THE DRYING AND WEIGHING OF POTASSIC BROMIDE. 37 



As a matter of fact, the potassic bromide always gave a perfectly clear 

 aqueous solution except once, namely, in analysis 14. In this case several 

 small flakes of platinum were noticed in the salt both before and after the 

 salt was dissolved in water. 



The solutions were always perfectly neutral to phenolphthalem, a fact 

 which is adequate proof of the retention of all the bromine during fusion, 

 and also of the absence of oxalate from the potassic bromide crystals. 



The third sample of potassic bromide had contained ammonic bromide, 

 which was expelled during the fusion. During the expulsion of this salt, 

 the boat was placed near the exit end of the porcelain tube, and there 

 gradually heated to redness in a current of nitrogen. During the entire 

 operation the middle portion of the porcelain tube was kept red-hot in 

 order to prevent backward diffusion of the ammonic bromide, which by 

 condensing might later cause a contamination of the potassic bromide. 

 All the ammonic bromide having been sublimed, the boat was pushed into 

 the middle of the tube, and the potassic bromide fused as described above. 

 There being a possibility that even at the fusing-point of potassic bromide 

 some ammonic bromide might still be retained, another portion of the same 

 third sample of bromide was fused under approximately the same condi- 

 tions as in the analyses. The solution of the bromide thus obtained was 

 submitted to the Nessler test, being compared with a tube of pure water 

 containing the same amount of reagent. No color was visible in either 

 tube, hence no appreciable amount of ammonic bromide remained to con- 

 taminate the potassium salt. 



The salt thus prepared for analysis was weighed in its glass-stoppered 

 weighing-bottle by substitution, using as the substituting tare a precisely 

 similar weighing-bottle. Successive weighings of the same specimen were 

 always practically identical. The balance and weights were similar to 

 those used with sodic and potassic chlorides, and all the precautions were 

 the same. At each weighing the heights of the barometer, the temperature 

 of the balance-room, and the humidity as indicated by a hygrometer were 

 recorded. The density of the displaced air could thus be determined for 

 each case. The variations from the average were, however, not enough to 

 affect essentially any of the results, as weighings were not made under 

 any abnormal atmospheric conditions. The weights were of course stand- 

 ardized by the usual Harvard method. 



The specific gravity of potassic bromide must be known to within five 

 one-hundredths of a unit in order to correct the weighings to the vacuum 

 standard with sufficient accuracy. Its determination 1 has been undertaken 

 with a variety of methods, with a variety of results, ranging from 2.20 to 



1 For the literature see Landolt u. Bornstein (Meyerhoffer). Physikalisch-Chem- 

 ische Tabellen (1905), and Clarke, Constants of Nature (part 1), Macmillan (1888). 



