444 Thome — Barium Bromide by Hyolrobromic Acid. 



So it appears that precipitation is complete in presence of a 

 sufficiency of either acid and that the precipitate will fall 

 chiefly as the bromide or as the chloride according to the pro- 

 portions of hydrobromic acid and hydrochloric acid present. 



It was thought a matter of interest in this connection to test 

 the constitution of the precipitates when incomplete precipita- 

 tion is brought about by addition of one of these acids, the 

 other being present in considerable proportion, though in 

 amount wholly insufficient to produce by itself precipitation 

 in the volume of water used for solution of the barium salts. 



Following is the record of experiments in which 1 grm. of 

 barium chloride, BaCl 2 . 2H 2 0, was dissolved in water, hydro- 

 chloric acid added to incipient precipitation, and then an amount 

 of hydrobromic acid which by itself would produce no pre- 

 cipitation in the water solution. The precipitate was dried 

 and weighed, and the content in bromine determined by the 

 method of Baubigny and Rivals.* 



BaCl 2 . 



2H 2 



taken. 



grm. 



1. 



Water 



for 



solution. 



cm 3 . 



30 



HBr HCI 



added, added. 



cm 3 . cm 3 . 



3 22 



Precipitate, 

 grm. 



o-5037f 



Bromine 



in 



precipitate. 



grm. 



0-0129 



BaBr 2 



in 



precipitate 



grm. 



0-0240 



1. 



42 



5 31 



0-6241J; 



0-0141 



0-0263 



1. 



40 



5 30 



0"6338t 



0-0202 



0-0375 



The meaning of these results seems to be that the precipita- 

 tion of the bromide is induced by the action of the hydrogen 

 chloride upon the solvent, water. The production of free 

 bromine ions and barium ions to the amount of the solubility 

 product of barium bromide in water is, under the conditions, 

 an impossibility. If this be admitted, it seems highly probable 

 that precipitation of barium chloride is likewise conditioned 

 by the action of the hydrogen chloride upon the solvent. 



It has been customary on the part of some to explain the sim- 

 ilar precipitation of other chlorides soluble in water, like sodium 

 chloride, by large amounts of hydrochloric acid upon the assump- 

 tion that insolubility is due to increased concentration of the 

 chlorine ions, and such processes have been held to be typical 

 of processes in which precipitation is affected by concentration 

 of the free ions. It seems more probable, however, that it 

 is the action of the hydrogen chloride upon the solvent which 

 is the effective thing in such precipitations, as in the precipita- 

 tion of barium bromide, after addition of hydrochloric acid, by 

 an amount of hydrobromic acid wholly insufficient to cause 

 precipitation in the water solution. 



I wish to express my thanks to Prof. Grooch for suggestions 

 and advice given during the progress of this work. 



* Compt. rend., cxxv, 527, 607. 



f Dried three months in desiccator over sulphuric acid. 



% Dried G hours at 80° and 12 hours in desiccator over sulphuric acid. 



