which should be free 

 from the suspicion of introducing any impurity with the metal desired, 

 either basic or acid. Potassic nitrate, which had served so well in the case 

 of the chloride, was not suitable for the present purpose, because the 

 destruction of the nitric acid involves too great a sacrifice of laboriously 

 prepared hydrobromic acid. After due consideration, potassic oxalate 

 was tentatively chosen. It was necessary to prove that this salt can be 

 easily and certainly purified by recrystallization, and especially to show 

 that the other alkali metals can thus be separated. The following tests 

 accomplished this proof. 



A saturated solution of potassic oxalate containing a purposely added 

 admixture of 10 per cent of sodic oxalate was crystallized and the crystals 

 drained centrifugally. By the flame test, a pronounced difference was 

 noticeable between the amount of sodium present in the mother liquor 

 and that in the crystals. A second recrystallization and whirling was 

 still more satisfactory ; the crystals gave no sodium test to the eye, while 



Richards and Wells, Proc. Am. Acad., 41, 435 (1906) ; Zeit. phys. Chem., 56. 

 348 (1906). 



