Tri-Bro7nide Equilibria. 109 



_ (^IHt), (IV) 



and similarly for a bromide. 



Preparation of Materials. 



Carbon bisulphide was purified by the methods of Cloez* 

 and of Obach,f which involved two distillations, one over quick- 

 lime, the other over mercuric chloride, after previous digestion 

 with metallic mercury. The carbon tetrachloride employed 

 was free from carbon bisulphide, as shown by the copper xan- 

 thogenate test. 



Of the various salts used, potassium bromide and iodide, cad- 

 mium iodide, zinc iodide, and mercuric iodide, were all tirst 

 quality reagents, and were used as purchased without further 

 purification. The rest were prepared as described below. 



Since the nature of the experiments called for only moderate 

 accuracy in the standardization of the halide solutions, these 

 were, when practicable, made up of the desired strength by 

 direct weighing of the salt. This method was followed with 

 cadmium iodide, potassium bromide and iodide, and mercuric 

 iodide. Zinc iodide solutions, after dilution to the required 

 strength and several days' standing, were filtered to remove 

 the zinc hydroxide thrown out by hydrolysis, and the total 

 iodide concentration then determined gravimetrically by weigh- 

 ing as silver iodide. The zinc content was then found by pre- 

 cipitation as the carbonate and weighing as ZnO, and the 

 proportion of hydrogen iodide was calculated by difference. 



Nickel iodide was prepared by the action of a mixture of 

 iodine and w^ater upon commercial '^ pure nickel." After 

 diluting to the desired concentration the solution was allowed 

 to stand for some days and the resulting precipitate, consisting 

 of nickel hydroxide and ferric hydroxide (from iron present as 

 impurity in the metal), was filtered off. A little hydriodic 

 acid was then added to prevent further hydrolysis. Solutions 

 so prepared showed very little iron by the sulphocyanate test, 

 but contained free iodine as well as hydriodic acid. In stand- 

 ardizing these solutions free iodine was titrated with thiosul- 

 phate, total (iodide + iodine) estimated by the method of Gooch 

 and Browning,^ and nickel determined both by weighing as 

 the salt of dimethyl glyoxime and by electrolysis. From the 

 data so obtained the concentrations of nickel iodide and of 

 hydrogen iodide could be calculated. 



* Jahresbericht der Chemie, 1869, 243. 



t Jour. prak. Chem. (2), xxvi, 282, 1882. 



iThis Journal (3), xxxix, 188, 1890, and xlv, 334, 1893. 



