EICHARDS AND BONNET. — DISSOLVED CHROMIC SULPHATE. 19 



by this salt-t It seemed possible, however, that since the complex green 

 sulphate is so tardy in the attainment of its equilibrium (a fact indi- 

 cated by the preceding work), it might also not form at once from the 

 complex green chloride. In this case it might be possible to precipitate 

 baric sulphate from a solution of chromic chloride before the green sul- 

 phate had been allowed time to form. 



The answering of this question was experimentally easy. Pure solu- 

 tions of sodic sulphate, chromic cliloride, and baric chloride were pre- 

 pared of convenient concentrations, which corresponded to 1.558 gram 

 of baric sulphate from the volumes used. 



In a first experiment, the chromic chloride and sodic sulphate were 

 heated separately to the boiling point, quickly and thoroughly mixed, 

 and at once precipitated by baric chloride. After the precipitation the 

 mixture was diluted four-fold from its small initial volume to about two- 

 tenths of a litre, and was digested for several hours on the steam bath in 

 order to facilitate filtering. The precipitate was weighed and its per- 

 centage of chromium found by fusion with alkaline carbonate and nitrate, 

 reduction with alcohol, and precipitation with ammonia. 



In this way, the baric sulphate was found to weigh 1.562 gram and to 

 contain 0.0100 gram of chromic oxide. An experiment similar in every 

 way, except that an hour elapsed between the mixing of chromic chloride 

 with sodic sulphate and the precipitation, yielded only 1.472 grams of 

 precipitate, containing 0.0582 gram of chromic oxide. Evidently far 

 more chromic salt was occluded when time for the formation of the 

 complex sulphate was allowed. 



As would be expected, this difference became still more evident when 

 the mixture and precipitation was carried on at 20°. A precipitate 

 which had been formed as soon as possible after the mixing of the 

 chromic and sulphate solutions appeared perfectly white, and contained 

 only 5.5 milligram of chromic oxide ; while fifteen minutes, an hour, and 

 three hours' delay in precipitation led to greener and greener precipitates 

 containing respectively 9.4, 13.6, and 14.7 milligrams of chromic oxide 

 respectively. 



These experiments were all verified by repetition, so that the following 

 conclusions seem to be indubitable : — 



(1) The chief occluded body is neither the simple chromic ion nor a 

 complex chloride, but rather a sulphate. 



t Richards and Parker, These Proceedings, 31, 67 (1894) ; Zeit. anorg. Chem., 

 8, 413 (1895). 



