lyo RESEARCHES UPON ATOMIC WEIGHTS. 



Hydrohromic Acid. — This substance was prepared in conjunction with Mr, 

 F. B, CoflSn, who was engaged in a parallel research upon the atomic weight of 

 arsenic, which has been described in the seventh paper of this collection. Com- 

 mercial bromine was converted into potassium bromide by addition to recrys- 

 talhzed potassium oxalate. In a concentrated solution of this bromide, in a 

 distilling flask cooled with ice, bromine was dissolved, and distilled from the 

 solution into a flask cooled with ice. A portion of the purified bromine was 

 then converted into potassium bromide with pure potassium oxalate as before, 

 and the remainder of the bromine was distilled from solution in this pure potas- 

 sium bromide. The product obtained was thus twice distilled from a bromide, 

 the bromide in the second distillation being essentially free from chlorine. This 

 treatment has already been proved sufiicient to free bromine from chlorine.^ 

 Hydrobromic acid was synthesized from the pure bromine by bubbling hy- 

 drogen gas (made by the action of water on "hy drone") through the bromine 

 warmed to 40°-44° and passing the mixed gases over hot platinized asbestos 

 in a glass tube. The apparatus was constructed wholly of glass. The hydrogen 

 was cleansed by being passed through two wash-bottles containing dilute sul- 

 phuric acid, and through a tower filled with beads also moistened with dilute 

 sulphuric acid. The hydrobromic-acid gas w^as absorbed in pure Water contained 

 in a cooled flask. In order to remove iodine the solution of hydrobroinic acid 

 was diluted with water and twice boiled with a small quantity of free bromine. 

 Then a small quantity of recrystallized potassium permanganate was added 

 to the hydrobromic acid solution, and the bromine set free was expelled by 

 boiling. Finally the acid was distilled with the use of a quartz condenser, the 

 first third being rejected. It was preserved in a bottle of Nonsol glass pro- 

 vided with a ground-glass stopper. The purity of the hydrobromic acid was 

 tested by a quantitative synthesis of silver bromide. The silver used, which 

 was kindly furnished by Mr. G. S. Tilley, had been prepared with all the neces- 

 sary precautions for work on the atomic weights of silver and iodine.^ The 

 procedure used by Baxter ^ for the synthesis of silver bromide from a weighed 

 amount of silver was followed in detail. In this experiment 6.02386 gm. of 

 silver yielded 10.48627 gm. of silver bromide; hence, silver bromide contains 



57.4452 per cent of silver, while Baxter found as the mean of 18 determinations 



57.4453 per cent. The hydrobromic acid was evidently pure. 



Silver Nitrate. — Crude silver nitrate was reduced with ammonium formate, 

 made by passing ammonia gas into redistilled formic acid. The reduced silver 

 was washed with the purest water, until the wash-waters no longer gave a test 

 for ammonia with Nessler's reagent, and was fused on sugar charcoal. The 



' Baxter: Proc. Amcr. Acad., 42, 201 (1906); Jour. Amer. Chem. Soc, 28, 1322; Zeit. anorg. 

 Chem., 50, 3S9. (See page 60.) 



2 Baxter and T'lWey: Jour. Amer. Chem. Soc, 31, 201 (1909); Zeit. anorg. Chetn., 61, 293. 

 (See page 120.) 



* Baxter, Loc. cit. 



J 



