110 Ya7i Name and Broion — Tri-Iodide and 



The strontium iodide, for reasons already mentioned, was 

 prepared with especial care. Iodine resublimed from potassium 

 iodide was converted by the hydrogen sulphide method into 

 hydriodic acid, which was purified by two distillations, and 

 finally treated with an excess of pure strontium carbonate. 

 The filtered solution was diluted to the desired extent and then 

 standardized by precipitation as strontium sulphate in the 

 presence of alcohol, and weighed on asbestos in a platinum 

 crucible. 



To prepare the lanthanum iodide a portion of the hydri- 

 odic acid solution just mentioned was allowed to act upon an 

 excess of ignited lanthanum oxide,* and after diluting and 

 filtering, the iodide content w^as found by determining the free 

 iodine (of which a small amount was present) with thiosulphate, 

 and the total (iodide + iodine) by the method of Gooch and 

 Browning. This solution showed only a faint acid reaction, 

 indicating that the amount of hydrolysis was very small, and 

 for this reason no attempt was made to determine the concen- 

 tration of free hydriodic acid. 



Cadmium bromide was prepared by dissolving the pure 

 metal in a mixture of water and bromine ; mercuric bromide in 

 the same way, except that the product was further purified by 

 several recrystallizations. In both cases the solutions prepared 

 were standardized by precipitation and weighing as silver 

 bromide. 



The solutions of the double iodides and double bromides 

 were made up by simple mixture of the constituents in weighed 

 quantities or in measured volumes of standard solutions. 



Experimental Method. 



The experiments were conducted in glass stoppered bottles 

 of about 200'"^' capacity. In tliese were placed 40-60'°^' of the 

 solution under investigation, and 20-25^"'^ of a strong solution 

 of iodine in carbon bisulphide or, in the case of bromides, of 

 bromine in carbon tetrachloride. Carbon bisulphide was used 

 with all iodides except mercuric potassium iodide. In this case, 

 on account of the solubility of mercuric iodide in carbon bisul- 

 phide, carbon tetrachloride was substituted. 



The bottles, with their necks covered by water-tight rubber 

 caps, were rotated in a thermostat at 25° until equilibrium had 

 been reachedf and then removed from the stirring axle and 



* This oxide had only a faint brownish color indicating that it was com- 

 paratively free from the oxides of the didymium earths. 



\ This requires only a ver}" short time. Our experience was in agreement 

 with that of Jakowkin (Zeitschr. phys. Chem. xviii, 585) w^ho found five min- 

 utes vigorous shaking sufficient. 



