Gooch and Hill — Purification of Barium Sulphate. 313 



by weighing out 20 gpm. of the BaCl, . 2H 2 0, transferring it to 

 a 2-liter measuring flask, and filling the flask to the mark with 

 distilled water. The quantity of the solution used in each 

 experiment was determined by drawing portions of 50 cm3 from 

 a burette into previously weighed glass-stoppered Erlenmeyer 

 flasks and weighing. Since 0*01 grm. of the solution contained 

 only O'OOOl grin, of BaCl 3 . 2H 2 0, it was not necessary to weigh 

 more accurately than to one centigram. However, the weigh- 

 ing was usually made to a milligram. 



In the first series of experiments determinations of barium 

 in the absence of salts of other metals were made in the usual 

 manner, and the effect of the treatment of the precipitate of 

 barium sulphate with sulphuric acid in the way described was 

 tried. The procedure in detail was as follows : One cubic centi- 

 meter of strong sulphuric acid was added to 100 cm3 of water in a 

 250 or 350 cm3 beaker, and the mixture w T as heated to boiling. 

 The weighed amount of barium chloride solution was washed 

 from the Erlenmeyer flask into the hot acid solution. The 

 beaker was then allowed to stand on the steam-bath for a few 

 hours, and in most cases at least one night intervened between 

 the precipitating and the filtering of the material. The pre- 

 cipitate was Altered on ashless paper (ash 0*00011 grm.) with- 

 out using pressure, ignited with the paper in a weighed plati- 

 num crucible, and brought to constant weight with a Bunsen 

 burner. From 2 to 5 cm3 of sulphuric acid were then added, the 

 gauze cone fitted in the crucible, the evaporation made as in 

 the previously described experiments, and the crucible with its 

 contents, the gauze, and cover, weighed. Only a trifling 

 change of weight would be expected, since the only solid mate- 

 rial that could be occluded in the original precipitate is barium 

 chloride. As this would take the place of an equivalent amount 

 of the sulphate, and the molecular weights of the chloride and 

 sulphate do not differ greatly, no very great error could be 

 introduced in this way. In the reverse process — the determi- 

 nation of sulphate by precipitation with excess of barium chlo- 

 ride — " the occlusion of barium chloride in the precipitate of 

 barium sulphate may lead to very serious error." * Table III 

 gives the results of the experiments just described, corrected 

 for the filter ash, 0*0001 grm. 



In the next series of experiments, 10 cm3 portions of a 20 per 

 cent solution of potassium chloride were added to the 100 cm3 

 of water and the l cn ' 3 of sulphuric acid before the precipitation 

 was made. The procedure just described was then followed 

 to the point where the evaporation with sulphuric acid was 

 completed. The material was then washed from the crucible 

 * Richards and Parker, Proc. Amer. Acad. Sci., vol. xxxi, p. 76. 



